PSYCH
...e review of the relevant behavioral genetic literature: "psychologically relevant environmental influences make children in a family different from, not similar to, one another"( Plomin & Daniels, 1987, p. 1 ). Although the absence of shared environmental influences may characterize many, perhaps even most, psychological traits, there are some psychological characteristics that do appear to be susceptible to shared environmental influence. Specifically, although shared environmental influences appear to be trivial or nonexistent for indicators of internalized feelings and dispositions, they appear to be at least modest for indicators of actual behavior. Thus, for example, the influence of shared environmental factors is near zero for dispositional measures of extraversion and anxiety ( Tellegen et al., 1988 ) but moderate for self-reports of delinquent behavior ( Gottesman & Goldsmith, 1995 ) and alcohol abuse ( McGue, 1994 ). These findings suggest that shared environmental influences, when they exist, act primarily by modulating the relationship between underlying dispositions and actual behavior. In the research presented here, we sought to further test this hypothesis by evaluating the degree of shared environmental influence on a battery of measures that included both dispositional and behavioral indicators. The magnitude of shared environmental influence is also likely to vary over the life course. Much of the evidence reviewed by Plomin and Daniels (1987) was based on samples of adults, and for adult samples there is indeed little evidence for shared environmental effects. A somewhat different picture emerges when reviewing evidence from samples of adolescents and children, however. With IQ, for example, there is evidence for substantial shared environmental influences in childhood or adolescence, accounting for approximately 25% to 30% of the total IQ variance, but little evidence for shared environmental influences in adulthood ( McGue, Bouchard, Iacono, & Lykken, 1993 ) It may be that for those cases in which shared environmental factors do contribute to sibling similarity, these contributions do not endure much past the age of common rearing. In the present study, we investigated shared environmental influences in a sample of adolescents during a stage in the life course when shared environmental influences might be expected to be greater than in adulthood. The behavioral genetic notion of genotype—environment correlation is fundamental to many current conceptualizations on the joint influence of genetic and environmental factors. Both Plomin, DeFries, and Loehlin (1977) and Scarr and McCartney (1983) have described three processes by which genetic and environmental effects can become correlated: passive, evocative, and active genotype—environment correlation. The possible existence of evocative (i.e., correlations that arise because genetically distinct individuals evoke different reactions from, for example, their parents, peers, and teachers) and active (i.e., correlations that arise because genetically distinct individuals will actively select different ex...