Previously, we showed that most parent-child similarity on substance use disorders and antisocial behavior can be attributed to the transmission of a general liability to a spectrum of externalizing disorders5,9. Here, we extended this finding by leveraging the strengths of the twin and adoptive family designs in a single analysis, showing a medium effect for this general transmission among biological parents and offspring, but a near zero effect for adoptive parents and offspring. Especially persuasive was that this pattern was found in families that included both biological and adoptive offspring. That is, even among children who shared the same rearing environment, biological offspring exhibited greater similarity than adoptive offspring to the same rearing parents. This indicates that—for the most part—the mechanism of parent to child transmission is not only general, but also genetic in nature. That is, rather than disorder-specific risk, what parents pass on to their (biological) offspring is a non-specific, genetic liability to multiple externalizing disorders.
