A sample of 6,257 adult Australian male and female twins that included monozygotic (MZ) and dizygotic (DZ) pairs as well as twins from incomplete pairs, were used. The full paired sample consisted of 494 MZ and 395 DZ same-sex male pairs and of 698 MZ and 513 DZ same-sex female pairs. Data on 661 DZ opposite sex pairs and 736 twins from pairs where a co-twin did not participate was also available – all twins were used for analyses. Twins were aged 24–36 (mean 30 [2.5] years) years at the time of interview, which were the source of data for the analyses presented here. All twins were born between 1964 and 1971 and were initially recruited through the Australian school systems and via mass media appeals. Parents initially registered the twins when they were children in 1980–1982 and the twins themselves were interviewed via telephone in 1996–2000, after informed consent was obtained from all participants, as approved by the Institutional Review Boards of the Washington University in St. Louis, USA and the Queensland Institute of Medical Research, Australia,. Further details
