The only other study to use a sample of twin pairs identified from birth records (Kendler et al. 1992) included only female same-sex pairs.4 Alcoholism was assessed using standard diagnostic criteria as defined by the Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised (DSM–III–R) (American Psychiatric Association 1987). The relatively high lifetime prevalence rates of alcohol dependence (i.e., 10.2 percent for DZ twins and 8.1 percent for MZ twins) are consistent with what has been reported for a national probability sample in the National Co-morbidity Survey (Kessler et al. 1994).5 As in any survey of a general community sample, the majority of cases may be expected to be mild (for example, see Heath et al. 1994). Once again, however, rates of alcoholism were significantly elevated in both MZ and DZ twins of alcoholics (i.e., risk ratios of 3.9 and 2.4, respectively), and evidence showed a higher risk to MZ than to DZ twins (although this result did not reach statistical significance).6
