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In This Article Expand or collapse the "in this article" section Twin Studies

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Twin Studies by Nancy L. Segal LAST REVIEWED: 19 March 2013 LAST MODIFIED: 19 March 2013 DOI: 10.1093/obo/9780199828340-0101

Twin research is an informative approach for understanding the genetic and environmental influences affecting behavioral, physical, and medical traits. The simple yet elegant logic of the twin method derives from the differences in genetic relatedness between the two types of twins. Identical (monozygotic or MZ) twins share 100 percent of their genes, while fraternal (dizygotic or DZ) share 50 percent of their genes, on average. MZ twins result when a fertilized egg (ovum) divides during the first two weeks following conception, while DZ twins result when a woman simultaneously releases two eggs that are fertilized by two separate sperm. MZ twins are always same sex, whereas DZ twins may be same-sex or opposite-sex. However, rare events occasionally produce unusual MZ and DZ twin variations. Twin researchers compare the resemblance between MZ and DZ twins with reference to a particular trait, such as height or weight. Greater resemblance between MZ twins than DZ twins demonstrates that the trait under study is under partial genetic control. There are also various ways that twins and their families can be used in research to increase the potential yield of a study. Sophisticated biometrical techniques can estimate the extent of difference among people associated with their genes, shared environments and nonshared environments. Twin research has proliferated in recent years. This is largely because the power of the twin method for understanding the origin and development of human traits has become increasingly appreciated by investigators representing diverse fields. Twinning rates have also increased dramatically since 1980, especially the rate of fraternal twinning as a consequence of fertility treatments. There have been stunning advances in quantitative mathematical methodology that continue to increase the value of twin studies. Lastly, there have been enormous developments in the molecular genetics and genomics fields with respect to associating genes posing increased risks for specific behaviors and disease. Twins will continue to play a prominent role in these endeavors. The sources presented in this article represent a wide range of areas and topics within twin research. General overviews of the field, both historical and current, are provided, as well as a listing of special collections in twin research, that is, books and journals focusing on a particular topic or theme and web addresses. The largest section includes topics reflecting the widening range of psychological, biological, and medical traits that have been examined via twin research methods. The section on twin-based perspectives provides sources treating unusual twin-related topics.

Twin research has had a successful yet controversial past, a trend that has continued through the present. Despite the wealth of information that has been derived from twin studies, various methodological and interpretive aspects continue to be questioned. The historical roots of twin studies, its acceptance into the mainstream of psychological and medical research, and its challenges are documented in a number of books, articles, and essays. The resources in this section span a wide range of twin-related topics. The five books are appropriate for experienced investigators and new scientists, as well as general audiences searching for information about the many ways twins are used in scientific studies. Johnson, et al. 2009 and Boomsma, et al. 2002 go more deeply into current trends in twin research but will interest anyone concerned with what twin studies have (and can potentially) reveal about the origins of variation in human behavioral and physical traits. The selections here include general overviews of the biological and psychological aspects of twinship ( Scheinfeld 1967 ), the nature-nurture debates ( Wright 1997 ), overviews of unusual topics in the study of twins ( Segal 2000 ), and cultural issues ( Stewart 2003 , Piontelli 2008 ). An older, but still informative account of the biology and psychology of twinning is also provided ( Bryan 1983 ).

Boomsma, Dorret, Andreas Busjahn, and Leona Peltonen. 2002. Classical twin studies and beyond. Nature Reviews (Genetics) 3:872–882.

DOI: 10.1038/nrg932

Describes and documents the potential of large twin registries to study complex human traits. Discusses various twin research designs (e.g., classic twin study, co-twin control, genotyping of marker loci) and their application in scientific research. Includes lists of twin registers in and outside European countries.

Bryan, Elizabeth. 1983. The nature and nurture of twins . London: Ballière Tindall.

A comprehensive examination of biological and psychological aspects of twinning by a British physician. Includes helpful information on twin types, twinning rates, and related topics. Also includes some specific topics not covered elsewhere, such as twin loss and twins with special needs.

Johnson, Wendy, Eric Turkheimer, Irving I. Gottesman, and Thomas J. Bouchard Jr. 2009. Beyond heritability: Twin studies in behavioral research. Current Directions in Psychological Science 18:217–220.

DOI: 10.1111/j.1467-8721.2009.01639.x

Makes the argument that the heritability of most behavioral traits is now known, yet twin studies retain a vital place in psychological research. Twin research should direct greater attention to environmental influences on behavior in a quest to identify its underlying mechanisms.

Piontelli, Alessandra. 2008. Twins in the world: The legends they inspire and the lives they lead . New York: Palgrave Macmillan.

Examines beliefs and practices regarding twinship from a cross-cultural perspective. The author’s background in neurology, psychiatry, psychoanalysis, and obstetrics substantially enriches the firsthand experiences she describes.

Scheinfeld, Amram. 1967. Twins and supertwins . Philadelphia: J. B. Lippincott.

An older, but complete survey of the history, biology, and psychology of twins before this became mainstream science. Often includes information that is difficult to find elsewhere.

Segal, Nancy L. 2000. Entwined lives: Twins and what they tell us about human behavior . New York: Plume.

A comprehensive overview of the background, methods, findings, and implications of twin research. Nine of the sixteen chapters address special topics such as athletic performance, legal circumstances, conjoined twinning, and noteworthy twin pairs. Written by a professor of psychology.

Stewart, Ellen. 2003. Exploring twins: Towards a social analysis of twinship . London: Palgrave Macmillan.

Addresses the social, societal, and cultural aspects of twinship. Also considers various views of twins from the perspectives of the twins, their family members, and society at large. Draws on sources from multiple disciplines.

Wright, Lawrence. 1997. Twins and what they tell us about who we are . New York: John Wiley.

An account of research concerning genetic and environmental events making MZ twins both alike and different in behavior. The focus is largely, but not exclusively, on separately raised twins. A very good starting point for work in this area, although more recent publications from the Minnesota Study of Twins Reared Apart should be consulted. Written by a well-known journalist.

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Published: 17 August 2018

Genetics, personality and wellbeing. A twin study of traits, facets and life satisfaction

Espen Røysamb 1 , 2 ,
Ragnhild B. Nes 1 , 2 ,
Nikolai O. Czajkowski 1 , 2 &
Olav Vassend 1

Scientific Reports volume 8 , Article number: 12298 ( 2018 ) Cite this article

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Heritable quantitative trait
Human behaviour

Human wellbeing is influenced by personality traits, in particular neuroticism and extraversion. Little is known about which facets that drive these associations, and the role of genes and environments. Our aim was to identify personality facets that are important for life satisfaction, and to estimate the contribution of genetic and environmental factors in the association between personality and life satisfaction. Norwegian twins (N = 1,516, age 50–65, response rate 71%) responded to a personality instrument (NEO-PI-R) and the Satisfaction With Life Scale (SWLS). Regression analyses and biometric modeling were used to examine influences from personality traits and facets, and to estimate genetic and environmental contributions. Neuroticism and extraversion explained 24%, and personality facets accounted for 32% of the variance in life satisfaction. Four facets were particularly important; anxiety and depression in the neuroticism domain, and activity and positive emotions within extraversion. Heritability of life satisfaction was 0.31 (0.22–0.40), of which 65% was explained by personality-related genetic influences. The remaining genetic variance was unique to life satisfaction. The association between personality and life satisfaction is driven mainly by four, predominantly emotional, personality facets. Genetic factors play an important role in these associations, but influence life satisfaction also beyond the effects of personality.

Disentangling the personality pathways to well-being

Personality traits and dimensions of mental health

Predicting wellbeing over one year using sociodemographic factors, personality, health behaviours, cognition, and life events

Introduction.

Human wellbeing and life satisfaction are influenced by life events, health, economy and social relations 1 , 2 . Life satisfaction is also closely connected to personality traits 3 , 4 , but the nature of this relation is partly unknown. There is limited knowledge about which specific aspects, or facets, of personality are most important. Further, both personality and life satisfaction are influenced by genetic factors 5 , 6 , 7 , 8 , but we have inadequate understanding of the role of genetic and environmental factors in explaining the links between personality and life satisfaction. Which traits, and which particular facets, are most important for promoting or obstructing individual life satisfaction? Are the associations accounted for by genetic factors, environmental factors, or both? Does the genetic influence on life satisfaction stem entirely from personality related genetics, or do genetic factors for life satisfaction operate independently of personality?

The scientific study of the good life and wellbeing has prospered in recent years 9 , 10 , 11 . As the field has grown, a number of constructs and approaches have emerged. The construct of subjective wellbeing (SWB) occupies a central position, and is typically seen as comprising three components - frequent positive affect, infrequent negative affect, and presence of life satisfaction 12 . Life satisfaction represents a global evaluation of life, a mental summarizing of life as good, or not so good, according to the individual’s own values, norms, and ideals 13 . As such, life satisfaction constitutes the key cognitive component in SWB and positive mental health 12 , 14 . In parallel with SWB, the construct of psychological wellbeing (PWB) contains components such as engagement, personal growth, and flow-experiences, thereby focusing more on functioning well than feeling well 15 , 16 , 17 . Research on SWB and PWB represent two different, but complimentary traditions, focusing on distinguishable yet related dimensions of wellbeing overall. The dimensions of SWB and PWB have also been integrated into broader models, such as the tripartite model of mental wellbeing (MWB) including emotional, psychological and also social wellbeing 18 . Thus, in correspondence with research on taxonomies and the nature of psychopathology (i.e., illbeing), the wellbeing field today addresses several aspects of the good life. Life satisfaction represents a central component in SWB in particular, but features as an important aspect inherent in most models.

Genetics of Wellbeing

Genetic factors appear to play an important role in most human characteristics 19 and wellbeing is no exception. Heritability estimates for different conceptualizations of wellbeing typically range from 0.30 to 0.50 20 , 21 , 22 , 23 , 24 , 25 . A meta-analysis of 13 studies from seven different countries and including more than 30,000 twins, reported a weighted average heritability of 0.40 for wellbeing 5 . This meta-analysis also found substantial heterogeneity in heritability estimates across studies, beyond that expected by random fluctuations, thus verifying the theoretical notion that there is no fixed heritability for wellbeing. Rather, the share of variance accounted for by genetic factors varies across cultures, age groups, and the particular wellbeing phenomena studied. Another meta-analysis by Bartels 6 , with somewhat different inclusion criteria, samples and analytic strategy reported an average heritability of 0.36 for wellbeing.

There is evidence of a common genetic influence on different wellbeing components such as subjective happiness, life satisfaction, SWB and PWB 24 , 26 , but also genetic influences that are specific to the different components 26 , 27 . The genetic factors in wellbeing are partly related to the genetic influences on social support 28 , and inversely, depression 29 and internalizing disorders 30 , 31 , 32 . Additionally, longitudinal studies have shown that genetic factors account for most of the stability in wellbeing with heritability for the stable variance, or dispositional wellbeing, estimated in the 70–90% range 33 , 34 . By contrast, environmental factors constitute the major source of change in wellbeing 34 , 35 .

Despite clear evidence of substantial genetic influences on wellbeing in general, findings on life satisfaction are somewhat divergent, with heritability estimates ranging from zero to 0.59 24 , 32 , 35 , 36 , 37 , 38 . The meta-analysis by Bartels 6 examined heritability of life satisfaction specifically, and reported an average heritability of 0.32. Thus, life satisfaction appears to be somewhat more influenced by environmental factors than other dimensions of wellbeing. Further, although levels of life satisfaction commonly vary only moderately with age, there might be age-related moderation of genetic and environmental factors. As life satisfaction represents an evaluation of life-so-far, life at older age likely include more life events, adversaries and accomplishments than life at younger age, thereby suggesting stronger environmental than genetic effects. There is a need for more knowledge about the genetic and environmental influences on life satisfaction in the mature population, measured by validated and reliable multi-item instruments.

Recent advances in molecular genetics have contributed to our understanding of the genetic underpinnings of wellbeing – including heritability. Genome-wide Complex Trait Analysis (GCTA) uses genotyping of common Single Nucleotide Polymorphisms (SNPs) in unrelated individuals to estimate heritability. Rietveld, et al . 39 reported that up to 18% of the variance in wellbeing can be explained by cumulative additive effects of genetic variants that are frequent in the population. This suggests that common genetic polymorphisms account for nearly half of the overall heritability of SWB. Genome-Wide Association Studies (GWAS) are used to identify specific genetic variants associated with a phenotype. Recently, Okbay, et al . 40 used GWAS in a sample of 298,420 individuals, and identified three credible genetic loci associated with wellbeing. However, these three variants explained only a small fraction (4%) of the variance. Molecular genetic studies expand rapidly and are expected to provide important new insights into the genetics of wellbeing. However, it also seems clear that twin and family studies are unique in their ability to capture the total genetic and environmental factors involved, along with the overall overlap and specificity across different characteristics.

Personality and Life-Satisfaction

Personality refers to relatively stable and characteristic patterns of cognition, emotion, and behavior that vary across individuals. These patterns are commonly described in terms of specific personality traits. The most widely known trait models today are the five-factor and big five models 41 , 42 , which converge on five broad personality traits, including extraversion, neuroticism, openness, conscientiousness, and agreeableness. There is a well-established relationship between personality traits and wellbeing in general, and personality and life satisfaction in particular 3 , 4 , 43 . More specifically, the big five traits of neuroticism and extraversion consistently explain substantial amounts of variance in wellbeing. The findings are more mixed regarding the trait of conscientiousness, whereas agreeableness and openness seem to play a limited, or negligible role in wellbeing 3 , 43 , 44 .

The five-factor model of personality is hierarchical with the higher-order domains (traits) comprising a set of lower-order facets 45 . For example, in the NEO-PI perspective, as developed by Costa and McCrae 41 , the domain of neuroticism includes the facets of anxiety, hostility, depression, self-consciousness, impulsiveness and vulnerability to stress. Correspondingly, the domain of extraversion includes warmth, gregariousness, assertiveness, activity, excitement-seeking and positive emotions. Despite solid evidence for relations between the general big five factors and wellbeing, there is still limited knowledge about which facets of the traits that contribute the most to wellbeing.

Theoretically, inter-personal facets such as warmth and gregariousness (sociability) contribute to wellbeing indirectly by creating well-functioning social relationships that subsequently influence wellbeing. Social support and good social relations have quite consistently been found to correlate positively with wellbeing 28 , 46 , 47 , and may partly be influenced by personality traits and facets.

There is also theoretical reason to expect factors contributing to accomplishments and goal attainment , in the conscientiousness domain, to be important for life satisfaction 48 , 49 , 50 , 51 . Life satisfaction judgments consider the gap between actual states and ideal states. Personality facets such as competence, self-discipline, achievement-striving and dutifulness may be important in obtaining ideal states, and are thus likely to predict life satisfaction.

Finally, personality tendencies to certain emotional experiences, such as anxiety or positive emotions may similarly influence wellbeing as life satisfaction judgments are coloured by both current emotional states and by memories of past emotional episodes. For example, a personality disposition to experience positive emotions may contribute to many episodes of joy and enthusiasm. These episodes may constitute a basis for the subsequent evaluation of life so far 4 , 52 , 53 . Thus, from a theoretical perspective both interpersonal facets, accomplishment-related facets and emotional facets would be important in generating a good life.

Empirical examinations of relations between personality facets and life satisfaction are limited. However, a few studies have shed light on the issue. Schimmack and colleagues 52 found the depression facet of neuroticism, and the positive emotions facet of extraversion to be the strongest and most consistent predictors of life satisfaction. They concluded that depression is more important than anxiety or anger, and a cheerful temperament is more important than being active or sociable. Quevedo and Abella 54 found depression and the achievement striving facet of conscientiousness, but not positive emotions, to be the important facets, whereas Albuquerque, et al . 55 identified depression and positive emotions as central, and found an additional effect from the vulnerability facet of neuroticism. Finally, Anglim and Grant 56 reported significant semi-partial correlations between life satisfaction and the three facets of depression, self-consciousness and cheerfulness.

These studies have provided important knowledge about the nuanced associations between personality and life satisfaction, and point to some particularly important personality facets. Yet, findings so far are limited, as the results are partly divergent, and mostly based on (young) student samples and convenience samples. Consequently, there is a need for replication of findings and expansion of cultures and age groups studied.

Genetic and Environmental Factors in Personality and Life Satisfaction

Personality traits are relatively stable characteristics, and there is considerable evidence for genetic components 57 , 58 . Although associations between personality traits, and partly their facets, and wellbeing are established, there is limited knowledge about the mechanisms involved in these associations. Are the associations between personality and wellbeing due to common genetic factors, and is the entire heritability of wellbeing accounted for by the genetic factors in personality – is wellbeing genetically speaking a personality thing?

A few studies have addressed these questions at the level of broad personality traits. First, Weiss and colleagues 59 found a global SWB-measure to be accounted for by unique genetic effects for neuroticism, extraversion and conscientiousness, and by a common genetic factor that influenced all five personality domains. Environmental factors also contributed to the associations, but there were no genetic effects unique to SWB. In a similar vein, Hahn and colleagues 38 reported shared genetic effects for life satisfaction and the traits of neuroticism and extraversion, but not conscientiousness. Both additive and non-additive genetic effects contributed to the relation between personality and life satisfaction, and again the entire heritability of life satisfaction was accounted for by personality-related genetic factors. Finally, a study examining personality traits and flourishing found substantial genetic effects on the associations, but also identified a unique genetic influence on wellbeing, unrelated to personality 60 . This latter study was unique in its focus on the construct of flourishing as comprising both eudaimonic and hedonic aspects of wellbeing, based on Keyes’ tripartite model including emotional, psychological and social wellbeing 61 , and thereby also involving both feeling good and functioning well.

Thus, a few recent studies have reported exciting evidence of a substantial genetic contribution to the association between personality traits and wellbeing. However, several important questions remain to be addressed. First, no studies to date have examined genetic and environmental contributions to the associations between personality facets and wellbeing. Given the findings for broad personality traits, we hypothesize considerable genetic effects also for their facets. Yet, the magnitude of such effects is unknown. Second, only one study 38 has examined life satisfaction specifically – rather than global measures of wellbeing. Third, as previous studies have relied only on short-form measures of broader traits, there is a pressing need for examining both traits and facets in relation to life satisfaction by means of comprehensive, valid, well-established instruments. Fourth, findings from the few previous studies are divergent as to whether the entire genetic effect on wellbeing is due to personality-related genetic influences. Fifth, whereas prior studies have examined samples with broad age ranges, we wanted to examine a specific period in life – middle to late adulthood – to assess how relatively stable personality characteristics contribute to life satisfaction in a life course perspective. Finally, as previous studies have been inconclusive regarding sex-differences in the underlying etiology of wellbeing 6 , 62 , we also wanted to test for such differences.

The aims of the current study were to (a) identify personality traits and facets that contribute uniquely to life satisfaction, and thereby pinpoint the dispositional constituents of a happy personality, (b) estimate the heritability of life satisfaction in middle to late adulthood, (c) disentangle the genetic and environmental influences shared by personality traits/facets, and life satisfaction, and finally hence to (d) determine whether all of the genetic influence on life satisfaction is due to personality related genetic factors as suggested by some previous studies.

Correlation and Regression Analyses

As shown in Table 1 , neuroticism, extraversion, and conscientiousness were all significantly correlated with life satisfaction, while agreeableness and openness were not. The strongest correlation was found for neuroticism, yet with substantial associations also for extraversion and conscientiousness. In the multiple regression analysis including these three factors, only neuroticism and extraversion showed significant unique contributions. The effects remained when controlling for sex and age. A total of 24% of the variance in life satisfaction was accounted for.

We next examined the associations for all the 30 personality facets. Table 2 shows the resulting correlations. A total of 23 facets were significantly associated with life satisfaction. In the neuroticism domain, all facets showed significant correlations, ranging from −0.14 for impulsiveness to −0.51 for depression. Within extraversion, excitement seeking was virtually unrelated (0.05) to life satisfaction, whereas positive emotions (0.30) and activity (0.28) showed substantial associations. In the openness domain, only one facet, ideas, showed a significant but very modest correlation (0.08). The agreeableness domain was notable for a combination of positive and negative associations. Trust (0.17) and altruism (0.09) were positively associated with life satisfaction, while negative associations were shown for modesty (−0.08) and tendermindedness (−0.08). Finally, in the conscientiousness domain, all factors showed significant and positive correlations, and in particular competence (0.30) and self-discipline (0.28) appeared to be potentially important.

Next, regression analyses were conducted in which all 30 facets were tested simultaneously. Ten facets showed significant and unique effects (Table 2 ). In total, these facets explained 33% of the variance (adjusted R 2 = 32%) in life satisfaction. Four facets yielded substantial betas, that is above 0.10, and with p < 0.01, namely N1-anxiety, N3-depression, E4-activity and E6-positive emotions (label N1 refers to Neuroticism facet 1, etc). The remaining significant facets were found across all personality domains and included the openness facets of values and actions, the agreeableness facet of compliance, and the conscientiousness facets of order and deliberation. However, these effects were relatively minor (i.e., beta <0.10). Also, when performing Bonferroni correction and examining the False Discovery Rate 63 , only the four facets with betas >0.10 retained p < 0.01. Thus, for the biometric analyses disentangling genetic and environmental effects we focused on these four facets with substantial and significant effects. Summarizing the regression findings, the happy, or satisfied personality is given by the equation:

Biometric twin analyses

Twin-cotwin correlations across zygosity groups were calculated for the neuroticism and extraversion traits, the four major facets (i.e., anxiety, depression, positive emotion, activity) and life satisfaction. Table 3 shows the correlations. In general, the monozygotic (MZ) correlations were substantial, and in all cases higher than the corresponding dizygotic (DZ) correlations, indicating additive genetic effects.

Based on the findings from the regression analyses, we next tested a set of tri-variate Cholesky models including neuroticism, extraversion and life satisfaction. Table 4 (upper part, block I) shows the fit of the different models. Model 1 included additive genetic (A), common environmental (C) and non-shared environmental (E) factors, and allowed estimates to vary across sex. Model 2, which included only A and E effects, did not fit significantly worse (i.e., Δ − 2LL = 0.89, Δdf = 12, n.s.) and produced a lower AIC value. Further, models 3 and 4, involving scalar sex-limitation, yielded additional improvements in fit, that is, increasingly lower AIC values, no significant reduction in fit, and more parsimony. Finally, models 5 and 6, where parameters were constrained to be equal across sex, resulted in higher AIC and worse fit. Thus, model 4 yielded overall best fit, and included only A and E effects with standardized parameters similar for men and women. Figure 1 shows the Cholesky parameters of the model.

Biometric Cholesky model of neuroticism, extraversion and life satisfaction. A = Additive genetic factor; E = Non-shared environmental factor; All parameters: p < 0.05, except one parameter (n.s.) in parenthesis and dotted arrow.

Heritability estimates (with 95% CIs) were 0.53 (0.46; 0.60) for neuroticism, 0.49 (0.41; 0.56) for extraversion and 0.32 (0.23; 0.41) for life satisfaction. Based on the Cholesky model we also calculated the genetic and environmental correlations between the two personality traits and life satisfaction. Genetic correlations were −0.70 (−0.58; −0.83) for neuroticism, and 0.53 (0.37; 0.68) for extraversion. Correspondingly, environmental correlations were −0.32 (−0.23; −0.40) for neuroticism and 0.15 (0.08; 0.24) for extraversion.

Moving from the big five factors to the personality facets, again we tested a set of models including the four facets found to be most strongly predictive of life satisfaction. Table 4 (lower part, block II, models 7–12) shows the results. Again, the best fitting model included only A and E effects (model 10), and standardized estimates did not differ across sex. Figure 2 shows the parameter estimates of the best model.

Biometric Cholesky model of four personality facets (anxiety, depression, activity and positive emotions) and life satisfaction. A = Additive genetic factor; E = Non-shared environmental factor; N = Neuroticism; E = Extraversion. All parameters: p < 0.05, except three parameters in parentheses and dotted arrows.

In this best-fitting model, heritabilities were estimated to 0.47 (0.40–0.54) for anxiety, 0.46 (0.38–0.53) for depression, 0.42 (0.33–0.49) for activity, 0.40 (0.32–0.48) for positive emotions, and 0.31 (0.22–0.40) for life satisfaction. As can be seen in Fig. 2 , genetic factors from both the neuroticism and extraversion facets uniquely influenced life satisfaction. However, after the effect of latent factor A1 (reflecting the genetic variance in anxiety) was accounted for, there was no additional genetic effect from the unique genetic factor of depression (A2). Likewise, the genetic variance in activity (A3) influenced life satisfaction, but there was no additional genetic effect from positive emotions (E4). Thus, the genetic variance in each of the two personality domains, which influenced life satisfaction, appeared to be shared by the facets within their respective domain (neuroticism or extraversion), and the facet-specific influences on life satisfaction appeared to be driven by environmental effects. Notable is also the unique genetic factor (A5) influencing life satisfaction after all the genetic effects of the facets were accounted for.

A total of 20% of the variance in life satisfaction was accounted for by personality-related genetic factors, and 11% was explained by a genetic factor unrelated to personality. Thus, of the total heritability of life satisfaction ( h 2 = 0.31), about 65% was driven by personality genetic factors, and the remaining 35% was due genetic influences independent of personality. Further, the combined effect of personality facets on life satisfaction also involved environmental effects, accounting for 11% of the variance. Finally, 58% of the variance in life satisfaction was environmental in origin and unrelated to personality. This environmental component includes random measurement error (1-alpha = 9%), thus implying an estimated true non-shared environmental component of 49%. Figure 3 shows the decomposed sources of variance for life satisfaction, along the corresponding variance components of the four facets.

Life satisfaction: Sources of origin decomposed. Genetic and non-shared environmental components, divided into personality-based and non-personality sources. Estimated random error (1-α) also shown for life satisfaction. For facets, additive genetic (A) and non-shared environmental variance (E) shown.

Based on the best-fitting model, we also calculated genetic and environmental correlations for the variables, shown in Fig. 4 , above and below the diagonal, respectively. Generally, the genetic correlations within personality domains were high, and the genetic correlations between facets and life satisfaction were moderate to high. The corresponding environmental correlations were generally lower, but suggested also important associations due to environmental factors.

Genetic and environmental correlations, above and below diagonal, respectively.

We set out to delineate etiological factors involved in the associations between personality and life satisfaction. Personality traits are well-established predictors of wellbeing in general and life satisfaction in particular 3 , 43 . The issue of why personality traits influence life satisfaction was addressed along two paths: First, we examined the broad personality traits and the specific personality facets that drive the effects from traits. Second, we examined the role of genetic and environmental factors in the link between personality and life satisfaction.

Personality and life satisfaction

At the level of broad traits, neuroticism and extraversion were uniquely predictive of life satisfaction, in line with previous studies 3 , 4 . Further, four facets of unique importance for life satisfaction were identified, namely anxiety and depression from the neuroticism domain, and positive emotions and activity from the extraversion domain. The happy, or satisfied personality thus seems to have low levels of anxiety and depression, and high levels of positive emotions and activity. The highly emotional nature of these facets is noteworthy. That is, three out of the four facets explicitly refer to affective tendencies, whereas the fourth facet (activity) adds vigor, energy and liveliness 41 . Thus, the cognitive evaluation of life satisfaction is partly based on emotional tendencies inherent in the big five model. Our findings accord with previous studies in identifying depression, and partly positive emotions, as central predictors of life satisfaction 52 , 55 . However, whereas prior studies have found facets such as vulnerability, excitement-seeking 52 , and achievement striving 54 to be significant, in this population based sample covering middle to late adulthood, we found anxiety and activity to be important.

Although a high number of facets were correlated with life satisfaction at the zero-order level, most facets did not show unique effects on life satisfaction in the multivariate analyses. There were no unique effects from interpersonal facets such as warmth, assertiveness, gregariousness, trust or straightforwardness. Neither did we find effects from accomplishment-related facets such as competence, self-discipline or dutifulness. This does not imply that having warm and trustful relations, or high levels of competence, are inconsequential for wellbeing. Rather, we interpret the findings to suggest that the predominantly emotional facets are underlying tendencies accounting for some of the zero-order associations between other facets and life satisfaction.

Why and how do depression, anxiety, positive emotions and activity play such important roles in generating a good – or not so good – life? We believe that a dual set of mechanisms are involved. First, from a top-down perspective 64 , 65 , life satisfaction is influenced by a general way of seeing life, the glasses through which we perceive the world. Therefore, negative and positive affective tendencies might color our ongoing evaluations of what life has been like.

Second, and in accordance with a bottom-up perspective 64 , positive and negative affective tendencies over time contribute to life experiences that are taken into account when performing a current evaluation. That is, a person with a strong tendency to experience positive emotions and activity/energy, combined with a low tendency to depression and anxiety, might recall a high number of episodes characterized by such experiences, and thereby summarize life as mostly good. In contrast, a person prone to anxiety and depression, who experiences few positive emotions and low activity/energy, might have a mental album comprising of numerous episodes and life periods that are less satisfactory.

Importantly, depression (sadness/distress), anxiety (fear) and positive emotions are represented in most models of basic emotions 66 , 67 , 68 , 69 . These basic emotions are seen as evolutionary adaptive and functional responses to environmental exposures. Although we are all equipped with the potential to experience such emotions, from a personality perspective there are individual differences in our tendency to activate them, and as such they are encompassed as facets in the five-factor personality model. Adding the facet of activity (energy) to the equation we have four basic building blocks, inherent in our personality, that contribute uniquely to a good life. In a dual-process model these tendencies operate both by coloring current perceptions of life-so-far, and by having contributed to a number of positive and/or negative experiences throughout the life lived.

In the wellbeing-illbeing structural model (WISM), wellbeing is conceptualized as comprising both well-staying and well-moving, and illbeing is correspondingly divided into ill-staying and ill-moving 15 , 23 . The model posits that humans have various goal states, and we may experience the presence of an obtained goal state (well-staying), we may be in a process towards a desired goal (well-moving), we may experience threats implying a risk of losing goals (ill-moving), and finally we may realize that a goal state is lost (ill-staying). The current findings are noteworthy in identifying personality facets that have certain connections to these four goal-state conditions. Positive emotions can be seen as indicative of well-staying, activity is potentially important for well-moving, anxiety is a core feature of ill-moving and depression is a characteristic of loss and ill-staying. Thus, our findings lend support to the notion of well-staying, well-moving, ill-staying and ill-moving as fundamental human scenarios that all are important for generating or obstructing good lives.

The role of genetic and environmental factors

The estimated heritability for life satisfaction was 0.31. This is in the lower range of previous estimates for general wellbeing 24 , 35 , and below a meta-analysis estimate of 0.40 5 . However, although findings are divergent, several studies have reported heritability estimates for life satisfaction that are moderately lower than for other wellbeing constructs 32 , 70 , 71 , and the meta-analysis by Bartels 6 reported a heritability of 0.32 for life satisfaction. Our study is one of the first to examine life satisfaction beyond midlife specifically, with a well-established instrument. The findings point to both genetic and environmental influences – yet with the latter clearly being the most important. As such, life satisfaction appears to be more about the environmentally influenced life course, events and relationships, than about a genetically driven tendency. Such an interpretation also implies potentials for change in life satisfaction, and possibly substantial benefits of wellbeing interventions 35 , 72 .

We tested models examining sex-differences in the genetic and environmental sources of wellbeing. In line with several studies 6 , 21 , 70 , but in contrast to some others 62 , 73 , we found the heritability, and the environmental component, to be of similar magnitude for females and males. Although the total variance might vary, our findings provide evidence that the relative contribution of genetic factors is similar across sex.

While genetic factors seem to play only a moderate role for the total variability in life satisfaction, genetic factors appear to have a major role in the association between personality and life satisfaction. Both at the levels of broad traits and more specific facets, genetic factors were highly important in explaining the effect of personality on life satisfaction. That is, there are genetic factors influencing personality that also influence life satisfaction, whereas environmental factors play a more limited role in this relationship. More specifically, the genetic dispositions to experience a low degree of depression and anxiety, and a high degree of positive emotions and activity contribute to a life experienced as good and satisfactory.

To our knowledge this study is the very first to examine genetic factors in the association between personality facets and life satisfaction. In general, our finding of genetic factors playing a key role accord with the few previous studies examining broad personality traits and wellbeing in genetically informative samples 38 , 59 , 74 . However, whereas two of these previous studies found the entire heritability of wellbeing to be due to personality-related genetic factors 38 , 59 , in line with Keyes et al . 60 we identified a unique genetic factor influencing life satisfaction beyond the effect of personality, accounting for 11% of the total variance. We can only speculate on the genetic mechanisms involved. Theoretically, there could be a specific, genetically driven, tendency to having a positive outlook on life that is not captured within the five-factor model. Alternatively, there could be influences from conditions such as mental abilities or somatic disorders – both of which have substantial genetic influences 19 – that also are outside the personality domain. Further studies are required both to address this aspect of life satisfaction, and generally to delineate the complex processes starting with DNA-molecules and ending up with a person evaluating her life as good – or not.

The findings also accord with a recent molecular genetic study of the association between wellbeing and neuroticism. Okbay, et al . 40 used GWAS and bivariate Linkage Disequilibrium Score regression, and reported a genetic correlation of −0.75 between wellbeing and neuroticism. Despite the limited variance explained in the GWAS it is noteworthy that the correlation corresponds highly with the current estimate of genetic correlations of −0.70 for neuroticism, −0.74 for the anxiety facet, and −0.71 for the depression facet.

It is also noteworthy that there was a common genetic factor for anxiety and depression that contributed to life satisfaction, and there was no unique genetic variance in depression that predicted life satisfaction beyond that shared with anxiety. The facet-specific influences appear to be driven by environmental effects. Corresponding findings were seen for extraversion; a common genetic factor for activity and positive emotions contributed to the genetic variance in life satisfaction.

Strengths of the current study include a population based sample, a fairly high response rate, and well-established extensive measurements. Nevertheless, some limitations should be noted. First, as with any twin study, heritabilities and genetic correlations are not fixed figures, but are estimated for a certain population, and only future studies can validate the findings across other societies and age groups. Second, the sample size implies limited ability to identify small effects – potentially common environmental factors or sex differences. Third, although the NEO-PI-R is a well-established instrument, the reliabilities of the facets were partly limited. Measurement error is captured in the E-factor in the biometric analyses, and might contribute to reduced environmental, but not genetic, correlations.

The findings replicate previous studies of wellbeing and life satisfaction as influenced by genetic factors – with heritabilities in the 30–40% range 5 , 6 . We also replicate substantial associations between wellbeing and personality, both for the general traits of neuroticism and extraversion, and for specific facets 3 , 52 , 56 . Moreover, we identified four personality facets that appear to play an important role in driving the associations between personality and life satisfaction. These facets include basic emotional tendencies, and point to the importance of emotions as sources of direct and indirect pathways that contribute to good lives. Roughly two thirds of the genetic variance in life satisfaction was found to be due to these facets. In addition, we found a certain genetic component in life satisfaction unrelated to personality traits or facets. Finally, the findings provide solid evidence of the role of environmental factors in generating good lives – also by contributing to associations between personality and life satisfaction.

Twins were recruited from the Norwegian Twin Registry (NTR). The registry comprises several cohorts of twins 75 , 76 , and the current study drew a random sample from the cohort born 1945–1960. In 2010, questionnaires were sent to a total of 2,136 twins. After reminders, 1,516 twins responded, yielding a response rate of 71%. Of the participants, 1,272 individuals were pair responders, and 244 were single responders. Zygosity has previously been determined based on questionnaire items shown to classify correctly 97–98% of the twins 77 . The cohort, as registered in the NTR, consists only of same-sex twins, and the study sample consisted of 290 monozygotic (MZ) male twins, 247 dizygotic (DZ) male twins, 456 MZ female twins and 523 DZ female twins. The age range of the sample was 50–65 years (mean = 57.11, sd = 4.5). The study was approved by the Regional Committee for Medical and Health Research Ethics of South-East Norway, and informed consent was obtained from all participants. All methods were performed in accordance with relevant guidelines and regulations.

Life satisfaction was measured with the Satisfaction With Life Scale (SWLS) developed by Ed Diener and colleagues 78 , 79 . The SWLS contains five items, such as “I am satisfied with my life”. Response options range from 1 = strongly disagree to 7 = strongly agree . The SWLS is widely used in wellbeing research, and has well-established psychometric properties 80 . Cronbach’s alpha in the current sample was 0.91.

Personality was measured by the NEO-PI-R 45 , 81 . The NEO-PI-R contains 240 items tapping the five general factors of personality, namely neuroticism, extraversion, openness to experience, agreeableness and conscientiousness. Within each of these factors, or domains, the NEO-PI-R measures six facets, or sub-factors (see results section for overview of all 30 facets). Each of these facets is measured by eight items. Response options range from 1 = strongly disagree to 5 = strongly agree . The NEO-PI-R is a well-established instrument, with sound psychometric properties 41 . In the current sample alphas for the five factors were 0.92 (neuroticism), 0.87 (extraversion), 0.88 (openness), 0.84 (agreeableness) and 0.87 (conscientiousness). Alphas for the facets ranged from 0.47 (C5 self-discipline) to 0.85 (N1 anxiety), with a mean of 0.67.

Correlations were used to examine the bivariate associations between life satisfaction and personality traits and their facets. Next, we used regression analyses to (a) examine the unique contributions from the five broad personality traits, and to (b) identify the facets that are important for the association between personality and life satisfaction. Due to the non-independence of observations within twin pairs we used Generalized Estimating Equations (GEE) to account for the paired structure to obtain correct standard errors and significance levels. Further, to adjust for multiple testing we performed subsequent analyses with Bonferroni correction and the False Discovery Rate (FDR) approach 63 .

Based on the regression analyses we conducted two sets of multivariate biometric analyses to estimate the genetic and environmental contributions to the associations between personality and life satisfaction. The first set examined the relation between the major big five factors and life satisfaction. The second set of analyses focused on the specific facets that uniquely predicted life satisfaction. In order to focus on facets with substantive effects, we chose to retain only facets yielding regression betas >0.10, and with p < 0.01.

Standard Cholesky models 82 , 83 were used to estimate the genetic and environmental contributions to variance and covariance in personality and life satisfaction. All models were run with the OpenMx package in R 84 . The biometric models take advantage of the basic premise that MZ twins share 100% of their genes, whereas DZ twins share on average 50% of their segregating genes. Generally, the models allow for estimating three major sources of variance, including additive genetic factors (A), common environment (C) and non-shared environment (E). In addition, non-additive genetic effects (D) may be tested, but are only indicated if the observed MZ-correlations are more than twice the DZ-correlations. A Cholesky model is a structural equation model comprising the measured variables as observed phenotypes and the A, C and E components as latent factors (for illustration see Fig. 1 ). Models are constrained so that latent A-factors correlate perfectly among MZ-twins, and at 0.5 among DZ-twins. C-factors are correlated at unity for both zygosity groups, and E-factors are by definition uncorrelated. Different models are compared to determine the presence of the genetic and environmental effects (e.g., the fit of an ACE model is compared to an AE model) or sex-differences. In line with standard practice, we tested different types of sex-limitation models 85 . First, common sex-limitation models allow parameter estimates to vary across sex, involving differences in magnitude for genetic and environmental effects. Second, scalar sex-limitation allows the unstandardized variance-covariance matrices to vary across sex, but standardized parameters (e.g., heritabilities) are constrained to be equal. Finally, the sex-limitation models were compared with models having all parameters constrained to equal across sex. To assess models and identify the best fitting model we used the minus2LogLikelihood difference (Δ − 2LL) test, and the Akaike Information Criterion (AIC) 86 .

Data availability

The dataset analyzed during the current study may be requested from the Norwegian Twin Registry. Restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Information about data access is available here: https://www.fhi.no/en/studies/norwegian-twin-registry/

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Twins in Behavioral Science Research

Reviewed by Psychology Today Staff

The information gathered from the study of twins lays the groundwork for future genetic research, including the degree to which any aspects of life are determined by genetics, the location of specific genes, and the prevention and treatment of diseases and disorders. All of which makes them valuable to study .

Twins share everything from their in-utero environment and parents to birthdate and the classroom setting. One unusual case study involved two-and-a-half years old identical twins, one of whom nearly drowned in cold lake water. Researchers wondered whether he would suffer intellectual deficits. Later, the twins completed a number of tests that measured different skills. Surprisingly, meaningful cognitive differences between the two brothers were not found.

There is much to celebrate in the research of twins. Even though many share environments before and after birth, no two twins are exactly the same. These conditions make every identical twin pair a mini-experiment. Close attention has been paid to identical twins reared apart by researchers such as Nancy Segal .

In 1875, Sir Francis Galton, a half-cousin of Charles Darwin, published research in the UK publication Fraser’s Magazine . His article, “The History of Twins, As a Criterion of the Relative Powers of Nature and Nurture,” describes the study of twins and how nature and nurture affect development.

Briana Mezuk and colleagues recruited some 50 pairs of identical twins from the Mid-Atlantic Twin Registry, the largest such registry in the US. The researchers found that studying twins can tell us how stress and mental health contribute to the risk of diseases like type-2 diabetes. And at King’s College London, the study of twins has focused on the genetics of healthy aging, examining the risk of diseases that include cardiovascular, metabolic, musculoskeletal, and ophthalmologic disorders.

In studies on mood, researchers have used inflammation indicators to help predict the severity of depressive symptoms. A study that examined twins, identicals who share 100 percent of their genes, found that the twin who registered a higher measure of inflammation was more likely to develop depression five years later.

This is a method in twin research that happens naturally, experimentally, or by chance when two identical twins have different experiences, and allows for a key comparison within the pair. Natural co-twin control happens when identical twin partners show differences in their appearance, health, or behavior that are not caused by any form of intervention. Experimental interventions occur when a researcher subjects one twin to a specific treatment or training program, and provides the other twin with a different experience or no treatment at all. For example, one twin may have been bullied, while the other was not, or one twin may have been exposed to the measles, while the other was not.

Throughout the years, many researchers have been examining twins. Twin registries are found throughout the world, all following thousands of identical and fraternal twins. Registries can be found in the United Kingdom, Norway, Sweden, and Finland, and the U.S. In this country alone, we have the National Academy of Sciences-National Research Council Twin Registry, the Minnesota Twin Registry, the Vietnam-Era Twin Registry, among many others.

Psychoanalyst Peter Newbauer directed the separation of four sets of identical twins and one set of identical triplets, as part of a longitudinal research project conducted in the 1960s and 1970s, which was funded by the NIMH. Newbauer was an NYU psychiatry professor who also consulted with a Jewish adoption agency; they arranged to have the newborn babies assigned to deliberately different adoptive parents--affluent versus non-affluent. This study was reported in various publications, as well as the documentary, Three Identical Strangers.

Astronaut Scott Kelly spent nearly a year in low-Earth orbit on the International Space Station in 2015-2016; meanwhile, his twin brother Mark, also an astronaut, stayed on Earth. Researchers wanted to see the effects of space travel in a genetically controlled situation. When compared with Mark, they found numerous changes in Scott such as bone density and altered telomeres—which protect the ends of chromosomes from the aging process. These changes, for the most part, returned to normal after his return to Earth.

The best-known research on twins reared apart was founded and run by psychologist Thomas Bouchard and his colleagues. The study has examined variance in traits—such as height, weight, IQ, rate of speech, and many other traits—in twins separated at birth and reared apart.

One of the world's largest twin research studies is The Swedish Twin Registry at the Karolinska Institute. It has data on 194,000 twins, including identicals and fraternals. The 60-year-old effort studies twins to help us understand health and disease including research on cancer, cognitive function, depression, substance use, ADHD, among many other areas. It has information on every twin born in Sweden since 1886.

This registry, founded in 1993, follows 12,000 twins, mostly made up of females and middle-aged or older. The registry has collected blood, urine, and tissue samples from identical and fraternal twins over the decades for comprehensive analysis of the biological makeup of twins.

In the early 20th century, the work of Josef Mengele as well as his mentor Otmar Freiherr von Verschuer included twins research. Mengele, known as the Angel of Death, conducted many experiments in Auschwitz during World War II. Both Mengele and von Verschuer used studies on twins to advance racial discrimination. Reportedly, Mengele sent specimens of eyes and blood samples from the 200 twins on which he conducted unethical human research, to von Verschuer for analysis. Only 10 percent of those twins survived Mengele's experimentation.

Family outsiders often do not believe that twins fight and don't get along.

Large research projects involving twins suggest a strong link between genetics and the likelihood of being unfaithful to a romantic partner.

In the worst of times, competition over which twin looks best can lead family members to pull their hair out or at least take a really long walk.

How are twin studies used to answer questions related to the nature-and-nurture debate?

The only way to handle this competition or sharing of friends problem is for parents and significant others to see it as a serious identity issue.

It is a steep learning curve for twins to accept and enjoy being different.

Here are some thoughts that twins have shared with me about the desire to connect in a peaceful manner.

Separation anxiety creates more conflict and more incentive to be different and unique from their twin.

Holidays for twins and their families are loaded with unfortunate and hard-to-avoid obstacles related to comparisons.

The side effects of too much advice are fighting and estrangement.

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Twin Studies

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Studies of twins ; Twin research

Research utilizing twins, usually with the goal of assessing the approximate contributions of heredity and environment toward a characteristic by comparing identical and fraternal twins (APA Dictionary of Psychology n.d. )

Introduction

Since they started almost a century ago, twin studies have been integral to the field of behavioral genetics by providing valuable information about the relative influence of genetics and the environment on different complex traits, behaviors, and illnesses. This is possible because twin pairs can come in two forms: monozygotic and dizygotic. Monozygotic (MZ) twins, or identical twins, share 100% of their genetic makeup. Since their genetics are the same and held constant, differences found between a pair of MZ twins are likely the result of environmental influences that only one twin has experienced, or a unique, nonshared environment. On the other hand, dizygotic (DZ) twins, or fraternal twins, share...

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Key Study: The Minnesota Twin Study of Twins Reared Apart

Travis Dixon February 11, 2019 Biological Psychology , Key Studies , Studies and Theories

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Understanding how and why twin studies are used is an important topic in biological psychology because they can give us important insights into the extent to which our behaviour is nature (genetics) or nurture.

Is our behaviour a product of nature or nurture? In other words, are we born the way we are, or have we become this way through years of experiences? This is the classic question in psychology and one that Bouchard and his colleagues have attempted to answer since 1979 at their Minnesota Centre for Twin and Family Research (MICTFR) .

In this time, over 100 sets of reared-apart twins and triplets have participated in the research. Rigorous analysis of scores of data has enabled the researchers to draw some strong conclusions. The following “study” is similar to the Vietnam Head Injury Study ( read more here ) in that it is one report from an ongoing longitudinal study.

Twin and kinship studies are used to determine heritability – the extent to which variations in behaviour can be attributed to genetic factors. e.g. 50% heritability for IQ means that differences in IQ are half (50%) due to genetics, and the other half is environment.

Monozygotic (identical) twins have 100% of their DNA in common, whereas dizygotic (fraternal) twins have on average 50% of their DNA in common. Twin studies compare similarities in behaviour between MZ and DZ twins. Comparing similarities in behaviour between MZ and DZ twins allows researchers to see the extent to which these variations are based on genetics.

If two identical twins grow up to be completely different from one another, we can assume that their environments were more influential in their behaviour than genetics. However, if they grow up similar despite very different childhoods, we can . conclude genetics is more important. This is basically how twin studies work, although they are more scientific in their approach.

In adoption studies (like the one below), the can also compare MZT twins (monozygotic together twins) – those who have been raised in the same household and those who have been raised apart (MZA). This allows researchers to compare the variable of the environmental influences on behaviour (because genetics is a constant variable between these two groups).

The behaviour we will focus on in this particular review is intelligence (IQ) .

The researchers for this study (and continual studies) don’t gather all their data at once, but continually, as they find participants to take part in the research. The participants come from mainly the UK and USA, but have also included Australia, Canada, China, New Zealand, Sweden and Germany.

They become involved either because:

the twins, a friend or family member finds out about the research
someone involved in the adoption process works with the MICTFR to put them in contact with the twin
Or…one twin becomes aware that they may have a twin somewhere and they contact the MICTFR and ask for their help in finding their separated twin.

Twin study by Bouchard et al. (1990)

For this study, the average age of the twins when they participate in this study was 41, which is important because most twin research prior to this focused on adolescents. The twins spent an average of 5 months together before being separated and reunited (on average) around 30 years of age.

Physical and psychological data was gathered in a number of different ways, which took around 50 hours. Methods were triangulated, either using researcher triangulation or methodological triangulation. For instance, when measuring IQ, three different IQ tests were used to gather and triangulate the data. And two different researchers conducted similar tests on the same participants. Controls were also established, like conducting the IQ tests on the twins at the same time but in different rooms under strict supervision by researchers.

Can you think of any ethical issues with this study?

To control for confounding variables in the environment, rigorous data was gathered on the childhood environments of the participants. For instance, a “Moos Family Environment Scale” was used to compare the impressions of the participants’ childhoods and a questionnaire was given to measure access to physical facilities, such as material possessions and cultural, mechanical and scientific goods. For instance, were their dictionaries, artworks and power tools in the house when they were growing up? This type of data enables researchers to draw conclusions regarding socio-economic environment of the families and where the participants grew up.

The analysis of the data revealed no significant difference between MZA twins (reared apart) and MZT twins (reared together) in regards to personality measures such as temperament, hobbies, interests, career pursuits or social attitudes.

Similar to previous research, this study also concluded that about 70% of differences between IQs in twins is due to genetic variation (70% heritability); the remaining 30% of difference is caused by environmental factors, which is similar to previous research.

There was also evidence from this study that suggested that twins that spend more time together after they are re-united are more similar. However, the data also suggested that it is the level of similarity between the twins that determines how much time they spend together, not the other way around.

Through their analysis of genetic and environmental variance, Bouchard et. al. concluded that genetics are an important factor in determining behaviour, but environment is also important. In addition to other research that suggests IQ similarities between children and adults increase over time , the researchers conclude that it can be our genetics that determines our environmental experiences. For example, if we have a naturally introverted disposition due to genetics, this will influence our psychological and personal experiences in life. If we think about neuroplasticity (the brain’s ability to change as a result of experience), you should be able to see how our intelligence may be malleable and this malleability is caused by a combination of genetic and environmental factors.

Critical Thinking Questions

How do the results of this study show intelligence is influenced by genetics and/or our environment? ( Application)
What are some relevant ethical considerations particular to this study? ( Analysis)
What are the strengths and limitations of this longitudinal study? ( Evaluation)

Bouchard, Thomas J, Jr. Lykken, David T. McGue, Matthew. Segal, Nancy L and Tellegen, Auke. “ Sources of Human Psychological Differences: The Minnesota Study of Twins Researched Apart.” Sciences, New Series, Vol. 250 (1990), pp223-8. Accessed from web.missouri.edu

Travis Dixon is an IB Psychology teacher, author, workshop leader, examiner and IA moderator.

A Brief History of Twin Studies

As NASA dives into the data from astronaut twins, take a look back at the famous, and infamous, results we’ve seen from this popular research tool

On Tuesday, NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko touched down in Kazakhstan after spending a whopping 340 days aboard the International Space Station (ISS).

As part of NASA’s "Year in Space" project, Kelly and his Earth-bound identical twin brother, retired astronaut Mark Kelly, provided samples of blood, saliva and urine and underwent a barrage of physical and psychological tests designed to study the effects of long-duration spaceflight on the human body.

Studies of identical and fraternal twins have long been used to untangle the influences of genes and the environment on particular traits. Identical twins share all of their genes, while fraternal twins only share 50 percent. If a trait is more common among identical twins than fraternal twins, it suggests genetic factors are partly responsible.

"Twins studies are the only real way of doing natural experiments in humans," says Tim Spector , a professor of genetic epidemiology at Kings College, London. "By studying twins, you can learn a great deal about what makes us tick, what makes us different, and particularly the roles of nature versus nature that you just can't get any other way.”

Spector is director of the TwinsUK Registry , which includes data from 12,000 twins and is used to study the genetic and environmental causes of age-related complex traits and diseases. He estimates that twins research is currently being conducted in more than 100 countries, and that most of those projects draw upon information contained in large databases such as the TwinsUK Registry.

While it may be a while before we see results from the astronaut twins, researchers are hopeful that the opportunity will yield some unique insights into human health. Here are some examples of what we've learned from past twins studies—both famous and infamous:

The Birth of Eugenics

Victorian scientist Francis Galton , a half-cousin of Charles Darwin, was one of the first people to recognize the value of twins for studying the heritability of traits. In an 1875 paper titled "The History of Twins," Galton used twins to estimate the relative effects of nature versus nature (a term that Galton himself coined). But his firm belief that human intelligence is largely a matter of nature led him to a darker path: He became a vocal proponent of eugenics (another term that he coined) and the idea that "a highly gifted race of men" could be produced through selective breeding.

Genes and I.Q.

In 2003, Eric Turkheimer , a psychology professor at the University of Virginia, took a fresh look at the research on the heritability of I.Q., which relied heavily on twin studies. Turkheimer noticed that most of the studies that found I.Q. is largely due to genetics involved twins from middle-class backgrounds, and he wondered what the pattern was among poorer people. When he looked at twins from poor families, he found that the I.Q.s of identical twins varied just as much as the I.Q.s of fraternal twins. In other words, the impact of growing up poor can overwhelm a child's natural intellectual gifts.

Genetic Basis for Everyday Diseases

Working with data and biological samples in the TwinsUK Registry, Spector and his colleagues have shown in more than 600 published papers that many common diseases such as osteoarthritis, cataracts and even back pain have a clear genetic basis to them. "When I started in this field, it was thought that only 'sexy' diseases [such as cancer] were genetic," Spector says. "Our findings changed that perception."

Heritable Eating Disorders

One of the newer twin registries to come online, the Michigan State University Twin Registry (MSUTR) was founded in 2001 to study genetic and environmental influences on a wide range of psychiatric and medical disorders. One of the most surprising findings to come out of the group's research is that many eating disorders such as anorexia have a genetic component to them.

"People thought for the longest time that it was due entirely to culture, the media and social factors,” says MSUTR co-director Kelly Klump . "Because of twins studies, we now know that genes account for the same amount of variability in eating disorders as they do in schizophrenia and bipolar disorder. We would have never known that without twins studies."

The Genetics of Obesity

A classic twin study conducted by geneticist Claude Bouchard in 1990 looked at the importance of genes for body-fat storage. Bouchard, now at Louisiana State University, housed a dozen lean young male twins in a dormitory and overfed them by 1,000 calories a day for three months. Although every participant was heavier by the end of the experiment, the amount of weight and fat gained varied considerably, from 9 pounds to 29 pounds. Weight gain within pairs of twins was much more similar than weight gain between different twin pairs, and the twins in each pair tended to gain weight in the same places, whether it be in the abdomen, buttocks or thighs.

Untangling the "Gay Gene"

Numerous twin studies have attempted to elucidate the importance of genes in sexual orientation. In 2008, researchers led by Niklas Langström , a psychiatrist at the Karolinska Institute in Stockholm, drew upon the treasure trove of twin data contained in the Swedish Twin Registry, the largest in the world, to investigate genetic and environmental influences that determine whether or not a person is gay. The scientists found that genetics accounted for only 35 percent of the differences between identical and fraternal gay men and even less—roughly 18 percent—in gay women.

The study, one of the most comprehensive to date, indicates that a complex interplay of genetics and environmental factors work together to shape people’s sexual orientations . But like other twins studies on this controversial subject, Langström’s study was criticized for possible recruitment bias, since only 12 percent of the males in the Swedish registry were included in the study.

Twins Reared Apart

In 1979, Thomas Bouchard conducted what is perhaps the most fascinating twin study yet. Then director of the Minnesota Center for Twin and Family Research , Bouchard looked at identical and fraternal twins separated in infancy and reared apart. He found that identical twins who had different upbringings often had remarkably similar personalities, interests and attitudes. In one of the most famous examples, Bouchard came across twins who had been separated from birth and reunited at the age of 39.

"The twins," Bouchard later wrote, "were found to have married women named Linda, divorced, and married the second time to women named Betty. One named his son James Allan, the other named his son James Alan, and both named their pet dogs Toy."

But MSUTR's Klump is quick to point out that Bouchard's findings are not proof of genetic determinism. "What they show is that we we enter the world not as random beings or blank slates,” Klump says. “As we walk through life, we have a lot of free choice, but some portion of that free choice is probably based on things that we're really good at and things that we like to do. Bouchard's study tells us that there is a dynamic interplay between what we like, what we want and the environments that we choose."

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Ker Than | | READ MORE

Ker Than is a freelance science writer living in the Bay Area. He has written for National Geographic, New Scientist, and Popular Science.

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IResearchNet

Twin Studies

The classic twin design involves comparing the similarity of identical and fraternal twins. If identical twins are more alike in intelligence, personality, or physical skills this demonstrates that the trait is probably influenced by genetic factors. Some people have objected that identical twins are alike because people treat them alike, not because of their shared genes. However, careful studies have ruled out this criticism after finding that identical twins who are treated alike are not more similar than identical twins who are treated differently.

There are many ways to study twins. A powerful method is studying identical twins reared apart from birth. Reared-apart identical twins resemble one another only because of their shared genes. Interestingly, research shows that identical twins reared apart and together are about equally similar in personality traits such as aggression and traditionalism. The twin-family method includes identical twins, their spouses, and their children. The children of identical twins are cousins, but they are also “half-siblings” because they have a genetically identical parent. These children’s aunts and uncles are like their “mothers” and “fathers” because they are genetically identical to the children’s own parents. It is possible to compare the behavioral similarity of a twin mother and her daughter (who share genes and environments) and a twin aunt and her niece (who share genes but not environments). Research has shown that parent-child and aunt/uncle-niece/nephew similarity is the same on a spatial visualization test. A more recent research design uses a unique twin-like pair called virtual twins. Virtual twins are same-age individuals who are raised together, but are not genetically related. Virtual twins show modest similarity in intelligence, despite their shared environment, a finding that supports genetic influence.

The multiple birth rate (especially the fraternal twinning rate) has increased from 19.3 to 30.7 multiple births per 1,000 births in recent years. This is primarily due to new reproductive technologies but also to the fact that women are having children at older ages. The increased twinning rate is good news for researchers. However, the downside is that twins are more likely than non-twins to suffer from birth difficulties.

It is likely that twins will continue to play significant roles in psychological and medical research.

Identical twins differing in traits, such as novelty-seeking, schizophrenia, or breast cancer may help identify which genes are expressed and which genes are not expressed. Thus, twin studies can help clarify the origins of behavior in everyone else.

References:

Machin, G. A., & Keith, L. G. (1999). An atlas of multiple pregnancy: Biology and pathology. New York: Parthenon.
Segal, N. L. (2000). Entwined lives: Twins and what they tell us about human behavior. New York: Plume.
Segal, N. L. (2005). Indivisible by two: Lives of extraordinary twins. Cambridge, MA: Harvard University Press.
Social Psychology Research Methods

Biological Approach In Psychology

Saul Mcleod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

Learn about our Editorial Process

Olivia Guy-Evans, MSc

Associate Editor for Simply Psychology

BSc (Hons) Psychology, MSc Psychology of Education

Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.

On This Page:

Biological psychology, also called physiological psychology, is the study of the biology of behavior; it focuses on the nervous system, hormones and genetics. Biological psychology examines the relationship between mind and body , neural mechanisms, and the influence of heredity on behavior.

Assumptions

The biological approach believes behavior to be a consequence of our genetics and physiology. It is the only approach in psychology that examines thoughts, feelings, and behaviors from a biological and, thus physical point of view.

Therefore, all that is psychological is first physiological. All thoughts, feelings & behavior ultimately have a biological cause. A biological perspective is relevant to the study of psychology in three ways:

1. Comparative method : different species of animal can be studied and compared. This can help in the search to understand human behavior. 2. Physiology : how the nervous system and hormones work, how the brain functions, how changes in structure and/or function can affect behavior. For example, we could ask how prescribed drugs to treat depression affect behavior through their interaction with the nervous system. 3. Investigation of inheritance : what an animal inherits from its parents, mechanisms of inheritance (genetics). For example, we might want to know whether high intelligence is inherited from one generation to the next. Furthermore the biological approach argues that some of our behaviors and characteristics are passed on genetically because they enhance our survival such as attachment and memory.

Each of these biological aspects, the comparative, the physiological (i.e., the brain), and the genetic, can help explain human behavior.

Issues and Debates

Free will vs. determinism.

It is strongly determinist as it views behavior as caused entirely by biological factors over which individuals have no control.

Nature vs. nurture

The biological approach is firmly on the nature side of the debate; however, it does recognize that our brain is a plastic organ that changes with experience in our social world, so it does not entirely deny the influence of nurture.

Cross-cultural research involves studying a particular behavior (e.g., gender, facial expressions) across different cultures. If the behavior is found to be similar across cultures, psychologists conclude that differences in behavior are biologically (i.e., nature) based.

However, if the behavior is found to be different across cultures, then it is likely to be affected by the environment (i.e., nurture). Cross-cultural research is useful as it contributes to the nature-nurture debate in psychology.

Holism vs. reductionism

The biological approach is reductionist as it aims to explain all behavior by the action of genetic or biochemical processes.

It neglects the influence of factors such as early childhood experiences, conditioning, or cognitive processes.

Idiographic vs. nomothetic

The biological approach is nomothetic as it establishes laws and theories about the effects of physiological and biochemical processes that apply to all people.

Are the research methods used scientific?

The biological approach uses very scientific methods such as scans and biochemistry. Animals are often used in this approach as the approach assumes that humans are physiologically similar to animals.

Investigation of Inheritance

Twin studies provide geneticists with a kind of natural experiment in which the behavioral likeness of identical twins (whose genetic relatedness is 1.0) can be compared with the resemblance of dizygotic twins (whose genetic relatedness is 0.5).

In other words, if heredity (i.e., genetics) affects a given trait or behavior, then identical twins should show a greater similarity for that trait compared to fraternal (non-identical) twins.

There are two types of twins:

Monozygotic = identical twins (share 100% genetic information).
Dizygotic = non-identical twins (share 50% genetic information, similar to siblings).

Research using twin studies looks for the degree of concordance (or similarity) between identical and fraternal (i.e., non-identical) twins. Twins are concordant for a trait if both or neither of the twins exhibits the trait. Twins are said to be disconcordant for a trait if one shows it and the other does not.

Identical twins have the same genetic makeup, and fraternal twins have just 50 percent of genes in common.

Thus, if concordance rates (which can range from 0 to 100) are significantly higher for identical twins than for fraternal twins, then this is evidence that genetics play an important role in the expression of that particular behavior.

Bouchard and McGue (1981) conducted a review of 111 worldwide studies which compared the IQ of family members. The correlation figures below represent the average degree of similarity between the two people (the higher the similarity, the more similar the IQ scores).

Identical twins raised together = .86 (correlation).
Identical twins raised apart = .72
Non-identical twins reared together = .60
Siblings reared together = .47
Siblings reared apart = .24
Cousins = .15

However, there are methodological flaws that reduce the validity of twin studies. For example, Bouchard and McGue included many poorly performed and biased studies in their meta-analysis.

Also, studies comparing the behavior of twins raised apart have been criticized as the twins often share similar environments and are sometimes raised by a non-parental family member.

Methods of Studying the Brain

It is important to appreciate that the human brain is a highly complicated piece of biological machinery. Scientists have only just “scratched the surface” of understanding the many functions of the workings of the human brain. The brain can influence many types of behavior.

In addition to studying brain-damaged patients, we can find out about the working of the brain in three other ways.

Children begin to plan activities, make up games, and initiate activities with others. If given this opportunity, children develop a sense of initiative and feel secure in their ability to lead others and make decisions.

1 . Neuro Surgery

We know so little about the brain and its functions are so closely integrated that brain surgery is usually only attempted as a last resort.

H.M. suffered such devastating epileptic fits that, in the end, a surgical technique that had never been used before was tried out.

This technique cured his epilepsy, but in the process, the hippocampus had to be removed (this is part of the limbic system in the middle of the brain.)

Afterwards, H.M. was left with severe anterograde amnesia. I.e., He could remember what happened to him in his life up to when he had the operation, but he couldn’t remember anything new. So now we know the hippocampus is involved in memory.

2 . Electroencrphalograms (EEGs)

This is a way of recording the electrical activity of the brain (it doesn’t hurt, and it isn’t dangerous). Electrodes are attached to the scalp, and brain waves can be traced.

EEGs have been used to study sleep, and it has been found that during a typical night’s sleep, we go through a series of stages marked by different patterns of brain wave.

One of these stages is known as REM sleep (Rapid Eye Movement sleep). During this, our brain waves begin to resemble those of our waking state (though we are still fast asleep) and it seems that this is when we dream (whether we remember it or not).

3 . Brain Scans

More recently, methods of studying the brain have been developed using various types of scanning equipment hooked up to powerful computers.

The CAT scan (Computerised Axial Tomography) is a moving X-ray beam which takes “pictures” from different angles around the head and can be used to build up a 3-dimensional image of which areas of the brain are damaged.

Even more sophisticated is the PET scan (Positron Emission Tomography) which uses a radioactive marker as a way of studying the brain at work.

The procedure is based on the principle that the brain requires energy to function and that the regions more involved in the performance of a task will use up more energy. What the scan, therefore, enables researchers to do is to provide ongoing pictures of the brain as it engages in mental activity.

These (and other) methods for producing images of brain structure and functioning have been extensively used to study language and PET scans, in particular, are producing evidence that suggests that the Wernicke-Gerschwind model may not after all be the answer to the question of how language is possible.

The Voyage of the Beagle (1805 – 1836) – Darwin formulated his theory of natural selection by observing animals while traveling the world.

Harlow (1848): Phineas Gage brain injury case study provides neuroscience with significant information regarding the working of the brain.

Darwin (1859) publishes On the Origin of Species by Means of Natural Selection . 1,250 copies were printed, most of which were sold on the first day.

Jane Goodall (1957) began her study of primates in Africa, discovering that chimps have behaviors similar to those of all the human cultures on the planet.

Edward Wilson (1975) published his book, Sociobiology which brought together an evolutionary perspective to psychology.

The birth of Evolutionary Psychology begins with the publication of an essay, The Psychological Foundations of Culture , by Tooby and Cosmides (1992).

Critical Evaluation

Charles Darwin proposed the theory of natural selection. He argued that genetically determined characteristics or behaviors that enhance our chances of survival and reproduction would be passed on to the next generation and become more common in a population.

In contrast, traits that do not enhance survival will gradually disappear. Theories within the biological approach support nature over nurture. An example of behavior of evolutionary explanation is Bowlby’s theory of attachment .

However, describing behavior solely in terms of either nature or nurture is limiting, and attempts to do this underestimate the complexity of human behavior. It is more likely that behavior is due to an interaction between nature (biology) and nurture (environment).

For example, individuals may be predisposed to certain behaviors, but these behaviors may not be displayed unless they are triggered by factors in the environment. This is known as the diathesis-stress model of human behavior.

A strength of the biological approach is that it provides clear predictions, for example, about the effects of neurotransmitters or the behaviors of people who are genetically related. This means the explanations can be scientifically tested , replicated, and peer-reviewed.

A limitation is that most biological explanations are reductionist . They reduce behavior to the outcome of genes and other biological processes, neglecting the effects of childhood and our social and cultural environment. They don’t provide enough information to explain human behavior fully.

Furthermore, it could be argued that the biological abnormalities seen in mental disorders could be the result rather than the cause of the disorder as the brain is a plastic organ that changes with the way we use it, so it could be that, for example, the damage seen in the caudate nucleus is the result of anxiety rather than its cause.

Additionally, it could be argued that the unbalance in neurotransmitters such as low serotonin , in a depressed individual is the consequence rather than the cause of depression because the brain is a plastic organ that changes with the way we use it, so it could be that the depressed thinking causes the low level of serotonin observed.

It could be argued that twin studies do not separate nature and nature because twins are raised and live in the same environment, and the difference in the concordance rate found between MZ and DZ twins could be due to the fact that MZ twins are treated more similarly by their parents than DZ twins because they look more similar.

Also, we usually do not find a 100% concordance rate in MZ twins for mental disorders, indicating that environmental and social factors must be involved in developing these disorders.

The biological approach is determinist as it sees our behavior as caused entirely by biological factors over which we have no control. This encourages people not to take responsibility for their actions and blame their genetic makeup.

Bouchard, T. J., & McGue, M. (1981). Familial studies of intelligence: A review . Science, 212(4498) , 1055-1059.

Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1st ed.) . London: John Murray.

Harlow, J. M. (1848). Passage of an iron rod through the head. Boston Medical and Surgical Journal , 39, 389–393.

Tooby, J., & Cosmides, L. (1992). The psychological foundations of culture. In J. Barkow, L. Cosmides, & J. Tooby (Eds.), The adapted mind: Evolutionary psychology and the generation of culture . New York: Oxford University Press.

Wilson, E. (1975). Sociobiology: The New Synthesis . Harvard University Press

Challenging Old Assumptions: Twin Study Reveals Surprising Connection Between Psychedelics and Psychosis

Every month, the BCSP brings you one essential read to keep you up to date with the leading topics in psychedelic science. This month, postdoctoral researcher Sean Noah explains “ Adolescent Psychedelic Use and Psychotic or Manic Symptoms ,” published in 2024 in JAMA Psychiatry.

Researchers have rarely studied the potential of psychedelics to cause psychotic or manic symptoms, let alone prolonged episodes of psychosis. A new article, “Adolescent Psychedelic Use and Psychotic or Manic Symptoms,” published in the journal JAMA Psychiatry, reports the results of a study of psychedelics use among young people in the Swedish Twin Registry. By studying a large sample of twins, the researchers carefully assessed the relationships among psychedelics use and symptoms of psychosis and mania. Surprisingly, this study showed that psychedelics use was associated with lower rates of psychotic symptoms.

The researchers found that psychedelics use was associated with fewer self-reported psychotic and manic symptoms per individual among the participants in their study. Most intriguing, among pairs of identical twins, psychotic symptoms were found to be less common in the sibling who had used psychedelics than in their twin who had not used psychedelics. Importantly, because their study is observational rather than experimental, the researchers claim no causal relationship between psychedelics use and decreased reports of psychotic and manic symptoms. The reasons that psychedelics use is correlated with lower incidence of these symptoms will need to be examined in further research. Nonetheless, this study sheds new light on the murky issue of psychedelics use and psychosis.

The Study Design

The researchers collected self-reported questionnaire data from fifteen-year-old twins in the Swedish Twin Registry. 16,244 twins answered questions about their past use of the psychedelic compounds psilocybin and LSD. Among these participants, 15,862 answered questions about whether they experienced psychotic symptoms, and 15,717 answered questions about manic symptoms. For example, the participants were asked to respond whether they agreed with the statement “Other people read my thoughts” with the options “Never,” “Maybe,” or “Definitely.”

Studying twin siblings is a powerful way to conduct observational research because it eliminates many of the possible confounds and strengthens the conclusions that can be made from the data.

Later, 9,426 of the twins also provided genetic data, allowing the researchers to estimate individuals’ risk for schizophrenia and bipolar I disorder from a set of genetic variants associated with the disorders.

The researchers used a statistical analysis to identify whether psychedelics use before the age of fifteen was associated with psychotic or manic symptoms. Crucially, the researchers also conducted this analysis within just identical twin pairs, allowing for an even closer inspection of the effect of psychedelics use on the self-reported outcome measures.

Double Take

This study is observational rather than experimental. In many experimental studies, researchers intervene in the participants’ behavior and randomly assign groups of participants to different experimental conditions (for example, receiving an active drug vs. a placebo). In observational studies, researchers simply observe the participants’ natural behavior and look for statistical associations in the data. One drawback of observational studies is that it’s harder to be confident about what might be causing the observed effects because there are so many potential factors in a real-world setting that are left unaccounted for and uncontrolled. These hidden variables, which play an important role in explaining the study’s results but are not acknowledged in the study design, are called confounds.

Studying twin siblings is a powerful way to conduct observational research because it eliminates many of the possible confounds and strengthens the conclusions that can be made from the data. In particular, conducting this observational study with a random group of people, rather than a group of twins, would leave open the possibility that a participant’s genes and/or their upbringing affected whether or not that participant reported psychotic or manic symptoms.

Most intriguingly, among pairs of identical twins, psychotic symptoms were found to be less common in the sibling who had used psychedelics than in their twin who had not used psychedelics.

By conducting the study with twins, it’s possible to control for these confounds. Twin siblings share a familial upbringing, a very similar environment, and many genes. Identical twins share all their genes. So if two twins differ in both their use of psychedelics and their psychotic symptoms, the symptoms can be more confidently attributed to the behavior that differs between the siblings: the use of psychedelics. However, it’s important to note that it’s still possible that a third factor affects both psychotic symptoms and psychedelics use and that there is no direct causal relationship between psychotic symptoms and psychedelics use.

Surprising Conclusions

The researchers found that psychedelics use was associated with fewer psychotic and manic symptoms. And most intriguingly, among pairs of identical twins, psychotic symptoms were found to be less common in the sibling who had used psychedelics than in their twin who had not used psychedelics. Importantly, the researchers claim no causal relationship between psychedelics use and decreased reports of psychotic and manic symptoms, and the reason that psychedelics use is correlated with lower incidence of these symptoms will need to be examined in further research.

Despite some important limitations, such as the focus on psychotic and manic symptoms reported only at 15 years of age, this study significantly advances our understanding of psychedelics and psychosis. Before this study, little was known about how commonly psychedelic use leads to psychotic symptoms, and even less was known about this association in adolescents. Maybe with more research, psychedelics and psychosis won’t be so closely tied in the popular imagination.

Want to read more? Dive into the full article .

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'Stealing the past': A spat between twins leads to a theory of disputed memories

Gabriel Spitzer

Emily Kwong

Berly McCoy

Rebecca Ramirez

Water color of two silhouettes that face away from one another but are joined together by intertwined hair.

It's not unusual for siblings to quibble over ownership of something – a cherished toy, a coveted seat in the car, the last ice cream sandwich. As they were growing up in Quebec, Canada, Mercedes Sheen and her identical twin sister used to quarrel over who owned memories.

"I think the best example is my first kiss – what I perceived to be my first kiss," says Mercedes Sheen , now a professor of psychology at Heriot-Watt University in Dubai.

She remembers all the details: Summer camp in New Brunswick, the day spent canoeing, the walk back up the hill.

"And this guy called Jeff Levitt, who was the camp catch, the most gorgeous guy at camp, he pulled me aside and pulled me into a bush and kissed me on the lips, very briefly," she says.

It made a big impression. Then months later, when she brought up the story with Michaela, her twin insisted that it had been her whom Jeff had smooched, not Mercedes.

"We both felt that it was 100% us, when the event could have only happened to one of us," she says.

The sisters would run into this situation fairly often: They would remember the same experience but disagree on which twin it had happened to. Sheen says sometimes the disputes felt personal.

"Our memories tie us to our personal past, our 'this-is-us,'" she says. " My thesis was called 'Stealing the Past' because it really feels like someone's taking your history from you. ... One could say that I took my arguments with my twin to a great extent by doing a PhD on it."

Disputing ownership of a memory

As she pursued studies in psychology, Mercedes Sheen was still thinking about those questions of twins and memory.

"It's kind of bizarre to think that you have such a strong memory, and have all those visual properties, the sounds and smells, and then to think that it didn't happen," she says. "It kind of makes you think, 'Well, wow, so, what is real?'"

Sheen was considering various dissertation topics at Canterbury University in Christchurch, New Zealand, when she had lunch with a visiting professor – David Rubin , a Duke University neuroscientist and a noted expert on memory.

The fact that Sheen is an identical twin came up in conversation, and by the end of the lunch the two had plans to launch a research project together : They would test whether disputed ownership of memories might be a common trait in twins.

Working with the local multiple births society in Christchurch, Sheen recruited dozens of volunteers – a task she said was relatively easy to do at the time. "There was a big increase in women doing IVF in the early eighties, and they ended up with an influx of twins. So I had these beautiful participants that were there," she says.

Next, she asked them in various ways: Do you have any memories where you disagree on whose it is?

"I used 20 or 30 cue words," Sheen says, "words that would cue everyday experiences, like birthday, McDonald's, road trip, injury, things like that. And just by asking them both to come up with a memory in response to those cue words, they just happened spontaneously."

Once they found a disputed memory the researchers would ask a series of detailed questions: What do you remember seeing? What do you remember hearing? Do you see the memory from your own point of view or an observer's?

The researchers found that in most cases of disputed memories, based on the supporting details and their level of confidence, both twins were equally credible – even though the event could have only happened to one of them. In fact, each twin tended to remember a disputed memory with more certainty than they did with memories they agreed on.

Goats and Soda

How do you get siblings to be nice to each other these latino families have an answer.

It all felt very familiar to Mercedes Sheen – as did the study subjects' resulting squabbles.

"They all had the same types of arguments that I had with my twin – like, 'You always do this! You steal my memories,'" she says.

Follow-up experiments would show that in addition to identical twins, fraternal twins also experience this effect to a lesser extent, followed by non-twin same-sex siblings.

A reconstruction, not a recording

Sheen says another finding was that disputed memories tend to be self-aggrandizing . As in her tale of the contested first kiss, they tend to show the person remembering in a positive light or as the main character.

"Our memories are selective, you know," says Sheen. "Our memories are not a file that we pick out from our brain. They are reconstructions."

Experts say creating those reconstructions is a complicated, even messy neurological process.

"My dad used to say to me, if you've got a machine with many moving parts, like a car or whatever, there's just so many more ways it can go wrong. And memory is one of those machines with many moving parts," says Charles Fernyhough, professor of psychology at Durham University in the United Kingdom.

In his book Pieces of Light , Fernyhough explains that a memory is made up of different kinds of information: the facts of what actually happened, the subjective, sensory information connected to it, "semantic knowledge" of how the world generally works. When the brain remembers, Fernyhough says, it must recombine all that data.

"It takes those different kinds of information spread across all those different bits of the brain and it puts them together right here, right now, when you're being asked to remember. It reconstructs a version of the past according to the demands of the present," he says.

Each time the brain reconstructs a memory, it creates opportunities for that memory to be further shaped by factors besides what actually happened – emphasis can shift, other memories can leak in, and, according to Sheen's research, the protagonist's identity can change.

Why are disputed memories common in twins?

Sheen points to a handful of factors that might explain this tendency for twins to dispute ownership of a memory, most of which are unconscious and not necessarily selfish. One is something she calls "empathy inflation."

"If I see my twin in pain, I feel that pain as well as much as she does. So I'm more likely to take that on as a personal experience and then later subsequently remember that as something I experienced myself," she says.

There's also contamination of the memory by outside information, such as other people's version of what happened. Sheen says parents misremember all the time which child belongs in a given story, and the parents of identical twins likely make those errors even more frequently.

"Parents often make mistakes," she says, "and so they go, 'I remember when you did that.' And you kind of go, 'Oh yeah, that happened to me,' when it really didn't."

And then there are a series of what Sheen calls "source monitoring errors," where a person mixes up a memory's origins.

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All grown up, but still fighting why more siblings are turning to therapy, together.

"I did a study once on the confusion between real and dreamt experiences . Because you have so much imagery involved with dreams, you can actually remember it as [if it were] a real event," Sheen says.

Sheen's work builds on decades of research from people like Elizabeth Loftus, the University of California, Santa Cruz psychologist who has demonstrated how unreliable eyewitness testimony can be in a courtroom setting.

Studies show that the vast majority of wrongful convictions for rape and murder were based on eyewitness testimony.

Sheen says an eyewitness account's sensory details may sound convincing, but they're still often wrong. Her own research finds that twins tended to impute even more details to disputed memories than to ordinary ones.

"Twins often say, 'this is my memory, I remember his red hair. I remember ice cream melting in my hair. I remember the humiliation. I remember the sound it made or the smell of fire burning.' All these qualities that are used in eyewitness testimony are also used between twins when they want to argue about their memories," she says.

As for her own twin, Mercedes says her sister greeted her research with eye rolls.

"When I when I said I was doing this, she was like, 'what?'" says Mercedes. "She thought that I was very stubborn and that I took took an argument too far. We do laugh about it still."

Check out more of NPR's series on the Science of Siblings .

Curious about more science about memories? Email us at [email protected] .

Listen to Short Wave on Spotify , Apple Podcasts and Google Podcasts .

Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave .

Today's episode was produced by Berly McCoy. It was edited by Rebecca Ramirez. Gabriel Spitzer checked the facts. Patrick Murray and Stu Rushfield were the audio engineers.

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Twins in Behavioral Science Research
One of the world's largest twin research studies is The Swedish Twin Registry at the Karolinska Institute. It has data on 194,000 twins, including identicals and fraternals.
Beyond Heritability: Twin Studies in Behavioral Research
Abstract. The heritability of human behavioral traits is now well established, due in large measure to classical twin studies. We see little need for further studies of the heritability of individual traits in behavioral science, but the twin study is far from having outlived its usefulness. The existence of pervasive familial influences on ...
Behavioral Genetics--A second look at twin studies
Topics in Psychology. Explore how scientific research by psychologists can inform our professional lives, family and community relationships, emotional wellness, and more. ... The future of twin research will involve combining traditional twin studies with molecular genetics research, according to Hewitt, who believes that day is already here. ...
Twin Studies
Twin research is an informative approach for understanding the genetic and environmental influences affecting behavioral, physical, and medical traits. The simple yet elegant logic of the twin method derives from the differences in genetic relatedness between the two types of twins. Identical (monozygotic or MZ) twins share 100 percent of their ...
Maximizing the value of twin studies in health and behaviour
Human behaviour. In the classical twin design, researchers compare trait resemblance in cohorts of identical and non-identical twins to understand how genetic and environmental factors correlate ...
Twin Methodology in Psychological Studies
Importantly, large-scale twin studies have been established in different countries around the globe, and they have facilitated nature and nurture investigations across different geographical, social and cultural settings. However, when interpreting findings of twin studies, specific methodological assumptions and limitations need to be considered.
Genetics, personality and wellbeing. A twin study of traits, facets and
Scientific Reports - Genetics, personality and wellbeing. A twin study of traits, facets and life satisfaction ... Psychology, and Economics 5, 193-211 ... A Research Update. Twin Res Hum Genet ...
Why psychologists study twins
Why psychologists study twins. In the second of our on-going series of guest features for psychology students, Dr. Angelica Ronald of London's Institute of Psychiatry describes the use of twin studies in psychology. Psychologists are often trying to control one thing to look at its effect on something else.
Twins in Behavioral Science Research
It has data on 194,000 twins, including identicals and fraternals. The 60-year-old effort studies twins to help us understand health and disease including research on cancer, cognitive function ...
Twin Studies
Twin studies have been very important in examining the etiology of different mental disorders. A review of twin studies of psychiatric illnesses revealed behavioral traits could be more heritable in permissive rather than restrictive environments (Kendler 2001).The studies reviewed also suggested genes for psychiatric and drug abuse disorders likely worked through the traditional physiological ...
Twin studies of brain, cognition, and behavior
Twin studies of brain, cognition, and behavior. For many of the papers in this volume, and most of the studies reviewed within those papers, the empirical evidence presented and analyzed is primarily about the heritability of, and environmental influences on, individual phenotypes. These data and analyses are an important first step in any ...
Key Study: The Minnesota Twin Study of Twins Reared Apart
For this study, the average age of the twins when they participate in this study was 41, which is important because most twin research prior to this focused on adolescents. The twins spent an average of 5 months together before being separated and reunited (on average) around 30 years of age. Physical and psychological data was gathered in a ...
Speaking of Psychology: What studying twins can teach us about
Our guest today is Dr. Nancy Segal, a professor of psychology and director of the Twin Studies Center at California State University, Fullerton. She has spent her entire career studying twins, beginning with her work on the Minnesota Study of Twins Teared Apart in the 1980s and '90s. Today, she continues to study identical and fraternal twins ...
A Brief History of Twin Studies
In 2003, Eric Turkheimer, a psychology professor at the University of Virginia, took a fresh look at the research on the heritability of I.Q., which relied heavily on twin studies. Turkheimer ...
What can we learn from twin studies? A comprehensive evaluation of the
Instead, the results of studies of twins reared apart are essentially consistent with the results of classic twin studies (Rowe, 1995, p. 72). The most prominent study of twins reared apart in the US revealed moderate to high correlations on a variety of characteristics (Bouchard et al., 1990), such as IQ and personality.
Psychoanalysis—Science and Research: Twin Studies as a Paradigm
Psychoanalysis—Science and Research: Twin Studies as a Paradigm. Edward D. Joseph, M.D. View all authors ... (1961), Interim report on a twin research. In: Panel on "Research in Psychoanalysis," This ... Hartmann, H. (1934-1935), Psychiatric studies of twins. In: Essays on Ego Psychology. New York: International Universities Press, 1964 ...
Twin study
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders.Twin research is considered a key tool in behavioral genetics and in related fields, from biology to psychology. Twin studies are part of the broader methodology used in behavior genetics, which uses all data that ...
Twin Studies (SOCIAL PSYCHOLOGY)
Twin Studies. Twin studies can tell us about how genes and environments affect behavioral and physical development. There are two kinds of twins: identical and fraternal. Identical twins result when one fertilized egg splits during the first two weeks of pregnancy. These twins share all their genes and are always of the same sex.
Twin Studies: A Unique Epidemiological Tool
The Swedish Twin Registry (STR), managed by the Karolinska Institute, is the largest population-based twin registry in the world (containing approximately 1,70,176 twins in 85,088 pairs born 1886-2000). There are 1, 414 twins still alive and living in Sweden. (,) It is a unique resource for clinical, epidemiological, and genetic studies.
Biological Approach In Psychology
Dizygotic = non-identical twins (share 50% genetic information, similar to siblings). Research using twin studies looks for the degree of concordance (or similarity) between identical and fraternal (i.e., non-identical) twins. Twins are concordant for a trait if both or neither of the twins exhibits the trait.
Challenging Old Assumptions: Twin Study Reveals Surprising Connection
Studying twin siblings is a powerful way to conduct observational research because it eliminates many of the possible confounds and strengthens the conclusions that can be made from the data. Later, 9,426 of the twins also provided genetic data, allowing the researchers to estimate individuals' risk for schizophrenia and bipolar I disorder ...
An argument and the psychology of disputed memory : Short Wave : NPR
The fact that Sheen is an identical twin came up in conversation, and by the end of the lunch the two had plans to launch a research project together: They would test whether disputed ownership of ...
Adoption & Twin Studies AO1 AO2 AO3
Evaluate the use of twin studies in psychology. (8 marks) A 8-mark "evaluate" question awards 4 marks for describing the twin study method (AO1) and 4 marks for evaluating it (AO3). You need a conclusion to get a mark in the top band (7-8 marks). "Twin studies" turn up in the Specification for the Biological Approach but could also be used ...

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Twin Studies by Nancy L. Segal LAST REVIEWED: 19 March 2013 LAST MODIFIED: 19 March 2013 DOI: 10.1093/obo/9780199828340-0101

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Key Study: The Minnesota Twin Study of Twins Reared Apart

Twin study by Bouchard et al. (1990)

Bouchard, Thomas J, Jr. Lykken, David T. McGue, Matthew. Segal, Nancy L and Tellegen, Auke. “ Sources of Human Psychological Differences: The Minnesota Study of Twins Researched Apart.” Sciences, New Series, Vol. 250 (1990), pp223-8. Accessed from web.missouri.edu

A Brief History of Twin Studies

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Biological Approach In Psychology

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'Stealing the past': A spat between twins leads to a theory of disputed memories

Disputing ownership of a memory

Goats and Soda

A reconstruction, not a recording

Why are disputed memories common in twins?

Shots - Health News

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