Zygosity was, of course, known a priori in the opposite sex-pairs. As detailed elsewhere (Lichtenstein et al. 2002), zygosity in the same-sex pairs registry was assigned using standard self-report items obtained from mailed questionnaires which, when validated against biological markers, were 95–99 % accurate. As seen in Table 1, the prevalence of DA, CB and AUD are slightly lower in both males and females in monozygotic (MZ) and same sex dizygotic (DZ) pairs versus opposite sex twin pairs. This is almost surely because the former but not the latter were screened for level of cooperation. That is at least one member of the pair had to return a questionnaire to the twin registry to be able to assign a zygosity, and cooperation was lower in subjects with DA, CB and AUD.
