Structural equation modeling was used to estimate the general and specific transmission effects for externalizing disorders. The general liability to externalizing disorders was operationalized as a latent externalizing factor, defined by the covariance among symptoms of conduct disorder, adult antisocial behavior, and alcohol, nicotine, and drug dependence. General transmission effects were operationalized as the correlations among the latent externalizing phenotypes between parents and offspring (vertical transmission) and between siblings (horizontal transmission). We estimated disorder specific transmission effects by allowing the residual variance of the parent symptom counts (i.e., the variance of each disorder that was unrelated to the general externalizing liability) to covary with the residual variance of the corresponding offspring symptom counts (e.g., residual variance of mother alcohol dependence was allowed to correlate with the residual variance of offspring alcohol dependence). The same procedure was used to estimate specific transmission effects between siblings.
