Wave 4 interviews. As all twins (N=3,787) were 18 years of age or older at the time of recruitment for Wave 4, sensitive questions regarding their illicit substance use was queried. Therefore, we limited the sample to MOAFTS participants who completed wave 4 interviews, but data from other waves (including the subsequent Wave 5, conducted from 2005 to 2008), which were available for over 95% of Wave 4 participants, were integrated as well. The Wave 4 sample consisted of 1,038 monozygotic (MZ) twin pairs, 735 dizygotic (DZ) twin pairs and 241 twins whose co-twins did not participate.
