Our study used linked data from multiple Swedish nationwide registries and healthcare data. Linking was achieved via the unique individual Swedish 10-digit personal ID number assigned at birth or immigration to all Swedish residents. Our databases utilized the following registers: the Multi-Generation Registry, providing information on family relations; the Swedish Twin Registry, providing self-reported measures of zygosity validated against biological markers that enable us to differentiate monozygotic and dizygotic twins with 95–99% accuracy (Lichtenstein et al., 2002); the total population registry and the population and housing censuses providing individual household data (individuals sharing the same dwelling are assigned a unique number).
