Zygosity was assigned based on responses from two questions: (Q1) During childhood, were you and your twin partner as alike as ‘two peas in a pod’ or no more alike than siblings in general? and (Q2) How often did strangers have difficulty distinguishing between you and your twin partner when you were children? Twins were classified as MZ if both twins within a pair responded ‘alike as two peas in a pod’ for Q1 and ‘almost always’ or ‘often’ for Q2. Twins were classified as DZ if both twins responded ‘not alike’ for Q1 and ‘seldom,’ ‘almost never’, or ‘never’ for Q2. All other twins were classified as ‘not determined.’ This algorithm was validated using 47 single nucleotide polymorphisms in 198 randomly selected twin pairs (Lichtenstein et al., 2002). Ninety-five percent were correctly classified; ten pairs (8 MZ and 2 DZ) were misclassified.
