We successfully interviewed 84% of the families in the recruitment pool to determine eligibility in the study and to collect basic demographic information. The eligibility interview could not be scheduled with some families before our target recruitment goals were met or the twins had become too old (8.6%), and some families refused the eligibility interview (7.6%). Sixty-seven twin pairs in the unscreened sample and 205 in the screened sample did not meet basic inclusion criteria (i.e., disability-free, family lived within a day's drive of our lab). An additional 930 twin pairs in the screened sample did not meet the pre-determined symptom threshold. In all, 663 twin pairs were eligible to participate in the ES study and were invited to do so (336 in the unscreened sample and 327 in the screened sample). Eligible families who did not participate included those that refused (17%) as well as those we were unable to schedule before our recruitment goals were met (7%). The ES refusal rate is virtually identical to that for the original MTFS cohort (Iacono et al., 1999). Participation rates did not vary by screening status.
