The current report included 246 female–female and 238 male–male monozygotic (MZ) twin pairs, 181 female–female and 169 male–male dizygotic (DZ) twin pairs, and 392 opposite-sex twin pairs. Zygosity was determined based on computer algorithms of questionnaire responses created from analyses of twin pairs participating in the clinical study with known zygosity (Lichtenstein et al. 2007).
