Subsequent studies using samples ascertained from birth records have confirmed, without exception, a higher risk to MZ compared with DZ twins of alcoholics, although this difference has not always been significant. Hrubec and Omenn (1981) identified alcoholism cases in a followup of a series of male same-sex twin pairs born between 1917 and 1927, identified originally from birth records, in which both twins engaged in military service during World War II. The researchers reviewed Veterans Administration (VA) medical records of approximately 13,486 male twin pairs, all of whom were age 50 at the time of the record review, to identify cases of alcoholism or alcoholic psychosis. Only 2.6 percent of MZ twins and 3.1 percent of DZ twins were reported as having any treatment history for alcohol problems. If one twin had a history of alcohol problems recorded, however, the probability that his or her twin also did was 26.3 percent for MZ twins (i.e., a risk ratio of 10.0) and 11.9 percent for DZ twins (i.e., a risk ratio of 3.8). These risk-ratio differences were highly statistically significant.
