Behavior Genetics: What's New? What's Next?

What's new in behavior genetics? With widespread acceptance that nearly all behavioral variation reflects some genetic influence, current studies are investigating developmental changes in the nature and magnitude of genetic and environmental effects, the extent to which different behaviors are influenced by common genes, and different forms of gene-environment correlation and interaction. New designs, focused on assessment of unrelated children in the same households or neighborhood environments, and use of measured environmental variables within genetically informative designs, are yielding more incisive evidence of common environmental effects on behavior. What will be next? Behavior genetic techniques and analyses will be used to inform efforts to find genes altering susceptibility for disorder and dispositional genes affecting behavioral variation. The developing integration of behavioral and molecular genetics will identify genes influencing specific behavioral variation and enhance understanding of how they do so. Psychologists will play a pivotal role in communicating that understanding to the public and in facilitating consideration of the inevitable ethical issues then to be confronted.     

发展行为遗传学简介

发展行为遗传学是发展心理学与行为遗传学的交叉学科, 旨在探明遗传与环境对人类心理与行为发展是否存在影响, 如何产生影响, 以及该影响及其作用机制是否随年龄增长而发展变化的问题。该学科与行为遗传学在研究对象、设计和内容等方面存在不同; 开展发展行为遗传学研究需要综合运用心理测量法和行为遗传学研究方法; 未来研究应拓宽和深化候选基因与行为关联性的考察, 并着力探析基因与环境的相互作用机制。     

Effects of the family environment: gene-environment interaction and passive gene-environment correlation

The classical twin study provides a useful resource for testing hypotheses about how the family environment influences children's development, including how genes can influence sensitivity to environmental effects. However, existing statistical models do not account for the possibility that children can inherit exposure to family environments (i.e., passive gene-environment correlation). The authors introduce a method to simultaneously estimate the effects of passive gene- environment correlation and gene- environment interaction and use it to investigate the relationship between chaos in the home and verbal ability in a large sample of 4-year-old twins.     

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.     

Genetic analyses of focal aspects of infant temperament.

The authors explored the genetic and environmental underpinnings of individual differences in temperament with a sample of 604 3- to 16-month-old infant twins and their parents. Mothers completed Rothbart's Infant Behavior Questionnaire (IBQ), and a subsample of 140 9-month-old twins participated in behavioral assessment of temperament in the laboratory as well. For IBQ Smiling and Laughter and Duration of Orienting, both additive genetic and shared environmental effects were needed to best represent the data. Shared environmental effects fully accounted for cotwin similarity for IBQ Soothability, and conversely, additive genetic effects fully accounted for cotwin similarity for the IBQ Distress to Limitations, Distress to Novelty, and Activity Level scales. With the subsample, the authors fit a multivariate model to mother report, father report, and lab measures of stranger distress and found that genetic influences were most important for the covariation among these measures.     

Gene-environment interplay in the association between pubertal timing and delinquency in adolescent girls

Early pubertal timing places girls at elevated risk for a breadth of negative outcomes, including involvement in delinquent behavior. While previous developmental research has emphasized the unique social challenges faced by early maturing girls, this relation is complicated by genetic influences for both delinquent behavior and pubertal timing, which are seldom controlled for in existing research. The current study uses genetically informed data on 924 female-female twin and sibling pairs drawn from the National Longitudinal Study of Adolescent Health to (1) disentangle biological versus environmental mechanisms for the effects of early pubertal timing and (2) test for gene-environment interactions. Results indicate that early pubertal timing influences girls' delinquency through a complex interplay between biological risk and environmental experiences. Genes related to earlier age at menarche and higher perceived development significantly predict increased involvement in both nonviolent and violent delinquency. Moreover, after accounting for this genetic association between pubertal timing and delinquency, the impact of nonshared environmental influences on delinquency are significantly moderated by pubertal timing, such that the nonshared environment is most important among early maturing girls. This interaction effect is particularly evident for nonviolent delinquency. Overall, results suggest early maturing girls are vulnerable to an interaction between genetic and environmental risks for delinquent behavior.     

The new look of behavioral genetics in developmental psychopathology: gene-environment interplay in antisocial behaviors

This article reviews behavioral-genetic research to show how it can help address questions of causation in developmental psychopathology. The article focuses on studies of antisocial behavior, because these have been leading the way in investigating environmental as well as genetic influences on psychopathology. First, the article illustrates how behavioral-genetic methods are being newly applied to detect the best candidates for genuine environmental causes among the many risk factors for antisocial behavior. Second, the article examines findings of interaction between genes and environments (G x E) associated with antisocial behavior, outlining steps for testing hypotheses of measured G x E. Third, the article envisages future work on gene-environment interplay, arguing that it is an interesting and profitable way forward for psychopathology research.     

The persistence of dichotomies in the study of behavioral development

The inadequacies of dichotomous views of behavioral development that oppose learned and innate behavior, or genetic and environmental determinants of behavior, have long been recognized. However, they continue to exert a powerful influence on current thinking about development, often by way of metaphors that simply recast these old ideas in a more modern technical vocabulary. The idea that the information for behavior can be attributed to either genetic or environmental sources was originated by Lorenz and provides the basis for many current dichotomous accounts of behavioral development. Lorenz's “sources of information” metaphor for development is fundamentally flawed, however, as are those more recent accounts that are based on it. The alternative interactionist account of development, most clearly articulated by Lehrman, is a far more powerful and coherent theoretical framework for development, but it has not been broadly assimilated into psychology and continues to be widely misunderstood. In particular, the interactionist account does not involve a radical environmentalism, does not attribute all behavior to the effects of learning, and does not interpret development as a gene-environment interaction. The attractive simplicity of dichotomous thinking encourages its continued application to the study of development despite the fact that it is clearly inadequate to the complexities of developmental analysis.     

Higher perceived life control decreases genetic variance in physical health: evidence from a national twin study

Physical health has been linked consistently with both income and sense of control, and the authors previously demonstrated that genetic variation in physical health measures decreased with increasing income (see W. Johnson & R. F. Krueger, 2004). Using a nationwide sample of 719 twin pairs from the MacArthur Foundation National Survey of Midlife Development in the United States, in this study the authors show that genetic variation in physical health measures (number of chronic illnesses and body mass index) also decreases with increasing sense of control. The authors integrate findings for income and control by demonstrating an interaction between genetic influences on sense of control and income in explaining physical health. They hypothesize that the mechanism underlying the interaction is the known biological relationship between metabolic efficiency and adaptation to stressful environments.     

The Association between Positive Parenting and Externalizing Behavior

The present study examined the role of positive parenting on externalizing behaviors in a longitudinal, genetically informative sample. It often is assumed that positive parenting prevents behavior problems in children via an environmentally mediated process. Alternatively, the association may be due to either an evocative gene-environment correlation, in which parents react to children's genetically-influenced behavior in a positive way, or a passive gene-environment correlation, where parents passively transmit a risk environment and the genetic risk factor for the behavioral outcome to their children. The present study estimated the contribution of these processes in the association between positive parenting and children's externalizing behavior. Positive parenting was assessed via observations at ages 7, 9, 14, 24, and 36 months and externalizing behaviors were assessed through parent report at ages 4, 5, 7, 9, 10, 11, and 12 years. The significant association between positive parenting and externalizing behavior was negative, with children of mothers who showed significantly more positive parenting during toddlerhood having lower levels of externalizing behavior in childhood; however, there was not adequate power to distinguish whether this covariation was due to genetic, shared environmental, or nonshared environmental influences.     

Youth Appraisals of Inter-parental Conflict and Genetic and Environmental Contributions to Attention-Deficit Hyperactivity Disorder: Examination of GxE Effects in a Twin Sample

Identification of gene x environment interactions (GxE) for attention-deficit hyperactivity disorder (ADHD) is a crucial component to understanding the mechanisms underpinning the disorder, as prior work indicates large genetic influences and numerous environmental risk factors. Building on prior research, children’s appraisals of self-blame were examined as a psychosocial moderator of latent etiological influences on ADHD via biometric twin models, which provide an omnibus test of GxE while managing the potential confound of gene-environment correlation. Participants were 246 twin pairs (total n = 492) ages 6–16 years. ADHD behaviors were assessed via mother report on the Child Behavior Checklist. To assess level of self-blame, each twin completed the Children’s Perception of Inter-parental Conflict scale. Two biometric GxE models were fit to the data. The first model revealed a significant decrease in genetic effects and a significant increase in unique environmental influences on ADHD with increasing levels of self-blame. These results generally persisted even after controlling for confounding effects due to gene-environment correlation in the second model. Results suggest that appraisals of self-blame in relation to inter-parental conflict may act as a key moderator of etiological contributions to ADHD.     

Sources of exposure to smoking and drinking friends among adolescents: a behavioral-genetic evaluation

Substance-using friends expose adolescents to models of, and opportunities for, substance use that may lead to its initiation or reinforce existing use. Using genetically informative data from the National Longitudinal Study of Adolescent Health (P. S. Bearman, J. Jones, & J. R. Udry, 1998), the authors examined whether adolescents' exposure to friends' tobacco smoking and alcohol drinking was better explained by family-social or genetic influences. To conduct analyses, the authors constructed substance use exposure scores for adolescent siblings from the responses of siblings' nominated friends to self-reported smoking and drinking items. Using behavioral-genetic analyses of these substance use exposure scores, the authors estimated that 64% of the variance in adolescents' exposure to friends who smoke and drink could be explained by genetic influences, whereas shared environmental influences were zero. These results provide evidence of active, evocative, or both types of gene-environment correlations. Genetic factors can influence the formation of friendships with substance-using peers, thereby contributing to adolescents' exposure to substance use behaviors.     

It does take a village: nonfamilial environments and children's behavior

Abstract - Family characteristics influence children's behavioral development, but so do variations in schools, neighborhoods, and communities. We documented extrafamilial environmental effects by fitting maximum likelihood models to questionnaire data collected from double dyads consisting of twins and their classmate controls. The classmate controls in each double dyad were genetic strangers living in separate households, but they shared school, neighborhood, and community environments with their yoked twin pair and with one another. At ages 11 to 12, the control classmates showed significant similarities in religious practices and smoking and drinking patterns, demonstrating that environmental influences outside the family affect children's behavioral development. Familial self-selection of residential neighborhoods may have contributed to these results, but direct effects of variation across communities, neighborhoods, and schools cannot be dismissed, and such effects warrant further study.     

Genetic and environmental influences on behavior: capturing all the interplay

Basic quantitative genetic models of human behavioral variation have made clear that individual differences in behavior cannot be understood without acknowledging the importance of genetic influences. Yet these basic models estimate average, population-level genetic and environmental influences, obscuring differences that might exist within the population and masking systematic transactions between specific genetic and environmental influences. This article discusses a newer, more sophisticated and powerful quantitative genetic model that articulates these transactions. Results from this model highlight the ways in which the gene- environment interaction and correlation are intertwined. They can be used to integrate findings from quantitative and molecular genetic studies and to understand the roles of genetic influences and social forces in manifested behaviors, even when DNA sequence variation is not relevant.     

Interactions between early parenting and a polymorphism of the child's dopamine transporter gene in predicting future child conduct disorder symptoms

Mounting evidence suggests that genetic risks for mental disorders often interact with the social environment, but most studies still ignore environmental moderation of genetic influences. The authors tested interactions between maternal parenting and the variable number tandem repeat (VNTR) polymorphism in the 3' untranslated region of the dopamine transporter gene in the child to increase understanding of gene-environment interactions involving early parenting. Participants were part of a 9-year longitudinal study of 4- to 6-year-old children who met criteria for attention-deficit/hyperactivity disorder (ADHD) and demographically matched controls. Maternal parenting was observed during standard mother-child interactions in Wave 1. The child's conduct disorder (CD) symptoms 5-8 years later were measured using separate structured diagnostic interviews of the mother and youth. Controlling for ADHD symptoms and child disruptive behavior during the mother-child interaction, there was a significant inverse relation between levels of both positive and negative parenting at 4-6 years and the number of later CD symptoms, but primarily among children with 2 copies of the 9-repeat allele of the VNTR. The significant interaction with negative parenting was replicated in parent and youth reports of CD symptoms separately.     

The etiology of behavioral problems and competencies in very young twins

Although genetic and environmental influences on behavior problems in middle childhood and adolescence have been well-studied, little is known about the etiology of behavior problems in very early childhood. The present study explores genetic and environmental contributions to individual differences in behavior problems and competences in an infant-toddler sample of twins. There were 1,950 twin pairs (mean age=23.8 months) who were rated by parents on the Infant-Toddler Social and Emotional Assessment. All four domains (Externalizing, Internalizing, Dysregulation, Competence) and 20 subscales-indices on the Infant-Toddler Social and Emotional Assessment displayed significant heritability. There were also substantial shared environmental influences operating on most of the domains and subscales. Compared with behavior problems, behavioral competencies were less heritable and more influenced by shared environments.     

As the Twig Is Bent? The Myth of Child-Rearing Influences on Personality Development

Social scientists and laypersons alike believe that child-rearing styles and family environments are formative of personality traits such as shyness and activity level, intellectual traits such as IQ, and psychopathology. After 100 years of behavioral genetic research, however, including new studies of twins raised apart and together, little factual support for this “common sense” proposition can be found. Nonintellectual traits seem to be determined instead by genetic influences and by relatively specific environmental influences, most of which are not particularly tied to the family or parental treatments. Intellectual traits show modest family environmental influence, but that influence may diminish in importance after childhood. I conclude that parents should be given less credit for children who turn out well and should take less blame for children who turn out poorly. Counselors who blame parents less for children's behavioral outcomes—especially extreme traits and psychopathology—may find those parents to be more willing helpers in those intervention processes that have been shown to work.     

Could stress cause psychosis in individuals vulnerable to schizophrenia?

It has long been considered that psychosocial stress plays a role in the expression of symptoms in schizophrenia (SZ), as it interacts with latent neural vulnerability that stems from genetic liability and early environmental insult. Advances in the understanding of the neurobiology of the stress cascade in both animal and human studies lead to a plausible model by which this interaction may occur: through neurotoxic effects on the hippocampus that may involve synaptic remodeling. Of late, the neurodevelopmental model of SZ etiology has been favored. But an elaboration of this schema that credits the impact of postnatal events and considers a role for neurodegenerative changes may be more plausible, given the evidence for gene-environment interaction in SZ expression and progressive structural changes observed with magnetic resonance imaging. Furthermore, new insights into nongliotic neurotoxic effects such as apoptosis, failure of neurogenesis, and changes in circuitry lead to an expansion of the time frame in which environmental effects may mediate expression of SZ symptoms.     

Beyond Heritability: Twin Studies in Behavioral Research

ABSTRACT— 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.     

Implications of sex differences in neonatal temperament for early risk and developmental/environmental interactions

Sex differences in neonatal temperament were examined in 30 pairs of full-term, opposite-sex twins. Behavior was assessed during feeding, during sleep, while awake for orienting responses, in response to a stressor, and for measures of spontaneous irritability and soothability. Composite and summary scales described irritability, resistance to soothing, reactivity, reinforcement value of the infant's behavior, activity during sleep, and activity while awake. Analyses of variance of paired comparisons performed separately for each behavioral area indicated that female neonates were more irritable and more difficult to soothe than male neonates. The results are discussed in terms of maturational effects on neonatal behavior, implications for infant-caregiver interaction, and their relation to environmental influences on development.