Some of the earliest findings suggesting heritable influences on substance use came from adoption studies in which offspring outcomes were compared with analogous measures in the biological and adoptive parents. Strong associations between biological parent and offspring behavior are interpreted as suggesting heritable influences, while strong associations between adoptive parent and offspring behavior suggest the role of rearing environment. While the seminal Iowa21 and Swedish22 adoption studies established strong correlations between biological parents and offspring diagnoses of alcohol and drug use disorders, they were based on adult samples.
