Twins born in Australia between 1964 and 1971 were recruited for the Australian Twin Registry through school systems and mass media appeals and registered by their parents from 1980 to 1982. Members of the twin registry were contacted as adults in 1989 by means of mailed questionnaires. They were subsequently administered diagnostic assessments via telephone between 1996 and 2000, when they were 24-36 years of age. (Participation in the telephone interview was not contingent on response to questionnaires mailed in 1989.) Interviews were conducted with 6,257 individuals (2,761 complete pairs and 735 singletons), 73% of the targeted sample. Informed consent was obtained prior to the start of interviews, as approved by the Human Research Protection Offices of Washington University and the Queensland Institute of Medical Research.
