Separated twins and the genetics of personality differences: a critique

This article discusses studies of separated twins, with special emphasis on the Minnesota Study of Twins Reared Apart (MISTRA), to determine whether they support the existence of an important genetic component in behavioral and personality differences. The methods and conclusions of the MISTRA team are discussed in the context of earlier studies of separated identical twins. I argue that volunteer-based studies are biased toward greater twin similarity. In addition, the MISTRA research team did not publish or share raw data and case history information. Reared-together and reared-apart monozygotic twins share important environmental similarities not controlled for by comparing personality correlations. I propose an alternative control group consisting of biologically unrelated pairs of strangers matched on all environmental factors common to pairs of separated monozygotic twins. I conclude that the evidence from studies of twins reared apart does not support the role of genetic factors in personality and behavioral differences.     

Nature,nurture and academic achievement: A twin study of teacher assessments of 7‐year‐olds

Background: Twin research has consistently shown substantial genetic influence on individual differences in cognitive ability; however, much less is known about the genetic and environmental aetiologies of school achievement. Aims: Our goal is to test the hypotheses that teacher‐assessed achievement in the early school years shows substantial genetic influence but only modest shared environmental influence when children are assessed by the same teachers and by different teachers. Sample: 1,189 monozygotic (MZ) and dizygotic (DZ) twin pairs born in 1994in England and Wales. Methods: Teachers evaluated academic achievement for 7‐year‐olds in Mathematics and English. Results were based on the twin method, which compares the similarity between identical and fraternal twins. Results: Suggested substantial genetic influence in that identical twins were almost twice as similar as fraternal twins when compared on teacher assessments for Mathematics, English and a total score. Conclusions: The results confirm prior research suggesting that teacher assessments of academic achievement are substantially influenced by genetics. This finding holds even when twins are assessed independently by different teachers.     

Sex differences in early verbal and non‐verbal cognitive development

The present study of over 3000 2‐year‐old twin pairs used a sex‐limitation model to examine genetic and environmental origins of sex differences in verbal and non‐verbal cognitive ability. Girls scored significantly higher on both measures (p<0.0001), although gender only accounted for approximately 3% of the variance in verbal ability and 1% of the variance in non‐verbal cognitive ability. For the verbal measure boys showed greater heritability than girls. Also the twin‐pair correlation is significantly lower for opposite‐sex twins than for non‐identical same‐sex twins. This indicates that individual differences in verbal ability include some sex‐specific factors. Non‐verbal cognitive ability did not differ in aetiology for boys and girls. We conclude that genetic and environmental influences differ for girls and boys for early verbal but not non‐verbal development.     

Genetic mediation of the relationship between social support and psychological well-being.

This study assessed genetic and environmental influences on the association between social support and psychological functioning using the combined adoption/twin design from the Swedish Adoption/Twin Study of Aging. A subsample of 424 twin pairs, 50 years and older, was used: 64 pairs of identical twins reared apart, 95 pairs of identical twins reared together, 132 pairs of fraternal twins reared apart, and 133 pairs of fraternal twins reared together. Multivariate model-fitting analyses indicated that the relationship between the perceived adequacy of social support and psychological well-being (depression and life satisfaction) was mediated in part by genetic factors.     

Task orientation, parental warmth and SES account for a significant proportion of the shared environmental variance in general cognitive ability in early childhood: evidence from a twin study

Prior research suggests shared environmental influences on cognitive performance are important in early childhood. However, few studies have attempted to identify the factors comprising this shared environmental variance. To address this issue, we examined the covariance between task orientation, parental warmth, socioeconomic status and general cognitive ability in a British twin study of 125 pairs of identical and same-sex fraternal twins. Task orientation correlated r =.41 with general cognitive ability Bivariate genetic analyses suggested that this correlation was mediated by shared environmental influences. Additional analyses suggested that SES and parental warmth mediated about two-thirds of the shared environmental covariance between task engagement and cognitive skills.     

Genetic and Environmental Effects on Openness to Experience, Agreeableness, and Conscientiousness: An Adoption/Twin Study

ABSTRACT Previous research has indicated that extraversion and neuroticism are substantially affected both by genotype and environment. This study assesses genetic and environmental influences on the other three components of the five-factor model of personality: Openness to Experience, Agreeableness, and Conscientiousness. An abbreviated version of the NEO Personality Inventory (NEO-PI) was administered to 82 pairs of identical twins and 171 pairs of fraternal twins reared apart and 132 pairs of identical twins and 167 pairs of fraternal twins reared together. Estimates of genetic and environmental effects for Openness and Conscientiousness were similar to those found in other studies of personality: Genetic influence was substantial and there was little evidence of shared rearing environment. Results for Agreeableness were different: Genetic influence accounted for only 12% of the variance and shared rearing environment accounted for 21% of the variance. Few significant gender or age differences for genetic and environmental parameters were found in model-fitting analyses.     

Relationships between parenting and adolescent adjustment over time: genetic and environmental contributions.

The predictive association between parenting and adolescent adjustment has been assumed to be environmental; however, genetic and environmental contributions have not been examined. This article represents one effort to examine these associations in which a genetically informative design was used. Participants were 395 families with adolescent siblings who participated in the Nonshared Environment in Adolescent Development (D. Reiss et al., 1994) project at 2 times of assessment, 3 years apart. There were 5 sibling types in 2 types of families: 63 identical twins, 75 fraternal twins, and 58 full siblings in nondivorced families and 95 full, 60 half, and 44 genetically unrelated siblings in stepfamilies. Results indicate that the cross-lagged associations between parental conflict-negativity and adolescent antisocial behavior and depressive symptoms can be explained primarily by genetic factors. These findings emphasize the need to recognize and examine the impact that adolescents have on parenting and the contribution of genetic factors to developmental change.     

Stability and change in temperament during adolescence

This study assessed genetic and environmental contributions to temperament during adolescence within the Nonshared Environment and Adolescent Development project (NEAD; D. Reiss, J. M. Neiderhiser, E. M. Hetherington, & R. Plomin, 2000). NEAD is a national study that includes twins and other sibling types who vary in regard to genetic relatedness. Seven hundred twenty sibling pairs (aged 12.1-13.5 years) participated at Time 1, and 395 sibling pairs (aged 14.7-16.2 years) participated again at Time 2. At both Times, mothers and fathers rated their children's temperament (emotionality, activity, sociability, and shyness). At Times 1 and 2, genetic and nonshared environmental factors accounted for variance in temperament, whereas shared environmental contributions were negligible. However, at Time 1, genetic contributions were inflated, and shared environmental contributions were masked if sibling contrast effects were not taken into account. At Time 2, sibling interaction effects had little impact on estimates of genetic and environmental contributions to temperament. Last, temperament stability was primarily explained by genetic factors, whereas both genetic and nonshared environmental factors accounted for change.     

The effect of being reared with an alcoholic half-sibling: a classic study reanalyzed

Data from an early study of alcoholism among half-siblings of alcoholic probands (17) was reanalyzed using log-linear analysis. The original study, which used chi-square analysis, identified genetic factors but found no environmental effects that contributed to the prediction of alcoholism. Log-linear analysis, however, reveals that being reared with a proband is associated with a reduced incidence of alcoholism in half-siblings. The results do not disconfirm the presence of genetic factors, but they do indicate that family dynamics make an independent and statistically significant contribution to the development of alcoholism not heretofore demonstrated. Similarities between these results and the results from studies of identical twins are discussed.     

Problems in Psychiatric Genetic Research: A Reply to Faraone and Biederman

This article examines evidence cited in favor of the operation of genetic factors in attention-deficit hyperactivity disorder (ADHD). Like other psychiatric conditions, a belief in the genetic basis of ADHD is derived from the results of family, twin, and adoption studies. Because family studies are widely believed to be confounded by environmental factors, primary emphasis is placed on twin and adoption studies. ADHD twin studies depend on the validity of the equal environment assumption (EEA), which holds that the environments of identical (MZ) and fraternal (DZ) twins are the same. Here it is argued that however the EEA is defined, it cannot be accepted. Therefore, the greater similarity or concordance of MZ twins when compared to DZ twins is plausibly explained by environmental factors. Adoption studies constitute a third method for investigating the role of genetic factors in ADHD. It is argued that these studies are greatly flawed by factors including non blinded diagnoses and the failure to study the biological relatives of adoptees. After an examination of the total weight of evidence in favor of a genetic basis or predisposition for ADHD, it is concluded that a role for genetic factors is not supported and that future research should be directed toward psychosocial causes     

The discrimination of dog odours by humans

The domestic dog is able to identify both individual conspecifics, i.e. other dogs, and individual interspecifics, e.g. humans, by smell. Whilst humans can recognise individual members of their own species using olfactory information, it is unknown whether they can identify members of any other species in this manner. We examined the ability of humans to identify individual dogs by smell. Twenty-six dog owners were required to smell two blankets, one impregnated with the odour of their own dog, the other impregnated with the odour of an unfamiliar dog. Participants were required to indicate which of the odours smelt the strongest, which smelt the most pleasant, and which of the odours belonged to their own dog. Most of the participants (88.5%) were able to recognise the odour of their own dog. They showed no significant bias, however, in responding which of the odours they thought smelt the strongest or most pleasant. The results indicate that dogs produce odours that are individually distinctive to their owners, and highlight the fact that humans can recognise members of another species using olfactory cues--an ability presumably acquired without conscious effort.     

General and supplementary factors of personality in genetic and environmental correlation matrices

Genetic and environmental correlation matrices among 7 personality scales were estimated, using samples of adult and young adult twins from Australia (2081 and 1178 pairs, respectively). A general factor of personality and two supplemental factors were obtained in each. The supplemental factors were tentatively identified as Social conformity and Other-dependence. Factors from the genetic and environmental correlations replicated well across samples, and they were similar in both the genetic and environmental covariation. It was concluded that the structure of personality is inherent in the evolved phenotype, and is not the immediate consequence of either genetic or environmental organizing factors.     

A DEVELOPMENTAL-GENETIC ANALYSIS OF CONTINUITY AND CHANGE IN THE BAYLEY MENTAL DEVELOPMENT INDEX FROM 14 TO 24 MONTHS:

Abstract— A developmental-genetic model was fitted to Bayley Mental Development Index (MDI) data to address questions concerning the origins of individual differences in MDI performance and the origins of change and continuity during infancy More than 350 pairs of identical and same-sex fraternal twins were studied longitudinally at 14, 20, and 24 months of age There was substantial genetic continuity of general cognitive ability from 14 to 24 months but significant new genetic variation also appeared at 24 months Shared family environmental influences were global across all three ages and not time-specific Finally, nonshared environmental influences did not contribute to the observed continuity of general cognitive ability, only to change.     

Genes Determine Stability and the Environment Determines Change in Cognitive Ability During 35 Years of Adulthood

ABSTRACT— Previous research has demonstrated stability of cognitive ability and marked heritability during adulthood, but questions remain about the extent to which genetic factors account for this stability. We conducted a 35-year longitudinal assessment of general cognitive ability using the Armed Forces Qualification Test administered to 7,232 male twins in early adulthood and readministered to a subset of 1,237 twins during late middle age. The proportion of variance in cognitive functioning explained by genetic factors was .49 in young adulthood and .57 in late middle age. The correlation between the two administrations was .74 with a genetic correlation of 1.0, indicating that the same genetic influences operated at both times. Genetic factors were primarily responsible for stability, and nonshared environmental factors were primarily responsible for change. The genetic factors influencing cognition may change across other eras, but the same genetic influences are operating from early adulthood to late middle age.     

Genetic and environmental causes of the interrelationships between self-reported fears. A study of a non-clinical sample of Norwegian identical twins and their families

The present study reports results from a study of the self-reported fears of identical twins and their spouses and offspring. Factor analysis with oblique rotation of questionnaire responses yielded four correlated fear dimensions: situational fears, illness-injury fears, social fears, and fear of small animals. Models allowing for genetic and cultural transmission, together with specially correlated environments for twins, were fitted, both for separate fears and across fears. Simple models with only genetic and uncorrelated environments were sufficient to account for each the fear dimensions considered separately. The cross-dimensional analyses revealed a genetic and an environmental factor common to the four fear dimensions, together with fear-specific genetic and environmental factors. The impact of the common genetic and common environmental factor varied across dimensions. No evidence of cultural transmission or specially correlated twin environments of the cross-dimensional environments was detected. It is concluded that both common and fear-specific genes and (individual-specific) common and fear-specific environments are necessary to account for the data. The results are discussed in terms of the prepared learning hypothesis and the expectancy bias hypothesis.     

Intellectual similarity of virtual twin pairs: Developmental trends

Virtual twins (VTs) are same-age unrelated siblings reared together from infancy who replicate twinship, but without the genetic relatedness. A 2005 report from the ongoing Fullerton Virtual Twin Study found an IQ intraclass correlation of .26 (p < .01, n = 113 pairs) and a within-pair difference of 13.22 IQ points. The average age of VTs in that study was 8.10 years (SD = 8.56, range: 4.01-54.84 years). An opportunity to retest members of 43 VT pairs, 1.70-8.96 years after their time 1 assessment, allowed additional analyses of genetic and environmental influences underlying general intellectual development. A decrease in the VT IQ correlation and an increase in the within-pair difference were indicated, consistent with increasing genetic and/or non-shared environmental influences and decreasing shared environmental influence on general intellectual development throughout childhood.     

Why aren't identical twins linguistically identical? Genetic, prenatal and postnatal factors

Results of twin studies clearly demonstrate that genetic factors play an important role in the rate of language acquisition and linguistic proficiency attained by normal and impaired children and adults [see Stromswold, K. (2001). The heritability of language: A review and meta-analysis of twin, adoption and linkage studies. Language, 77, 647-723.]. That said, twin-based heritability estimates for language rarely exceed .6 and monozygotic (MZ) twins (who are usually assumed to have identical genetic and environmental endowments) sometimes have very different linguistic profiles. In addition, twins are more likely to suffer linguistic delays and impairments than singletons. Postnatal factors, such as differences in linguistic input twins receive, are usually assumed to be the major reason for these findings. This paper discusses how genetic, epigenetic, and perinatal environmental factors can lower heritability estimates for language, cause MZ twins to be linguistically discordant, and increase the risk of language impairments in twins. We present results from our ongoing Perinatal Environment and Genetic Interaction (PEGI) study that suggest that perinatal environmental factors affect linguistic development more than postnatal factors, and that postnatal factors affect cognitive development more than perinatal factors. Because perinatal factors are overwhelming biological, whereas postnatal factors tend to be psychosocial (e.g., how and how much parents speak to their children), these results support nativist/biological theories of language and language development and call into question empiricist/emergentist theories. These results are also consistent with modularist theories of language. We end by suggesting new methods that can be used to tease apart the effects of prenatal and postnatal environment and to investigate how these factors interact with genetic factors.     

GENETIC AND ENVIRONMENTAL CONTRIBUTIONS TO VARIANCE IN AGE AT FIRST SEXUAL INTERCOURSE

Abstract— Little is known about the relative importance of genetic and environmental factors as determinants of age at first sexual intercourse. In this study, subjects were 5,080 individuals from the Australian Twin Registry (3,310 females, 1,770 males, age range 27–70 years, median 40 years) who completed a semistructured interview by telephone in 1992–1993. Self-reported age at first intercourse correlated higher for identical (monozygotic) twins than for nonidentical (dizygotic) twins. Structural equation model fitting found that the genetic contribution to variance was considerably greater among twins aged 40 years or less (72% for males and 49% for females) than for those aged from 41 to 70 years (0% for males and 32% for females). Among the older cohort, there was evidence that somewhat different aspects of the shared social environment influenced age at onset in males and females. In a more laissez-faire social climate in recent decades, it is likely that biological and psychological characteristics that are partly under genetic control significantly influence the age at which a person commences sexual activity.     

Discrimination of familiar human faces in dogs (Canis familiaris)

Faces are an important visual category for many taxa, and the human face is no exception to this. Because faces differ in subtle ways and possess many idiosyncratic features, they provide a rich source of perceptual cues. A fair amount of those cues are learned through social interactions and are used for future identification of individual humans. These effects of individual experience can be studied particularly well in hetero-specific face perception. Domestic dogs represent a perfect model in this respect, due to their proved ability to extract important information from the human face in socio-communicative interactions. There is also suggestive evidence that dogs can identify their owner or other familiar human individuals by using visual information from the face. However, most studies have used only dogs’ looking behavior to examine their visual processing of human faces and it has been demonstrated only that dogs can differentiate between familiar and unknown human faces. Here, we examined the dog's ability to discriminate the faces of two familiar persons by active choice (approach and touch). Furthermore, in successive stages of the experiment we investigated how well dogs discriminate humans in different representations by systematically reducing the informational richness and the quality of the stimuli. We found a huge inter-individual and inter-stage variance in performance, indicating differences across dogs in their learning ability as well as their selection of discriminative cues. On a group level, the performance of dogs significantly decreased when they were presented with pictures of human heads after having learned to discriminate the real heads, and when – after relearning – confronted with the same pictures showing only the inner parts of the heads. However, as two dogs quickly mastered all stages, we conclude that dogs are in principle able to discriminate people on the basis of visual information from their faces and by making active choices.     

Sex differences in the genetic and environmental influences on childhood conduct disorder and adult antisocial behavior

Sex differences in the genetic and environmental influences on childhood conduct disorder and adult antisocial behavior were examined in a large community sample of 6,383 adult male, female, and opposite-sex twins. Retrospective reports of childhood conduct disorder (prior to 18 years of age) were obtained when participants were approximately 30 years old, and lifetime reports of adult antisocial behavior (antisocial behavior after 17 years of age) were obtained 8 years later. Results revealed that either the genetic or the shared environmental factors influencing childhood conduct disorder differed for males and females (i.e., a qualitative sex difference), but by adulthood, these sex-specific influences on antisocial behavior were no longer apparent. Further, genetic and environmental influences accounted for proportionally the same amount of variance in antisocial behavior for males and females in childhood and adulthood (i.e., there were no quantitative sex differences). Additionally, the stability of antisocial behavior from childhood to adulthood was slightly greater for males than females. Though familial factors accounted for more of the stability of antisocial behavior for males than females, genetic factors accounted for the majority of the covariation between childhood conduct disorder and adult antisocial behavior for both sexes. The genetic influences on adult antisocial behavior overlapped completely with the genetic influences on childhood conduct disorder for both males and females. Implications for future twin and molecular genetic studies are discussed.