Fifth, the three externalizing syndromes shared genetic risk factors more strongly than shared environmental risk factors. For DA, CD and AUD in men, and DA and AUD in women, at least 75 % of the genetic risk came from the common factor. Only CB in women stood out from this pattern with more than 50 % of the genetic risk unique to that syndrome. By contrast, only for AUD in men, and DA and CB in women did a large majority of the shared environmental effects originate with the common factor. For DA and CB in males, and AUD in females a majority of the shared environmental influences were syndrome specific. These results suggest that the tendency for an individual with a high genetic liability to externalizing disorders to develop DA versus CD or AUD might be substantially influenced by shared environmental exposures. These results have implications for molecular genetic studies and would predict that many of the risk alleles found for individual externalizing disorders would be shared with a range of similar syndromes.
