Using as a criterion the requirement of at least one Temperance Board registration, the proportion of registered twins whose co-twins were also registered was significantly higher for the MZ than for the DZ pairs. Approximately 61 percent of MZ co-twins of twins with an alcohol problem and a significantly lower fraction (39 percent) of DZ co-twins of twins with an alcohol problem were registered. Based on national data, Kaij (1960) reported a lifetime alcoholism prevalence of 7.7 percent for males in the general population. Using this value gives a risk ratio of 9.1 for male MZ twins of registered co-twins and 6.2 for DZ co-twins. Again, these data are consistent with a genetic influence on alcoholism risk.
