Third, prior studies have produced contradictory evidence about the presence of shared environmental effects for AUDs. This is partly a problem of statistical power as in the presence of substantial heritable influences, quite large twin samples are needed to detect reliably modest shared environmental effects (Neale et al. 1994). Our meta-analysis suggests the presence of modest, though statistically significant, shared environmental influence on risk for AUDs. Our twin findings predict that the correlation in liability for AUDs in siblings should be equal to 0.49/2 + 0.10 = 0.35 of which approximately one third comes from shared environmental and two-thirds from genetic sources of resemblance.
