Do Polygenic Indices Capture “Direct” Effects on Child Externalizing Behavior Problems? Within-Family Analyses in Two Longitudinal Birth Cohorts

Failures of self-control can manifest as externalizing behaviors (e.g., aggression, rule-breaking) that have far-reaching negative consequences. Researchers have long been interested in measuring children’s genetic risk for externalizing behaviors to inform efforts at early identification and intervention. Drawing on data from the Environmental Risk Longitudinal Twin Study ( N = 862 twins) and the Millennium Cohort Study ( N = 2,824 parent–child trios), two longitudinal cohorts from the United Kingdom, we leveraged molecular genetic data and within-family designs to test for genetic associations with externalizing behavior that are not affected by common sources of environmental influence. We found that a polygenic index (PGI) calculated from genetic variants discovered in previous studies of self-controlled behavior in adults captures direct genetic effects on externalizing problems in children and adolescents when evaluated with rigorous within-family designs (βs = 0.13–0.19 across development). The PGI for externalizing behavior can usefully augment psychological studies of the development of self-control.