Considering Interactions between Genes, Environments, Biology, and Social Context
Kristen Jacobson received her Ph.D. in Human Development and Family Studies from the Pennsylvania State University in 1999. She spent a year as a postdoctoral scholar in psychiatric genetics under the direction of Dr. Kenneth Kendler at the Virginia Institute for Psychiatric and Behavioral Genetics, where she later served as faculty from 2000-2005. Dr. Jacobson is currently an Assistant Professor of Psychiatry at the University of Chicago, and serves as the Associate Director for Twin Projects and the Associate Director of the Clinical Neuroscience and Psychopharmacology Research Unit. Dr. Jacobson is a collaborator on a number of twin studies of children, adolescents, and adults, and is currently conducting a multidisciplinary, multi-level study of adolescent development, From Neighborhoods to Neurons and Beyond, funded by an NIH New Innovator Award . She is editor of a special issue of Behavior Genetics entitled Pathways between Genes, Brain, and Behavior (expected publication January, 2010). New areas of research involve pilot studies of epigenetics in both mice and humans.
Bronfenbrenner’s bioecological model (Bronfenbrenner & Ceci, 1994) highlights the need to consider interactions between individual, family, peer, school, and community characteristics in understanding individual differences in human development. In order to obtain a complete understanding of the processes involved in individual differences, multidisciplinary studies that measure risk and protective factors at multiple levels of analysis are required. With recent advances in human molecular genetics, the need to integrate environmental measures into genomic studies is of even greater importance. While the mapping of the human genome and the corresponding availability of genome-wide association analysis (GWAS) techniques has led to a flurry of research activity trying to discover “genes for” particular disorders and traits, a significant body of research, both historic as well as quite recent, cautions that efforts to uncover specific genetic variants that ignore the effects of social and contextual environments in genetic studies of individual differences in human behavior and traits may be futile. This essay briefly reviews some of the most interesting work regarding the interplay of genes and environments on individual differences in human development.
Nature versus Nurture
For years, behavioral genetic studies using twin or adoptive samples have been considered the gold standard for assessing the joint effects of nature and nurture in accounting for individual differences in human behaviors and traits. Decades of behavioral genetic research have demonstrated the importance of genetically-influenced characteristics on individual differences in child, adolescent, and adult behaviors and traits. At the same time, behavioral genetic studies have revealed that generally over half of the variation in individual behaviors and traits is due to environmental factors, typically environmental factors that are unique across people within the same family or that have different effects on behavior (i.e., nonshared environmental influence).
Genetic influence has been found on “environmental” measures, suggesting the presence of gene à environment correlations. Gene à environment correlations arise because exposure to certain risk and protective environments is not random, but rather is influenced by inherited characteristics of the individual, and also because children “inherit” both genes and environments from their parents. The role of genes and environments in mediating pathways between risk and behavior is complex, however. For example, recent quasi-longitudinal work using twins to understand the relationship between peer group deviance and adolescent problem behavior found that while genetic factors accounted for most of the relationship between earlier problem behavior and later peer group deviance (consistent with genetic characteristics of an individual relating to peer selection), the relationship between prior peer group deviance and later problem behavior was largely environmentally mediated (consistent with peer influence effects; (Kendler, Jacobson, Myers, & Eaves, 2008).
Nature and Nurture
While the nature versus nurture debate may have attenuated in recent years with consensus from many fields regarding the importance of both genes and environments, other areas of research have further identified interactions between nature and nurture as important components of individual differences. A host of adoption studies in the 1980s and 1990s have shown that genetic liability to antisocial behavior (as indexed through biological parent psychopathology and substance abuse) is only associated with the development of adult criminality and aggression under adverse adoptive environmental conditions, indicating that neither nature nor nurture was sufficient in and of itself to cause pathology (Cadoret, Yates, Troughton, Woodworth, & Stewart, 1995; Cloninger & Gottesman, 1987).
Alternatively, gene X environment (gXe) interactions may be implicated when the relative importance of genetic influence on behaviors and traits as measured through standard twin designs varies across social and ecological context. For example, a study by Rowe, Almeida, and Jacobson (1999) integrated genetically-informative regression models within a hierarchical linear modeling design to show that levels of parental warmth, measured at the aggregate school level, moderated the heritability (i.e., proportion of individual differences due to genetic factors) of adolescent aggression. Heritabilities of delinquent behavior are increased among adolescents living in families with high rates of dysfunction (Button, Scourfield, Martin, Purcell, & McGuffin, 2005), while the heritability of adolescent smoking decreases with higher levels of parental monitoring (Dick et al., 2007). Family and personal religiosity has been shown to decrease the importance of genetic variance on adolescent substance use behaviors (Koopmans, Slutske, Heath, Neale, & Boomsma, 1999; Timberlake et al., 2006), and urban-rural differences in the heritability of adolescent alcohol use were found to be mediated by contextual factors such as alcohol sales and neighborhood migration (Dick, Rose, Viken, Kapiro, & Koskenvuo, 2001). These latter areas of research may be of particular importance in generalizing results from prior twin studies to minority individuals or individuals in socially and economically disadvantaged environments, as most large-scale twin registries are based on primarily middle-class, Caucasian or Asian samples.
More recently, attention has turned to using measured genotypes and measured environments to investigate ”classic” gXe interactions for a number of important behaviors. Caspi et al.(2002) have elucidated an important and highly replicated (Kim-Cohen et al., 2006) gXe interaction using measured genotype (MAO-A gene) and environmental risk (child abuse) variables, demonstrating that the relationship between child maltreatment and various indices of aggressive and antisocial behavior is attenuated among individuals with the high MAO-A activity genotype.
Another highly replicated interaction has been found between a serotonin transporter gene (5-HTTPLR) and stressful life events in predicting depression (Canli & Lesch, 2007). Further studies have found interactions between the 5-HTTPLR genotype and socioeconomic status (SES) for aggression in preadolescents (Nobile et al., 2007), between the 5-HTTPLR genotype and lab-induced stress for lab measures of aggression in adult males (Verona, Joiner, Johnson, & Bender, 2006) and between life stress and the 5-HTTPLR genotype for individual differences in amygdala activation (Canli et al., 2006). There is also emerging evidence for environmental modification of dopaminergic genes related to impulsivity and aggression, with studies finding significant interactions among the DRD4-7 repeat polymorphism and caregiver quality in predicting higher levels of aggression and impulsive traits in infants and preschoolers (Bakermans-Kranenburg & van Ijzendoorn, 2006; Sheese, Voelker, Rothbart, & Posner, 2007), and interactions between SES and the DRD4 gene for aggression in pre-adolescents (Nobile et al., 2007). Thus, genes implicated in multiple neurotransmitter pathways work in conjunction with a host of social and environmental experiences to alter individual differences across multiple behaviors and traits.
Additional Gene-Environment Interplay
While the above section concerns statistical interactions between genes and environments which may represent genetic sensitivity to environmental stressors, or, alternatively, environmental exacerbation of genetic effects, another potentially important avenue for research concerns the dynamic interplay between genes and environments, that is, genetic influence on environments and environmental influences on genes. By now, it is fairly common knowledge that when measures of family environment are treated as ‘phenotypes’ in traditional behavioral genetic models, significant genetic influences on these measures are often detected (Plomin & Bergeman, 1991). Decades of behavioral genetic studies have provided considerable evidence for significant genetic influence for measures such as various dimensions of parenting, indices of SES such as income and educational level, social support, and stressful life events (see Kendler & Baker  for a recent review). What has been slower to develop, however, is the notion that environmental influences and experiences can have profound effects on genetic influence. While the underlying DNA structure and sequence individuals are born with does not change over time, a newer area of research in epigenetics is beginning to identify factors that may alter gene expression and function across the lifespan.
Epigenetics, defined formally as changes in gene expression caused by mechanisms other than changes in the underlying DNA sequence, offers an exciting new frontier in the study of human psychiatric and medical diseases, and psychological behaviors and traits. Epigenetic mechanisms include DNA methylation and chromatin remodeling, the latter via post-translational modifications (e.g. methylation, acetylation, phosphorylation and ubiquitylation) to histone proteins which form the scaffold for the DNA helix. Although some epigenetic processes are essential to organism function (e.g., differentiation of cells in the developing embryo during morphogenesis), other epigenetic processes can have major adverse effects on health and behavioral outcomes. While some epigenetic changes only occur within the course of one individual organism's lifetime, animal models suggest that other epigenetic changes can be inherited from one generation to the next (see Champagne  for a review), contributing, in part, to the heritability of behavioral traits and psychiatric disease.
However, a growing field of research suggests that environmental experiences, particularly those related to stress, have the capacity to alter biological and genetic mechanisms associated with increased risk of problem behavior. Again, the notion that environmental experience can change biological processes has important historical precedence. Harlow’s seminal deprivation studies of non-human primates have shown that disruptions in early rearing environments have the capacity to disrupt psychobiological regulatory functions, leading to behavioral changes. Other important animal research has begun to identify the precise mechanisms by which social environmental factors can alter epigenetic programming. Relatively recent research using animal models offers an elegant demonstration of how early environmental stressors can alter neurobiological responsivity to future stressful conditioning (Meaney, 2001). Meaney’s model highlights how individual differences in maternal behaviors can cause regulatory changes in the corticotropin releasing hormone (CRH) system at the level of the central nucleus of the amygdala, and how these changes relate further to changes in adrenocortical and autonomic effects of later stressful events. Importantly, his work suggests that these effects can be altered through intervention (Weaver et al., 2005). Differences in early maternal care have also been associated with differences in methylation of the glucocorticoid receptor gene promoter in the hippocampus (Meaney & Szyf, 2005). Most critically, a recent comparison of post-mortem brain tissue from a sample of patients with a history of child abuse and/or neglect and who died by suicide indicated DNA hypermethylation of the rRNA promoter region in the hippocampus relative to controls who experienced sudden, accidental death (McGowan et al., 2008), supporting the hypothesis that epigenetic changes due to social and environmental experiences are related to behavioral traits.
Other studies of monozygotic twins have identified variations in DNA methylation levels in certain target gene promoter regions. Because identical twins share identical genomes and experience many of the same family environmental factors, this indicates that environmental experiences that are not shared among children in the same family have an important causal role in gene expression, and may further be related to behavioral differences among identical twin pairs. Importantly, within-pair differences in DNA methylation and histone acetylation patterns were increased in older twin pairs, especially those who had different lifestyles and had spent fewer years of their lives together, strongly supporting epigenetic processes as a part of nonshared environmental influence on individual differences (Fraga et al., 2005). This suggests that epigenetic processes represent a fundamental gene-environment interface in the development and ongoing plasticity of the human brain.
While there is no doubt that genetic studies of individual behaviors and traits will increase our understanding of both normal human variation and pathological disorders, there is increasing recognition that the interplay between genes and environments is remarkably complex. Not only are both genes and environments important for both normal and abnormal human development, but genes and environments operate interactively to produce both risk and resilience to specific behavioral and psychiatric disorders. More importantly, emerging lines of research from epigenetics suggest that not only can nature alter nurture, but nurture, in turn, has the power to modify nature. Thus, genomic studies that incorporate a range of social and environmental influences will further our understanding of the complex dance between nature and nurture in human development.
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