In contrast to adoptive parents and offspring, we detected significant similarity between adoptive siblings on the general externalizing liability, which was solely attributable to environmental influences. This was reflected in the results of the biometric factor model, as we detected significant shared environmental influences on the general externalizing factor, more so than in previous twin studies6,7. Previous MTFS studies also identified shared environmental influences on a general externalizing factor in childhood9 and middle adolescence20, but this is the first to do so in adulthood. Though the mechanism of these shared environmental effects is not yet known, it seems sibling rather than parent-child similarity indexes shared environmental risk for multiple externalizing disorders.
