Fourth, as typical for twin studies, 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. (This was not needed for full sibling pairs). As expected, DA was associated with a reduced probability of returning questionnaires so the rate of DA was lower in pairs with known zygosity. This is a form of “concordance-dependent” ascertainment where the probability of known zygosity will be lowest in pairs concordant for DA, intermediate in those discordant for DA and highest in those where neither twin has DA (Kendler and Eaves, 1989). Simulations suggest that with the level of differential ascertainment expected in our data given the observed prevalence differences, biases in parameter estimates are unlikely to be very large and most probably would result in underestimation of genetic and shared environmental effects and overestimation of the individual-specific environment (Kendler and Eaves, 1989).
