Abstract
The aim of the present study was to estimate the genetic, shared and nonshared environmental contributions to self-reported and parent-reported internalizing and externalizing problems in a follow-up study of intercountry adopted young adults. Young Adult Self-Report ratings were obtained from 1475 adoptees aged 22 - 32 years and Young Adult Behavior Checklist ratings from 1115 adoptive parents. For the genetic analyses, a subset of 143 adopted biologically related and 295 unrelated siblings was used. The data were subjected to model fitting decomposing three sources of variance: genetic factors (A) shared environment (C) and nonshared environment (E). Genetic factors were of more importance in both self-reported (A(2) = 54%, C-2 = 0, and E-2 = 46%) and parent-reported (A(2) = 76%, C-2 = 15% and E-2 = 9%) internalizing problems. Environmental factors were of more importance in both self-reported (A(2) = 33%, C-2 = 17% and E-2 = 50%) and parent-reported (A(2) = 28%, C-2 = 27% and E-2 = 45%) externalizing problems. This was in contrast with findings from the first and second assessments in the same sample during adolescence when genetic factors were more important in explaining externalizing problems compared with internalizing problems. Our results suggest a developmental reversal in genetic and environmental influences on behavior problems from early adolescence into adulthood, which could be related to different underlying developmental trajectories.
