Data were collected for the Swedish Screening Across the Lifespan Twin (SALT) study, which is based on the Swedish Twin Registry (Lichtenstein et al. 2002; Pedersen et al. 2002) and has been described elsewhere (Kendler et al. 2006) (see also Supplementary Material, available online). The present analyses are based on same-sex twins of known zygosity (n = 27993) (Table 1). Zygosity was assigned as described elsewhere (Lichtenstein et al. 2002). Informed consent was verbally obtained prior to interviews and confirmed in a post-interview letter. The Swedish Data Inspection Authority, the Ethics Committee of the Karolinska Institute and the Institutional Review Board of the University of Southern California approved this project.
