Eric Turkheimer

Socioeconomic Status Modifies Heritability of IQ in Young Children

Scores on the Wechsler Intelligence Scale for Children were analyzed in a sample of 7-year-old twins from the National Collaborative Perinatal Project. A substantial proportion of the twins were raised in families living near or below the poverty level. Biometric analyses were conducted using models allowing for components attributable to the additive effects of genotype, shared environment, and non-shared environment to interact with socioeconomic status (SES) measured as a continuous variable. Results demonstrate that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.

Three Laws of Behavior Genetics and What They Mean

Behavior genetics has demonstrated that genetic variance is an important component of variation for all behavioral outcomes, but variation among families is not. These results have led some critics of behavior genetics to conclude that heritability is so ubiquitous as to have few consequences for scientific understanding of development, while some behavior genetic partisans have concluded that family environment is not an important cause of developmental outcomes. Both views are incorrect. Genotype is in fact a more systematic source of variability than environment, but for reasons that are methodological rather than substantive. Development is fundamentally nonlinear, interactive, and difficult to control experimentally. Twin studies offer a useful methodological shortcut, but do not show that genes are more fundamental than environments.

The role of the children of twins design in elucidating causal relations between parent characteristics and child outcomes.

BACKGROUND Determination of causal connections between parental measures and child outcomes using typical samples is limited by the inability to account for all confounds, both environmental and genetic. This paper discusses the strength of the Children of Twins (COT) design to highlight the role of specific environments. METHODS A new analytical model is presented which helps differentiate and quantify the environmental and genetic processes underlying associations between family-level risk factors and child adjustment. In order to illustrate the COT design, the relation between smoking during pregnancy and child birth weight (BW) is examined in a sample of female twins and their children from Norway and the United States. RESULTS The results illustrate that smoking during pregnancy is influenced by genetic factors. However, the Children of Twins model supports the claim that smoking during pregnancy has a direct environmental influence on BW and that genetic and shared environmental confounds cannot account for the association. CONCLUSIONS An assessment of the strengths and limitations of the Children of Twins design and a comparison with other research strategies suggest that the design plays a unique role in the study of developmental psychology and psychopathology. Finally, the authors describe how methodological advances and future applications of the design will provide additional insight into the causal processes underlying childrens adjustment to environmental stimuli.

Critical Need for Family-Based, Quasi-Experimental Designs in Integrating Genetic and Social Science Research

Researchers have identified environmental risks that predict subsequent psychological and medical problems. Based on these correlational findings, researchers have developed and tested complex developmental models and have examined biological moderating factors (e.g., gene-environment interactions). In this context, we stress the critical need for researchers to use family-based, quasi-experimental designs when trying to integrate genetic and social science research involving environmental variables because these designs rigorously examine causal inferences by testing competing hypotheses. We argue that sibling comparison, offspring of twins or siblings, in vitro fertilization designs, and other genetically informed approaches play a unique role in bridging gaps between basic biological and social science research. We use studies on maternal smoking during pregnancy to exemplify these principles.

Perceptions of people with personality disorders based on thin slices of behavior

Strangers made reliable judgments about personality traits after viewing one 30-s excerpt from interviews with anonymous target persons. Ratings were generated for 229 military recruits participating in a study of personality disorders. Approximately 28% of the recruits met DSM-IV criteria for a definite or probable personality disorder (PD). Several untrained undergraduate students rated each video clip with regard to the Big Five personality traits, physical attractiveness, and likeability. The students accurately rated people who exhibited features of paranoid, schizotypal, dependent, and avoidant PDs as being lower in extraversion. The raters also considered these people less likeable. Students accurately rated people who exhibited features of histrionic PD as being higher in extraversion and found them to be more likeable. Laypersons can make accurate judgments regarding some personality characteristics associated with personality disorders, even on the basis of minimal information. These perceptions may influence ways in which people respond to others with PDs.

Gene-Environment Correlation and Interaction in Peer Effects on Adolescent Alcohol and Tobacco Use

Peer relationships are commonly thought to be critical for adolescent socialization, including the development of negative health behaviors such as alcohol and tobacco use. The interplay between genetic liability and peer influences on the development of adolescent alcohol and tobacco use was examined using a nationally-representative sample of adolescent sibling pairs and their best friends. Genetic factors, some of them related to an adolescent’s own substance use and some of them independent of use, were associated with increased exposure to best friends with heavy substance use—a gene-environment correlation. Moreover, adolescents who were genetically liable to substance use were more vulnerable to the adverse influences of their best friends—a gene-environment interaction.

Emergence of a Gene × Socioeconomic Status Interaction on Infant Mental Ability Between 10 Months and 2 Years

Recent research in behavioral genetics has found evidence for a Gene × Environment interaction on cognitive ability: Individual differences in cognitive ability among children raised in socioeconomically advantaged homes are primarily due to genes, whereas environmental factors are more influential for children from disadvantaged homes. We investigated the developmental origins of this interaction in a sample of 750 pairs of twins measured on the Bayley Short Form test of infant mental ability, once at age 10 months and again at age 2 years. A Gene × Environment interaction was evident on the longitudinal change in mental ability over the study period. At age 10 months, genes accounted for negligible variation in mental ability across all levels of socioeconomic status (SES). However, genetic influences emerged over the course of development, with larger genetic influences emerging for infants raised in higher-SES homes. At age 2 years, genes accounted for nearly 50% of the variation in mental ability of children raised in high-SES homes, but genes continued to account for negligible variation in mental ability of children raised in low-SES homes.

A Genetically Informed Study of Marital Instability and Its Association With Offspring Psychopathology

Parental divorce is associated with a number of emotional and behavioral problems in young-adult offspring, but theoretical and empirical considerations suggest that the relation may be partially or fully accounted for by passive gene-environment correlation or environmental selection characteristics. The current study used the Children of Twins Design to explore whether shared environmental or genetic factors confound the relationship between parental marital instability and measures of psychopathology. Comparisons of the offspring of adult twins in Australia on 3 factors of abnormal behavior, including drug and alcohol, behavioral, and internalizing problems, suggest that environmental influences associated with divorce account for the higher rates of psychopathology. The results are consistent with a causal connection between marital instability and psychopathology in young-adult offspring.

Beyond Heritability Twin Studies in Behavioral Research

The heritability of human behavioral traits is now well established, due in large measure to classical twin studies. We see little need for further studies of the heritability of individual traits in behavioral science, but the twin study is far from having outlived its usefulness. The existence of pervasive familial influences on behavior means that selection bias is always a concern in any study of the causal effects of environmental circumstances. Twin samples continue to provide new opportunities to identify causal effects with appropriate genetic and shared environmental controls. We discuss environmental studies of discordant twin pairs and twin studies of genetic and environmental transactions in this context.

Traits Associated with Personality Disorders and Adjustment to Military Life: Predictive Validity of Self and Peer Reports

This study demonstrated that traits and features associated with several types of personality disorders are related to work performance and the ability to conform to requirements of military life. The Schedule for Nonadaptive and Adaptive Personality and a peer nomination procedure were used to assess traits associated with 10 types of personality disorders in a sample of 1,080 Air Force recruits (57% male) at the end of basic military training. Correlations between self-report and peer nomination scores for each set of traits ranged from 0.24 to 0.13, indicating only modest convergence. Follow-up data, collected 2 years later, indicated that several scores from the Schedule for Nonadaptive and Adaptive Personality and the peer nomination procedure were significantly related to early discharge from the military. Peers provide useful information that is not redundant with that provided by the individual. When possible, assessment of personality problems should be considered from information obtained from peers or other informants.