Third, we were only able to include same-sex twins whose zygosity was known as a result of at least one member responding to a mailed questionnaire but this does not apply to opposite-sex pairs. As expected, individuals with externalizing syndromes had a reduced probability of returning questionnaires so the rate of DA, CB and AUD were lower in both males and females from same-sex than from opposite sex pairs. This is a form of “concordance-dependent” ascertainment where the probability of known zygosity will be lowest in pairs concordant for an externalizing syndrome, intermediate in those discordant and highest in those where neither twin has an externalizing syndrome. Simulations suggest that with the moderate level of differential ascertainment expected in our data given the observed prevalence differences, biases in parameter estimates are likely to be modest and result in slight under-estimations of a2 and c2 and overestimation of e2 (Kendler and Eaves 1989).
