Second, two studies appear to be outliers, producing results at variance with the general trend. The estimate of the shared environmental contribution to alcoholism risk from the Kaij (1960) study is much greater than in all other studies. The reasons for this are unknown, although it is possible that in the work by Kaij some registrations were accidentally overlooked. However, once one twin from a pair was identified with a registration, the records were searched more thoroughly to determine whether the co-twin also had been registered. Cadoret’s study of four adoption agencies (Cadoret 1994; Cadoret et al. 1995) has yielded a high estimate of the genetic contribution to variability in alcoholism risk, which does not differ significantly from a probability of 100 percent (i.e., complete heritability). Further inquiry is needed to determine the cause of this result.
