However, almost half of the covariance is due to common shared- and twin- “environmental” factors. These factors may reflect passive gene-environment correlation, or children’s genetic factors being correlated with the environments they experience due to the fact that both are related to parents’ genetic factors. Alternatively, these factors may reflect true environmental effects. These processes cannot be distinguished in a child-based twin study. Therefore, while these results do not support environmental mediation, we cannot reject the hypothesis that the relationship between maltreatment and conduct problems is in part environmentally-mediated.
