Creating a social world: a developmental twin study of peer-group deviance.

CONTEXT Peer-group deviance is strongly associated with externalizing behaviors. We have limited knowledge of the sources of individual differences in peer-group deviance. OBJECTIVE To clarify genetic and environmental contributions to peer-group deviance in twins from midchildhood through early adulthood. DESIGN Retrospective assessments using a life-history calendar. Analysis by biometric growth curves. SETTING General community. PARTICIPANTS Members of male-male pairs from the population-based Virginia Twin Registry personally interviewed in 1998-2004 (n = 1802). MAIN OUTCOME MEASURE Self-reported peer-group deviance at ages 8 to 11, 12 to 14, 15 to 17, 18 to 21, and 22 to 25 years. RESULTS Mean and variance of peer-group deviance increased substantially with age. Genetic effects on peer-group deviance showed a strong and steady increase over time. Family environment generally declined in importance over time. Individual-specific environmental influences on peer-group deviance levels were stable in the first 3 age periods and then increased as most twins left home. When standardized, the heritability of peer-group deviance is approximately 30% at ages 8 to 11 years and rises to approximately 50% across the last 3 time periods. Both genes and shared environment contributed to individual differences in the developmental trajectory of peer-group deviance. However, while the correlation between childhood peer-group deviance levels and the subsequent slope of peer-group deviance over time resulting from genetic factors was positive, the same relationship resulting from shared environmental factors was negative. CONCLUSIONS As male twins mature and create their own social worlds, genetic factors play an increasingly important role in their choice of peers, while shared environment becomes less influential. The individual-specific environment increases in importance when individuals leave home. Individuals who have deviant peers in childhood, as a result of genetic vs shared environmental influences, have distinct developmental trajectories. Understanding the risk factors for peer-group deviance will help clarify the etiology of a range of externalizing psychopathology.