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.     

Nonshared environmental processes in social‐emotional development: an observational study of identical twin differences in the preschool period

Differences in mothers’ parenting behaviors toward their identical twin preschoolers were examined to identify nonshared environmental processes in social‐emotional development. The study included 62 pairs of 3½‐year‐old same‐sex identical twins. Indicators of each child’s social‐emotional development (temperament, prosocial behavior, behavior problems and noncompliance) and parenting environment (warmth and negativity, positive and negative control, responsiveness) were assessed using observers’, interviewers’, and parents’ ratings. Mothers treated their identical twins differently, and this differential treatment covaried in expected ways with identical twin differences in social‐emotional adjustment. The twin who received more supportive and less punitive forms of parenting was also higher in positive mood and prosocial behaviors and lower in negative mood and behavior problems when compared to her or his twin.     

Twin and sibling attachment in a nationally representative sample

The objective of this research was to evaluate the hypothesis that twin relationships are attachments, using data from a nationally representative sample. The results indicated that twin siblings were more likely than nontwin siblings to be attached to their siblings. Moreover, analyses indicated that both attachment theoretical and inclusive fitness perspectives are necessary for explaining these findings. Namely, twins were more likely to be attached than nontwin siblings, as expected from an attachment perspective. But identical twins were more likely than fraternal twins to be attached to one another, as might be expected from an inclusive fitness perspective. Cross-sectional analyses indicated that older people are less likely than younger people to use their sibling as an attachment figure compared to younger people and that married adults are less likely to use their sibling as an attachment figure than nonmarried people.     

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.     

Twins' reactions to delayed auditory feedback

10 pairs of identical and 10 pairs of fraternal twins, matched by age, spoke under conditions of 0.0-, 100-, 200-, 300-, 400-, and 500-msec. delayed auditory feedback. Length of spoken passages was controlled. Product-moment and intraclass correlations were calculated for speaking times and disfluencies. Significant Pearson rs for times were noted at 0.0 and 300 msec. for both groups and at 100, 200, and 400 msec. for identical twins, while fraternal twins' times were significantly correlated at 500 msec. Difference scores were significantly correlated at 100, 200, 300, and 400 msec. for identical twins. Disfluencies were significantly correlated for identical twins at 400 msec. Data were combined with those of Timmons' (1969) study, increasing subjects to 21 pairs per group. Intraclass correlations supported the contention that responses of identical twin pairs to delayed auditory feedback were more highly correlated than those for fraternal twin pairs.     

EAS temperaments during the last half of the life span: twins reared apart and twins reared together

In this first behavioral genetic study of personality in the last half of the life span, results are reported using the powerful adoption/twin design that compares identical and fraternal twins reared apart and identical and fraternal twins reared together. Traits studied were the EAS temperaments (emotionality, activity level, and sociability), traits that show substantial genetic influence in childhood. It was hypothesized that the EAS traits would also show significant genetic influence later in life and that most of the environmental variation would be nonshared--that is, twins reared together would show no greater resemblance for the EAS traits than twins reared apart. Both hypotheses are supported.     

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     

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.     

Hedonic possibilities and heritability statistics

Several influential psychologists have attempted to estimate to what extent human happiness levels are directly controlled by genes by comparing the happiness levels of identical twins raised apart. If we discover that the happiness levels of identical twins raised apart tend to be closer than the happiness levels of fraternal twins raised apart, this is taken as evidence that average happiness levels are largely controlled by genes. However, if it turns out that identical twins' happiness levels tend to be substantially different, as different as fraternal twins raised apart, it has been argued that this implies that the environment largely determines happiness levels. I contend that this interpretation of the data rests on a set of questionable, closely related assumptions: a) that pairs of identical twins raised apart are raised in substantially different environments; b) that genetically identical twins aren't identical in other pertinent ways; and c) that average levels of affect that are correlated with certain sets of genes in the environments the twin studies were conducted in will be correlated with those genes in other relevant environments because those genes govern average affect levels. In this paper, I will show that these assumptions are false and explain how to properly go about investigating the contingent causal links between genes and happiness. In the end, I argue that there is no good evidence that our genes delimit our hedonic potential and that the fact that this misinterpretation of the data has been widely disseminated is potentially harmful to human well-being.     

Parenting and child behaviour problems: a longitudinal analysis of non‐shared environment

This study examined potential non‐shared environmental processes in middle childhood by estimating statistical associations between monozygotic (MZ) twin differences in externalizing and internalizing problems and positive social engagement, and differential maternal positivity and negativity, over 1 year. Seventy‐seven pairs of identical twins participated (M=6.08‐years old, 65% male) in two annual home visits. Observers' ratings and maternal reports were gathered. At both assessments, the twin who showed more conduct problems (maternal report and observers' ratings) and less positive social engagement (positive affect, responsiveness) received more maternal negativity and less maternal warmth (self‐reports and observers' ratings), relative to his or her genetically identical co‐twin. The same patterns held over time, for the associations between change in differential MZ twin conduct problems and social engagement and change in differential maternal behaviour. Effects for child internalizing problems were not consistent within or across raters. Overall, these results indicated that differential maternal warmth and negativity—self‐perceived and observed by others—are important aspects of sibling differentiation for both problematic and adaptive behaviours during middle childhood. Copyright © 2009 John Wiley & Sons, Ltd.     

Multiple regression analysis of twin and sibling data

DeFries and Fulker (1985) employed a multiple regression analysis of twin data in which a cotwin's score is predicted from the proband's score, the coefficient of relationship (R = 1.0 for monozygotic twins and 0.5 for dizygotic twins), and their interaction to yield direct estimates of heritability (h²) and the proportion of variance due to common or shared environmental influences (c²). The purpose of the report is to extend this model to allow for the analysis of data from more than two familial relationships simultaneously. Data from identical twins, fraternal twins, and siblings pairs, in which one member of each pair was selected based on low reading performance, were analyzed. To analyze the data simultaneously, additional coefficients were added to the regression equation in order to differentiate shared environmental influences of twins and siblings. When the model was applied to the combined cognitive data, significant estimates for heritability were obtained in three of the six tests analyzed. The results also indicated a lack of justification for the separate twin and sibling environmental parameters, and a discussion of more parsimonious models is included.     

In situ observation of twin boundary migration in copper with nanoscale twins during tensile deformation

In situ transmission electron microscopy observations are reported of the dynamic process of twin boundary migration in Cu with nanoscale twins. The experiment provides the first direct evidence of twin boundary migration via Shockley partial dislocation emission from the twin boundary/grain boundary intersections, and reveals that such migration is the dominant deformation mechanism in the initial stage of plastic straining. The behaviour is discussed in comparison with molecular dynamics simulations and in terms of the unique characteristics of the sample microstructure.     

The effects of shared environment on adult intelligence: A critical review of adoption, twin, and MZA studies

There has been a vigorous debate for decades concerning the heritability of intelligence. In recent years, the debate has been focused on whether the components of IQ variability change with age and on separating environmental effects into shared and unshared components. Citing evidence from adoption studies, studies comparing identical and fraternal twins, and studies of identical twins raised apart, some prominent psychologists have concluded that the shared environment has a significant effect on the intelligence of children but little or no effect on the intelligence of adults. In this article, the evidence from such studies is reviewed. The article reaches the conclusion that while there is some evidence from adoption studies supporting the claim that shared environment has little or no effect on adult intelligence, that evidence is inconclusive and is inconsistent with evidence from twin studies and from studies of identical twins reared apart. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Pain-provoking behaviour as a driven reaction to psychological distress: the bio-psycho-social neurotic loop model

In clinical psychology and psychiatry, different complaints are generally perceived as problems with a different ethiology, despite the fact that it can also be hypothesised that they share the same dimension. We believe that many clinicians share the heuristic insight that a pure categorical approach to psychopathology underscores the potential value of a dimensional approach. The neurotic loop model attempts to explain how pain-provoking behaviour develops and how it persists, increases and expands in spite of continuing negative consequences. Pain-producing behaviour is postulated as a neurobiologically rooted reaction to the activation of distressing representations. In biologically vulnerable people, pain-provoking behaviour channels stress resulting from disturbing emotions. This behaviour will initially be very rudimentary and may become more sophisticated according to the operant history. In this way, the neurotic loop model postulates pain-provoking behaviour on a continuum from simple muscle contractions to more complex self-injurious behaviour. Negative consequences of pain-provoking behaviour will reactivate the distressing representations, resulting in a driven vicious circle: the neurotic loop. The model integrates the most influential models of learning theory with neurobiology and developmental aspects. The neurotic loop model attempts to provide insight into the dynamics of dysfunctional pain-provoking behaviour and yields implications for therapy.     

ABILITIES OF TWINS

The investigation concerns two main problems: (1) the distribution of ability test scores among twins in comparison with single-borns; and (2) intra-pair similarity among the various twin categories. More than 1000 individual twins were compared with 2700 single-borns on 21 test variables covering Swedish, Mathematics and English. The twin group consistently showed a lower average than the single-borns due to a displacement of the test score distribution as a whole among twins. The intra-pair similarities, expressed as intra-class correlations, run between 0.85 and 0.90 for identical pairs, whereas like-sex fraternals showed coefficients about 0.60. The unlike-sex fraternals had coefficients ten to twenty hundredths lower than those of the like-sex pairs.     

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.     

A Comparison of Selected Behaviors of Identical Twins,One Blind from Birth

The purpose of this study was to compare differences in mental-emotional, fine perceptual-motor and gross motor behaviors of a set of 13 year-old identical twins, one of whom was blind from birth. In general, the blind twin was superior in performances where total body involvements were minimal and fine manipulative movements of the hands and/or fingers were stressed. The blind twin also appeared to be more well-adjusted emotionally than the sighted twin.     

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.     

How do friends influence smoking uptake? Findings from qualitative interviews with identical twins

The smoking behavior of friends is a major risk factor for adolescent smoking uptake. To explore the social context of smoking experimentation and consolidation with a particular focus on friends, the authors interviewed both members of 14 young adult identical twin pairs who were discordant for smoking. The different smoking status of twins was connected to their different friendship groups and development of different identities. Smoking respondents gravitated to the behaviors and images of the peer group who smoked. Many nonsmokers felt strong pressure from their peers not to smoke and spoke about how the images conveyed by smoking were inconsistent with their peer group's image. Adolescents and young adults are aware of the messages that smoking can convey to others and exploit these images to construct a social identity.     

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.