Information on twin zygosity was ascertained via a self-report questionnaire assessing twins’ physical similarity and the frequency with which the twins were mistaken for each other as children. The questionnaire method of determining twin zygosity has been found to be 95% accurate (Eaves, Eysenck, & Martin, 1989). We were able to assess the accuracy of our zygosity assignments for the current study due to the availability of DNA for 241 of the same-sex twin pairs. The error rate for zygosity determination, based on 241 same-sex twin pairs in the current sample, was 2.5% (Slutske al., 2009).
