The adoption design has important limitations: adoption studies have typically relied on official records to characterize biological parent psychopathology (e.g., arrest, hospitalization) and are likely to identify only severe cases. Related to this, biological parents who give their children up for adoption are not representative of the general population of biological parents. Additionally, due to both adoption agency screening and self-selection by adoptive parents, the number of adopted individuals raised in high-risk environments is likely to be limited: compared with the general population, adoptive parents are likely to be older, more affluent and less likely to show high rates of psychopathology. Furthermore, while the link between biological parents and offspring are presumed to exclude shared environment, exposure to maternal interuterine environment as well as age at adoption can result in sharing of environmental factors between biological mothers and their offspring.23
