Reared-apart Twins Research Articles

Developmental trends in intelligence revisited with novel kinships: Monozygotic twins reared apart v. same-age unrelated siblings reared together

Prior research has reported convergence of monozygotic (MZ) cotwins' intelligence scores over time, with divergence observed between dizygotic (DZ) cotwins. These patterns have been variously explained with reference to the increased influence of genetic factors (MZ and DZ twins) and nonshared environmental factors (DZ twins), and the reduced effects of shared environmental events (DZ twins). Studies of unrelated siblings have found modest within-pair resemblance in intelligence, with increasing divergence over time. This subject is revisited in a study using three novel kinships: young reared-apart MZ twins from China (CTA-MZ), adult reared-apart twins from Denmark (D-MZA) and findings from a previous study of same-age unrelated siblings (virtual twins or VTs). Despite modest sample sizes, the anticipated trends described above were observed, replicating extant findings. Intraclass correlations for overall IQ score at Time 1 and Time 2, respectively, were ris = 0.51, 0.81 (CTA-MZs) and ris = 0.64, 0.74 (D-MZAs). The hypothesis that VTs would show score divergence at Time 2 was confirmed. Increased genetic influence (CTA-MZ, D-MZA and VT), reduced impact of shared environments (VT), and increased effects of nonshared environments (VT) appear to best explain the findings. These results also inform parent and educator expectations regarding twins' and siblings' academic performance.

Life-course socioeconomic differences and social mobility in preventable and non-preventable mortality: a study of Swedish twins.

BackgroundDespite advances in life expectancy, low socioeconomic status is associated with a shorter lifespan. This study was conducted to investigate socioeconomic differences in mortality by comparing preventable with non-preventable causes of death in 39 506 participants from the Swedish Twin Registry born before 1935.MethodsChildhood social class, own education, own social class and social mobility were used as separate indicators of socioeconomic status. These data were linked to the Swedish Cause of Death Register. Cause of death was categorized as preventable or non-preventable mortality according to indicators presented in the Avoidable Mortality in the European Union (AMIEHS) atlas. Using Cox proportional hazard models, we tested the association between the socioeconomic measures and all-cause mortality, preventable mortality and non-preventable mortality. Additional co-twin control analyses indicated whether the associations reflected genetic confounding.ResultsThe social gradient for mortality was most prominent for the adult socioeconomic measures. There was a social gradient in both preventable mortality and non-preventable mortality, but with an indication of a moderately stronger effect in preventable causes of death. In analyses of social mobility, those who experienced life-time low socioeconomic status (SES) or downward social mobility had an increased mortality risk compared with those with life-time high SES and upward social mobility. Adjustments for genetic confounding did not change the observed associations for education, social class or social mobility and mortality. In the co-twin control analyses of reared-apart twins, the association between childhood social class and mortality weakened, indicating possible genetic influences on this association.ConclusionsOur results indicate that there is an association between low adult socioeconomic status and increased mortality independent of genetic endowment. Thus, we do not find support for indirect social selection as the basis for mortality inequalities in Sweden

Pairs of Genetically Unrelated Look-Alikes

Relationships of physical resemblance to personality similarity and social affiliation have generated considerable discussion among behavioral science researchers. A "twin-like" experimental design (involving genetically unrelated look-alikes, U-LAs) explores associations among resemblance in appearance, the Big Five personality traits, self-esteem, and social attraction within an evolutionary framework. The Personality for Professionals Inventory (PfPI), NEO/NEO-FFI-3, Rosenberg Self-Esteem Scale, and a Social Relationship Survey were variously completed by 45U-LA pairs, identified from the "I'm Not a Look-Alike" project, Mentorn Media, and personal referrals. The mean U-LA intraclass correlations were negligible for all Big Five personality traits on the PfPI and NEO/NEO-FFI-3 (ri= -.02 and - .04, respectively). In contrast, mean ri values of .53 and .15 for monozygotic (MZA) and dizygotic (DZA) reared-apart twins, respectively, have been reported for these personality measures. The U-LA self-esteem correlation (ri= -.18) was also below the correlations reported for MZ and DZ reared-together twins (ri = .31 and .13, respectively). Finally, far fewer U-LAs expressed close social relationships (20%) than MZA (80%) and DZA (65%) twins. The present study extends earlier findings indicating that appearance is not meaningfully related to personality similarity and social relatedness. The criticism that MZ twins are alike in personality because their matched looks invite similar treatment by others is refuted. A more judicious interpretation is reactive genotype-environment correlation, namely that MZ twins' similar personalities evoke similar reactions from others. MZ twins' close social relations most likely derive from their perceptions of genetically based within-pair similarities that are lacking in U-LAs.

Identical twins doubly exchanged at birth: a case report of genetic and environmental influences on the adult epigenome.

Epigenetic comparisons within monozygotic twin pairs have enhanced our understanding of nongenetic mechanisms underlying disease etiology. We present epigenetic findings for a unique case of doubly exchanged Colombian male monozygotic twins raised in extremely different environments. Using genome-wide DNA methylation data from cheek swabs from which blood-specific differentially methylated probes had been removed, the individuals grouped by shared genetics rather than shared environment, except for one twin who presented as an outlier. Closer inspection of DNA methylation differences within both reared-apart twin pairs revealed several genes and genetic pathways likely to be influenced by the rearing environment. Together with our previous findings, we suggest that genetics, pre- and postnatal environments contribute to the epigenetic profile, although additional studies are needed to quantify these effects.

The trouble with twin studies: a reassessment of twin research in the social and behavioral sciences

The Trouble with Twin Studies questions popular genetic explanations of human behavioral differences based upon the existing body of twin research. Psychologist Jay Joseph outlines the fallacies of twin studies in the context of the ongoing decades-long failure to discover genes for human behavioral differences, including IQ, personality, and the major psychiatric disorders. This volume critically examines twin research, with a special emphasis on reared-apart twin studies, and incorporates new and updated perspectives, analyses, arguments, and evidence.

Genetic and environmental influences on applied creativity: A reared-apart twin study

Applied creativity involves bringing innovation to real-life activities. The first reared-apart twin study assessing genetic and environmental origins of applied creativity, via Draw-a-House (DAH) and Draw-a-Person (DAP) tasks, is presented. Participants included 69 MZA and 53 DZA twin pairs from the Minnesota Study of Twins Reared Apart. Drawings were evaluated by four artists and four non-artists. Genetic effects were demonstrated for the DAP (.38–.47), but not for the DAH. Creative personality showed genetic effects (.50), and modest, but significant correlations with scores on the two drawings (rs=.17–.26). Both genetic and nonshared environmental influences underlie variance in applied creativity. Individuals concerned with enhancing creativity among students and others may better understand individual differences in performance and training.

Resemblance for Age at Menarche in Female Twins Reared Apart and Together

Menarche is a significant developmental event in the lives of young females. Genetic and family environmental influences on the timing of its occurrence are explored in the first formal analysis using reared-apart and reared-together monozygotic (MZA, MZT) and dizygotic (DZA, DZT) twin pairs. Mean age at menarche was 12.50 years (SD = 1.67) for the reared-apart pairs and 12.86 years (SD = 1.49) for the reared-together pairs. Intraclass correlations for age at menarche were 0.56 for MZA twins, 0.16 for DZA twins, 0.70 for MZT twins, and 0.41 for DZT twins. The mean within-pair difference was 1.07 years (SD = 1.04) for MZA twins, 1.67 years (SD = 1.59) for DZA twins, 0.64 year (SD = 0.86) for MZT twins, and 1.43 years (SD = 1.34) for DZT twins. These results are consistent with genetic influence, although the lower correlations for reared-apart twins and their larger within-pair differences suggest that age at menarche is partly affected by common rearing environments. Feeling understood by one's father during the growing-up years was significantly associated with earlier age at menarche, and a comparable trend was found for feeling understood by one's mother. These findings are considered with reference to current theories of pubertal timing.

The similiarity of facial expressions in response to emotion-inducing films in reared-apart twins

While the role of genetic factors in self-report measures of emotion has been frequently studied, we know little about the degree to which genetic factors influence emotional facial expressions. Twenty-eight pairs of monozygotic (MZ) and dizygotic (DZ) twins from the Minnesota Study of Twins Reared Apart were shown three emotion-inducing films and their facial responses recorded. These recordings were blindly scored by trained raters. Ranked correlations between twins were calculated controlling for age and sex. Twin pairs were significantly correlated for facial expressions of general positive emotions, happiness, surprise and anger, but not for general negative emotions, sadness, or disgust or average emotional intensity. MZ pairs (n=18) were more correlated than DZ pairs (n=10) for most but not all emotional expressions. Since these twin pairs had minimal contact with each other prior to testing, these results support significant genetic effects on the facial display of at least some human emotions in response to standardized stimuli. The small sample size resulted in estimated twin correlations with very wide confidence intervals.

"Beyond the hedonic treadmill: Revising the adaptation theory of well-being": Comment on Diener, Lucas, and Scollon (2006).

Comments on the article by E. Diener, R. E. Lucas, and C. N. Scollon which cited a study by Tellegen et al. in which the Well-Being scale of the Multidimensional Personality Questionnaire (MPQ), administered to 44 pairs of monozygotic reared-apart (MZA) twins, yielded a within-pair correlation of .48. I contend that, in their subsequent proposed changes to subjective well-being (SWB) theory, Diener et al. failed to incorporate the implications of this heritability.

The extended genotype: the heritability of personality accounts for the heritability of recalled family environments in twins reared apart.

How are retrospective accounts of family rearing environments linked to adult personality? We addressed this question by measuring both domains in a sample of 180 reared-apart twins. Twins completed extensive measures of rearing environments (the Minnesota-Briggs History Record, the Block Environmental Questionnaire, the Family Environment Scale, and the Physical Facilities Questionnaire) and an omnibus measure of adult personality (the Multidimensional Personality Questionnaire). Retrospective accounts of family environments were partially heritable and all the heritable variance in environmental measures could be accounted for by heritable variance in personality. In addition, differences between twins in their accounts of their rearing environments (nonshared environmental factors) were not significantly linked to differences between twins in their personalities. Hence, family environmental measures appear to be heritable because personality genes influence the way people shape and recall their rearing environments. In addition, differences in reared-apart twins' retrospectively recalled rearing environments appear to have little impact on differences in their personalities in adulthood.

Evidence for the construct validity and heritability of the Wilson–Patterson conservatism scale: a reared-apart twins study of social attitudes

A 28 item version of the Wilson–Patterson Attitude Inventory was administered to 345 participants (twins, spouses, friends, and others) in the Minnesota Study of Twins Reared Apart. An overall conservatism measure demonstrated impressive construct validity. It had strong convergent validity, demonstrated by correlations of 0.72 and 0.58 with the Altemeyer Right Wing Authoritarianism scale and the Multidimensional Personality Questionnaire (MPQ) Traditionalism Scale, respectively. It also had strong discriminant validity, being largely uncorrelated with the other 10 MPQ Scales and WAIS Full Scale IQ. Heritabilities were calculated from fitting models to the variance–covariance matricies based on 54 pairs of monozygotic (MZ) and 46 pairs of dizygotic (DZ) twins reared apart. Conservatism demonstrated significant and sizeable genetic influence ( h 2=0.56), replicating the results reported in the Virginia 30,000 study of ordinary twins and family members and those of a large Australian twin study.

Periodontal bacteria in adult twins.

Both environmental and genetic factors are known to influence clinical measures of periodontal disease. The purpose of this study was to determine whether genetic factors similarly influence the presence of specific periodontal bacteria in subgingival plaque. Reared-together and reared-apart monozygous (MZ) and dizygous (DZ) adult twins were examined clinically. Demographic and behavioral information was obtained from each subject by questionnaire. Subgingival plaque samples were obtained from the index teeth, and the presence of P. intermedia, P. gingivalis, A. actinomycetemcomitans, E. corrodens, and F. nucleatum was determined using an immunoassay. Microbiological and clinical data were available for 169 twin pairs. The subject-based prevalences of the bacteria in the twin groups ranged from 11% for Porphyromonas gingivalis to 40% for F. nucleatum. For all species examined, the concordance rates were not significantly different (P > 0.05) between MZ and DZ twin groups. These findings were apparent despite similar smoking histories, self-reported oral hygiene practices, and antibiotic use in the twin groups. Furthermore, MZ twins reared together were not more similar than MZ reared-apart twins with respect to any bacterial species examined. These findings suggest that in a population with access to routine dental care, any effects that host genes and the early family environment have on the presence of specific bacteria in subgingival plaque are not apparent in adulthood. Most twins with disease in this study had early periodontitis. Results from this study may not necessarily be extrapolated to more advanced disease states.

Genetic and environmental influence on morningness–eveningness

Little is known about the causes of the wide variability in circadian rhythms in humans. We present the first report of genetic influence on a circadian phenotype, the morningness-eveningness disposition. Two hundred and five pairs of adult monozygotic reared-together twins, and 55 pairs of adult monozygotic and 50 pairs of adult dizygotic reared-apart twins completed a morningness-eveningness questionnaire. Biometrical model-fitting showed that genetic variability accounted for about 54% of the total variance in morningness-eveningness; age contributed to 3% of the variance; and the remaining variance was explained by nonshared environmental influences and measurement error. Shared rearing environmental effects were not significant. The husband-wife correlation for morningness-eveningness was 0.25. This correlation appears to be due largely to assortative mating (the tendency of like to marry like) at the time of marriage rather than a consequence of cohabitation during marriage.

Genetic and environmental influences on self-reported diet: a reared-apart twin study

HUR, Y.-M., T. J. BOUCHARD JR. AND E. ECKERT. Genetic and environmental influences on self-reported diet: a reared-apart twin study. PHYSIOL BEHAV 64(5) 629–636, 1998.—A sample of 66 monozyogtic twins reared apart (MZA) and 51 dizygotic twins reared apart (DZA), and 101 nontwin individuals (mostly spouses of the twins) who participated in the Minnesota Study of Twins Reared Apart (MISTRA) from 1979 to 1995 completed a self-report food frequency questionnaire. Intraclass-correlations and model-fitting analyses indicated that approximately 30% of the variance in the self-report of diet was attributable to genetic factors, with random environmental factors and measurement error responsible for the remaining variance. Spouse correlations were moderate. To investigate the effects of living together during marriage, the absolute differences between husband and wife on the dietary variables with years of marriage were correlated. None of the correlations were significant. Hierarchical multiple regression analyses also indicated that no convergence occurred during marriage. These results suggest that sharing a current family environment exerts minimal influence on individual differences in self-reported diet.

Individual differences in adult ego development: sources of influence in twins reared apart.

A behavior genetic analysis of the personality dimension of ego development, as assessed by the Washington University Sentence Completion Test, was carried out on a sample of 45 pairs of monozygotic twins and 28 pairs of same-sex dizygotic twins. The twins ranged in age from 16 to 70 years at the time of assessment, had been separated during infancy, and had been raised apart in adoptive homes before being reunited in adulthood. Analyses indicated that reared-apart twins were similar in trait levels of ego development and that such similarity could not solely be attributed to measured similarity in cognitive ability, including verbal reasoning. Estimates of broad similarity for ego development (46%) were comparable to values reported for other major personality dimensions. Phenotypic variance in adult levels of ego development appears to have substantial genetic and environmental sources.

Work values: Genetic and environmental influences.

The Minnesota Importance Questionnaire (MIQ), measuring work values, was administered to 23 monozygotic and 20 dizygotic reared-apart twin pairs to test the hypothesis that genetic factors are associated with work values. Both univariate and multivariate analyses were performed. In the univariate analysis, intraclass correlations were computed to estimate the proportion of variability in work values associated with genetic factors for each of the 20 MIQ scales and for the 6 higher order work value scales. The multivariate analysis used maximum likelihood estimation to separate the genetic and environmental factors for the correlated higher order scales. Results from both analyses indicated that, on average, 40% of the variance in measured work values was related to genetic factors.

A Twin Study of Genetic Variation in Proportional Radiographic Alveolar Bone Height

The purpose of this study was to estimate the genetic variance for alveolar bone height by means of the classic twin method and the study of monozygous twins reared apart. Panoramic radiographs were obtained from 120 pairs of adult twins (mean age = 40.4 years, S.D. = 10.4 years), for comparison of 62 pairs of monozygous twins reared together (MZT), 25 pairs of like-sexed dizygous twins reared together (DZT), and 33 pairs of monozygous twins reared apart (MZA). Mesial and distal bone heights were determined as a proportion of tooth length. A full-mouth bone score was computed for each twin by averaging these proportions from all measurable teeth. Between-pair (B) and within-pair (W) variances were computed for each twin group. The population variances (B + W) of the MZT and DZT twin groups were similar, which validated a basic assumption of the twin model. Intraclass correlations and heritability estimates were also computed for the reared-together and reared-apart twin groups. Boot-strap sampling was used to provide estimates and confidence limits for these values. The intraclass correlations for the twin groups were: MZT = 0.70, DZT = 0.52, and MZA = 0.55. The results of this study suggest that there is significant genetic variance in the population for proportional alveolar bone height.

Social contact and sibling similarity: Facts, issues, and red herrings

We consider evidence that shared experience is associated with sibling resemblance and report new analyses of our earlier data to assess the magnitude and meaning of that association. We cite recent Swedish studies of early separated twins and present new data from reared-apart twins in Finland. Results of these analyses confirm our earlier conclusion that, for some dimensions of personality and life-style, twins who cohabit longer into adulthood and twins who retain closer contact after separation are more alike. Further, new analyses here reported are consistent with our earlier inference that, for neuroticism and consumption of alcohol, cotwin differences in contact precede, and causally contribute to, differences in behavioral similarity.

Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart

Since 1979, a continuing study of monozygotic and dizygotic twins, separated in infancy and reared apart, has subjected more than 100 sets of reared-apart twins or triplets to a week of intensive psychological and physiological assessment. Like the prior, smaller studies of monozygotic twins reared apart, about 70% of the variance in IQ was found to be associated with genetic variation. On multiple measures of personality and temperament, occupational and leisure-time interests, and social attitudes, monozygotic twins reared apart are about as similar as are monozygotic twins reared together. These findings extend and support those from numerous other twin, family, and adoption studies. It is a plausible hypothesis that genetic differences affect psychological differences largely indirectly, by influencing the effective environment of the developing child. This evidence for the strong heritability of most psychological traits, sensibly construed, does not detract from the value or importance of parenting, education, and other propaedeutic interventions.

Genetic Influence on Adults' Ratings of Their Current Family Environment

The genetic and environmental origins of individual differences in adults' ratings of their current family environment have not previously been studied. An abbreviated version of the Moos Family Environment Scales (FES) was completed by 50pairs of identical twins reared apart, 92pairs of identical twins reared together, 129 pairs of fraternal twins reared apart, and 115 pairs of fraternal twins reared together. The average age of the sample was 59 years. Maximum-likelihood estimates of model parameters indicated that, on average, about 25% of the variance of adults' scores on the FES is due to genetic differences among them. Of the eight FES scales, CulturalIntellectual Orientation showed the most genetic influence (40% heritability) and Achievement Orientation showed the least (12% heritability). Reared-together twins were no more similar than reared-apart twins, suggesting that the environment in which they are reared has little effect on adults' ratings of their family environment.