Do polygenic indices capture "direct" effects on child externalizing behavior? Within-family analyses in two longitudinal birth cohorts.

Externalizing behaviors/disorders are important but difficult to predict and address. Twin models have suggested that externalizing behaviors are heritable (∼80%), but it has been difficult to measure genetic risk factors directly. Here, we go beyond heritability studies by quantifying genetic liability for externalizing behaviors using a polygenic index (PGI) and employing within-family comparisons to remove sources of environmental confounding typical of such polygenic predictors. In two longitudinal cohorts, we find that the PGI is associated with variation in externalizing behaviors within families, and the effect size is comparable to established risk factors for externalizing behaviors. Our results suggest that genetic variants associated with externalizing behaviors, unlike many other social-science phenotypes, primarily operate through direct genetic pathways.