Twin pair zygosity was determined on the basis of responses to standard questionnaires about physical similarity and confusion of the twins by parents, teachers, and strangers: methods that have been found to give better than 95% agreement with results of genotyping. Information about shared early experiences (e.g., shared peers, being in the same classes at school) was also gathered to allow conservative adjustment for excess similarity of environmental experiences in monozygotic (MZ) compared to dizygotic (DZ) peers.
