The sample for the current study was comprised of members of the Australian Twin Registry. Twins born in Australia between 1964 and 1971 were recruited for the registry through school systems and mass media appeals. They were registered by their parents from 1980 to 1982 and initially contacted in 1989 through mailed questionnaires, then interviewed by telephone with standard psychiatric assessments between 1996 and 2000, when twins ranged in age from 24-36 years (mean = 29.9 years). Approximately 73% of the targeted sample was successfully recruited into the study. Interviews were conducted with 6,257 individuals, representing 2,761 complete twin pairs and 736 respondents whose co-twins did not take part in the study. Breakdown by sex and zygosity of complete pairs are as follows: 698 monozygotic (MZ) female, 494 MZ male, 513 dizygotic (DZ) female, 395 DZ male, and 661 DZ opposite sex twin pairs. Informed consent was obtained prior to the start of interviews, as approved by the Human Research Protection Offices of Washington University School of Medicine in the U.S. and the Queensland Institute of Medical Research in Australia.
