Differential experiences of siblings in the same family as predictors of adolescent sibling personality differences

In the area of personality development, environmental influences operate to make siblings in the same family different rather than similar to each other. The goal of the present study was to determine whether differential experience of siblings can be used to explain the marked personality differences of siblings. The Sibling Inventory of Differential Experience (SIDE) along with personality information (the EAS Temperament Inventory and questions about career expectations) was administered to 50 biological sibling pairs and 98 adoptive sibling pairs in adolescence and young adulthood. The results indicated that differential sibling interaction and differential peer characteristics as self-reported on the SIDE explain 6%-26% of the variance in sibling personality difference scores. For example, the sibling who reports more sociability as compared to his sibling also experiences more sibling closeness and more peer popularity as compared to his sibling. Comparison between adoptive and biological siblings indicates that the SIDE relations are mediated environmentally rather than genetically.     

Left or right? Sources of political orientation: the roles of genetic factors, cultural transmission, assortative mating, and personality

In this study, we used an extended twin family design to investigate the influences of genetic and cultural transmission as well as different sources of nonrandom mating on 2 core aspects of political orientation: acceptance of inequality and rejecting system change. In addition, we studied the sources of phenotypic links between Big Five personality traits and political beliefs using self- and other reports. Data of 1,992 individuals (224 monozygotic and 166 dizygotic twin pairs, 92 unmatched twins, 530 spouses of twins, 268 fathers, and 322 mothers) were analyzed. Genetically informative analyses showed that political attitudes are genetically but not environmentally transmitted from parents to offspring and that a substantial proportion of this genetic variance can be accounted for by genetic variance in personality traits. Beyond genetic effects and genotypic assortative mating, generation-specific environmental sources act to increase twins' and spouses' resemblance in political beliefs. The results suggest multiple sources of political orientations in a modern democracy.     

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.     

Mothers' representations of their 13-month-old twins and child-raising attitudes

The purpose of this study was to describe maternal representations of the twin sibling relationship and of the children's differences and similarities. Mothers' mental representations were also analyzed with regard to two aspects of their child-raising attitudes: individualization of childcare routines, and the question of differentiation by “external signs of twinship” (dress, toys, and first names). Data from interviews with 44 mothers of twins conducted 13 months after the birth were analyzed in terms of 8 categories—complicity, rivalry/jealousy, agonistic relations, dominance, imitation, physical appearance, development, and personality. A multiple correspondence analysis of the data pointed out that the first dimension describing the mothers' representations concerns the contrast between the “unalike” representation of dizygotic twins and the “alike” representation of monozygotic twins. The second dimension corresponds to the casual, twinship-minimizing representations generated by the twin condition. Two other dimensions express the issues of aggression and dominance. The relationship between the child-raising behavior of the mothers and their representations of the twin couple turned out to be more complex than expected. The results are discussed in terms of the specificity of the psychological issues facing mothers of twins. Defense mechanisms used by some mothers to cope with the exceptional nature of the twin phenomenon are also suggested. © 1998 Michigan Association for Infant Mental Health     

What does a general factor of personality look like in unshared environmental variance?

A general factor of personality (GFP) was obtained solely from unshared environment, by deriving it from the intercorrelations of monozygotic twin differences. This GFP was compared with a GFP derived from correlations reflecting all three components of variance—genes, shared environment, and unshared environment. In two large samples of adult Australian twins, for the items of two questionnaires, the two approaches gave virtually identical GFPs. However, in a sample of adolescent Australian twins, the two GFPs were correlated but distinct.     

Maternal expressed emotion predicts children's antisocial behavior problems: using monozygotic-twin differences to identify environmental effects on behavioral development

If maternal expressed emotion is an environmental risk factor for children's antisocial behavior problems, it should account for behavioral differences between siblings growing up in the same family even after genetic influences on children's behavior problems are taken into account. This hypothesis was tested in the Environmental Risk Longitudinal Twin Study with a nationally representative 1994-1995 birth cohort of twins. The authors interviewed the mothers of 565 five-year-old monozygotic (MZ) twin pairs and established which twin in each family received more negative emotional expression and which twin received more warmth. Within MZ pairs, the twin receiving more maternal negativity and less warmth had more antisocial behavior problems. Qualitative interviews were used to generate hypotheses about why mothers treat their children differently. The results suggest that maternal emotional attitudes toward children may play a causal role in the development of antisocial behavior and illustrate how genetically informative research can inform tests of socialization hypotheses.     

Differential parent-child relationships and adolescent externalizing symptoms: cross-lagged analyses within a monozygotic twin differences design

Research has indicated that differential parental treatment is linked to differences in externalizing symptomology (EXT) across siblings, even those siblings who are genetically identical. However, the direction of causation and longitudinal significance of this relationship remains unclear. Thus, in the present study, the authors examined 486 monozygotic twin pairs, assessed at ages 11, 14, and 17 years, within a cross-lagged twin differences design. Results revealed that differential parent-child conflict at age 11 years uniquely contributed to differential sibling EXT 3 years later but only in the most discordant twin pairs. In the full, unselected sample, this relationship was not significant. These results suggest that markedly different parent-child conflict has an environmentally mediated impact on child behavior through mid-adolescence, findings that yield insights into environmental influences on behavior.     

Growing up and growing apart: a developmental meta-analysis of twin studies

Developmental change in twin similarity was examined with age contrasts in a meta-analysis of twin studies from 1967 through 1985. Intraclass rs were coded from 103 papers that included data for monozygotic or dizygotic twins, or for both, on personality or intelligence variables. Analyses indicated that there was a general tendency for some intraclass rs to decrease with age. In other words, as twins grow up, they grow apart. There were also developmental differences associated with components of variance for heritability, the shared environment, and the nonshared environment. Mechanisms through which the nonshared environment may operate are discussed.     

On facial expertise: processing strategies of twins' parents

Parents of monozygotic twins typically learn to recognise their own children and also to tell them apart on photographs. However, it is unknown whether such exemplar expertise can be generalised to unfamiliar faces of equal similarity (ie other twins). In the present study, parents of monozygotic twins were compared to 'control' parents whose children were non-twin siblings. In a same/different task with familiar and unfamiliar twin faces as stimuli, twins' parents were faster than control parents in distinguishing their own children, but slower when distinguishing unfamiliar twin faces. These unexpected findings can be interpreted as a generalisation of a habitual perceptual strategy that is efficient only with familiar face exemplars. When pictures of the twins' faces were inverted by 180degrees, the typical inversion costs for familiar faces were observed, but the twins' parents recognised unfamiliar twins' faces better when these were inverted than when upright. This inverted inversion effect could be caused by the high similarity of the face pairs as well as by experience with familiar faces of equally high similarity. We conclude that the facial expertise of twins' parents is idiosyncratic to their own children and not based on an enhanced general facial expertise.     

A possible genetic basis of accuracy in personality perception

An analysis was conducted to assess whether individuals are more accurate in perceiving those personality characteristics of other' that have a larger genetic component. Adult monozygotic (MZ) twins (63 pairs) and same-sex dizygotic (DZ) twins (35 pairs) completed the Personality Research Form-E (PRF) and the Sibling Evaluation of Personality Characteristics (SEPC) which consists of 20 sets of adjectives corresponding to the 20 trait scales of the PRF. Subjects also evaluated their co-twin with the SEPC. Self-other agreement correlations were then correlated with the heritability estimates of the PRF traits calculated on the same twin sample. The results suggest that accuracy in perceiving the co-twin's personality does increase with the heritability component of the personality dimension.     

Genetic and experiential influences on behavior: Twins reunited at seventy-eight years

Twins living in different countries offer opportunities to explore associations between observed differences and experiential effects. This report compared the life histories, cognitive abilities, personality traits, psychomotor skills, medical characteristics, job satisfaction, social support and social relations of dizygotic (DZ) female twins reunited at 78, the world’s longest separated set. The twins’ advanced age also enabled a study of how co-twin differences in aging may be associated with current behavioral and social differences. Consistent with previous studies, these dizygotic reared apart (DZA) twins showed discordance across some, but not all, traits. Their different rearing situations and life histories may explain current differences in their responses to meeting their twin. This case highlights the importance of both genetic and rearing factors on behavior, but does not allow firm conclusions regarding the extent to which these sources explain individual developmental differences. However, such data contribute to the growing number of cross-culturally separated twins, generating novel hypotheses that may be assessed using larger samples.     

Beyond the heritability of life satisfaction – The roles of personality and twin-specific influences

In a behavior genetic design, we investigated individual differences in life satisfaction and its relation to personality with respect to both internal and external influences. We questioned the absence of shared environment and examined the specific contribution of additive and non-additive genetic influences. We also tested for twin-specific environmental influences in a total sample of 1308 dyads including identical and fraternal twins, siblings, mother–child and grandparent–child pairs. The results showed substantial shared environmental influences on life satisfaction that varied between twins and non-twins, supporting twin-specific environmental influences. Additive and non-additive genetic influences on life satisfaction were completely shared with personality. The remaining variance could be explained by unique non-shared environmental influences for life satisfaction independent of personality.     

A comparison of twins for degree of closeness and field dependency

An examination of Witkin's construct of field dependency was undertaken with a sample of adolescent twin pairs to determine if the degree of closeness in twin relationships would be reflected in the twins' degree of field dependency. Sixteen pairs of monozygotic (MZ) adolescent male twins and the same number of dizygotic (DZ) same-sexed male twins were administered three measures: Nichols and Bilbro's (1966) zygosity questionnaire, Paluszny and Beit Hallahmi's (1974) semantic differential measure of closeness, and Jackson's (1956) short form of the Embedded Figures Test. No significant differences were found between the MZ and DZ twin groups with regard to their expressed closeness to one another, as measured with Paluszny and Beit-Hallahmi's index of closeness; in addition, there were no significant differences between the two twin groups on the measure of field dependency. However, significant relationships of these two variables with intelligence were found and possible reasons for such an occurrence were offered.     

Tacit coordination in monozygotic twins, dizygotic twins and virtual twins: Effects and implications of genetic relatedness

Game theorists have long been interested in why some partners are more successful at achieving mutual goals than others. The present study was conducted to determine if tacit coordination (non-negotiated consensus) varies as a function of genetic relatedness between social partners. The sample included 7–13-year-old monozygotic (MZ) twin pairs (n = 53), dizygotic (DZ) twin pairs (n = 85) and virtual twin (VT) pairs (n = 42; same-age unrelated siblings). Co-twin agreement on questions completed under individual and coordination conditions was compared across the three groups. MZ twins showed significantly greater overall agreement than DZ twins and VTs, using multiple criteria. The implications of these findings are considered with reference to behavioral-genetic and evolutionary theories of behavior.     

The effect of classroom environment on problem behaviors: A twin study

Children's behavior problems, both internalizing and externalizing, are a function of both genetic and environmental influences. One potentially important environmental influence is the classroom environment. This study of 1941 monozygotic twin pairs examined whether children whose parents rated them as similarly high or low on a number of problem behaviors were rated in the same way by teachers when they were together versus separated in their classrooms at school. Results showed that twins in the same classrooms were rated by their teachers more similarly on each dimension of problem behavior than were twins who were separated into different classrooms, suggesting that the classroom environment is important in predicting differences between twins in terms of problem behaviors at school. In addition, parents' ratings of problem behaviors were lower for twins placed in the same classroom versus twins in different classrooms. Thus, there is reason to consider whether, at least during the early school years, parents should be allowed to make the decision to not separate twins at school.     

Genetic and Rearing Environmental Influences on Adult Personality: An Analysis of Adopted Twins Reared Apart

ABSTRACT We report a genetic and environmental analysis of California Psychological Inventory (CPI) scale scores gathered on a sample of 45 sets of monozygotic twins reared apart (MZA) and 26 sets of dizygotic twins reared apart (DZA) Analysis of twin intraclass correlations and the results of models fit to the twin data demonstrate that the heritability of most scales and five factors of the CPI is about 50 When compared to results from studies of adult MZ and DZ twins reared together few of the scales demonstrate any common family environmental influence Placement coefficients on the Family Environment Scale (FES) can explain only a minor portion of the correlations for twins reared apart The influence of specific rearing environmental factors on adult personality was evaluated by analyzing the relationship between the FES and the CPI in this adoptee sample One FES factor (Cohesion vs Conflict) does correlate substantially with the CPI factor of Consensuality and may account for up to 24% of the variance in that factor, but the retrospectively gathered     

Genetic and environmental influences on observed personality: evidence from the German Observational Study of Adult Twins

Previous behavior-genetic research on adult personality relied primarily on self-reports or peer reports that may be subject to contrast effects, resulting in biased estimates of genetic and environmental influences. In the German Observational Study of Adult Twins (GOSAT), personality traits of 168 monozygotic (MZ) and 132 dizygotic (DZ) twin pairs were rated on 35 adjective scales, largely markers of the Big 5. The ratings were provided by 120 judges who never met the twins but observed videotaped behaviors of 1 twin of each pair in 1 of 15 different settings. The aggregated video-based trait ratings were highly reliable, and substantial correlations were obtained between MZ as well as DZ twins. Model-fit analyses suggested about 40% genetic, 25% shared environmental, and 35% nonshared environmental influence. Extraversion was the only trait that seemed not to be influenced by shared environment.     

Sharing genes fosters identity fusion and altruism

Researchers have shown that the more genes twins share, the more they care about one another. Here, we examine a psychological mediator of such genetic influences, “identity fusion” (a visceral sense of oneness with them). Results supported this hypothesis. Relative to dizygotic twins, monozygotic twins reported stronger fusion and elevated desire to have contact and share experiences with their twin (Study 1), to forgive and grant favors to their twin after being disappointed by him/her (Study 2), and willingness to make sacrifices for their twin (Study 3). Fusion with the twin mediated the impact of zygosity on these outcomes. These findings demonstrate that genetic relatedness fosters a powerful feeling of union with one’s twin that predicts sharing, tolerance, and self-sacrificial behavior toward him or her.     

Whose life is it anyway? Adoption of each other's autobiographical memories by twins

Anecdotal evidence suggests that twins may dispute ownership of autobiographical experiences. We investigated the frequency and characteristics of such disputed memories in comparison to memories with undisputed ownership. In the present study, monozygotic twins (MZ), dizygotic twins (DZ), and siblings were asked to remember disputed and non‐disputed memories. They also dated each memory and provided a rating on the following variables: level of imagery, detail, rehearsal, and importance. Results showed that disputed memories were more common among MZ twins than in DZ twins and siblings. The frequencies of undisputed memories were not different among the three groups. When compared to non‐disputed memories, disputed memories were more likely to come from events experienced during preschool years. We consider disputed memories as instances of false memories, at least partly influenced by social interactional processes, and event characteristics. Copyright © 2003 John Wiley & Sons, Ltd.     

Genetic probes of three theories of maternal adjustment: II. Genetic and environmental influences.

This is the first report of the Twin Mom Study, an investigation of three hypotheses concerning influences on maternal adjustment. These hypotheses concern the role of the marital and parent-child relationships in mediating genetic influences on maternal adjustment and on the importance of the mothers' marital partners as a specifiable source of influences on their adjustment not shared with their sisters. The study's sample of 150 monozygotic (MZ) twins and 176 dizygotic (DZ) twins was drawn randomly from the Swedish Twin Registry and is, with some small exceptions, likely to be representative of women in the Swedish population. The sample included the marital partners of these twins and their adolescent children. Self-report and coded videotapes were a source of information about family process. Results reported in this first report focus on comparability of American and Swedish samples on scales measuring psychiatric symptoms, and on an analysis of genetic and environmental influences on nine measures of mothers' adjustment. Results suggest comparability between the US and Sweden. Genetic influences were found for all measures of adjustment, particularly in the psychological manifestations of anxiety and for smoking. The pattern of findings also underscored the importance of influences unique to each sibling within the twin pair, thus focusing attention on the potential role of marital partners in maternal adjustment. Results also suggested that experiences shared by the twin sisters, experiences unrelated to their genetic similarity, may influence their fearfulness and alcohol consumption. Our model did not include these influences and thus must be amended.