Participants were women from monozygotic (MZ) and same-sex dizygotic (DZ) twin pairs who participated in the Swedish Twin study of Adults Genes and Environment (STAGE). STAGE, a population-based prospective study of Swedish twins born 1959–1985, assessed more than 30 different health and demographic topics (Furberg et al., 2008; Lichtenstein et al., 2006). Data were collected on-line in 2005; twins were between 20 and 47 years old. The Regional Ethics Committee of Karolinska Institutet and the University of North Carolina’s Biomedical Institutional Review Board approved this study.
