Second, some twin studies suggest that shared environmental risk factors contribute to the familial aggregation of AUDs (McGue et al. 1992; Kendler et al. 1997) while others do not (Heath et al. 1997; Prescott et al. 1999). The magnitude of the shared environmental variance, however, is typically small (but see Kaij, 1960 for an exception). In the presence of substantial genetic influences, quite large samples of twins are needed to detect shared environmental effects (Neale et al. 1994). A large meta-analysis would be much better powered to identify shared environmental effects for AUDs than an individual study, if indeed they are present.
