This 20-year longitudinal study showed that the young adult offspring of teen mothers are at risk for a range of adverse outcomes including early school leaving, unemployment, early parenthood, and violent offending. We tested how much the effect of teen childbearing on offspring outcomes could be accounted for by social selection (in which a woman's characteristics that make her an inadequate parent also make her likely to bear children in her teens) versus social influence (in which the consequences of becoming a teen mother also bring harm to her children, apart from any characteristics of her own). The results provided support for both mechanisms. Across outcomes, maternal characteristics and family circumstances together accounted for approximately 39% of the effect of teen childbearing on offspring outcomes. Consistent with a social-selection hypothesis, maternal characteristics accounted for approximately 18% of the effect of teen childbearing on offspring outcomes; consistent with a social-influence hypothesis, family circumstances accounted for 21% of the teen childbearing effect after controlling for maternal characteristics. These results suggest that public policy initiatives should be targeted not only at delaying childbearing in the population but at supporting individual at-risk mothers and their children.