We estimated heritability from 220 twin pairs. Their zygosity was initially determined by a twin questionnaire adapted from Cohen et al.25, and confirmed using a 16 marker microsatellite panel following the protocol outlined by Yang et al.26 DNA extracted from saliva was analyzed for short tandem repeats using the AmpFLSTR® IdentifilerTM PCR Amplification kit (PE Applied Biosystems, Foster City, CA, USA), a panel consisting of 15 autosomal, codominant, unlinked loci, and the sex-determining marker, amelogenin amplified in a single PCR26. We classified twin pairs as monozygotic (MZ) if all 16 markers were identical between the pair; otherwise, they were classified as dizygotic (DZ)26. We had four sets of DZ triplets. We randomly selected two siblings from each triplet to be a DZ twin pair and excluded the other sibling. Our final twin sample included 60 MZ twin pairs (50% male) and 160 DZ twin pairs (60 male, 33 female, and 67 opposite-sex pairs). The mean age of the twins was 10.5 years (SD 2.6) and no individuals had a reported diagnosis of OCD.
