Importance Twin-family studies have shown that parent-child resemblance on substance
use disorders and antisocial behavior can be accounted for by the transmission of a general
liability to a spectrum of externalizing disorders. Most studies, however, include only
biological parents and offspring, which confound genetic and environmental transmission
effects. Objective To examine the familial transmission of externalizing disorders among
both adoptive (genetically unrelated) and biological relatives to better distinguish genetic …