The twins were 47.0% female and ranged in age from 6 to 10 years, although a small handful (n = 14 pairs) had turned 11 years old by the time the family participated [mean age for full sample = 8.2 (s.d. = 1.46) years]. Zygosity was established using physical similarity questionnaires administered to the twins’ primary caregiver (Peeters et al. 1998). On average, the physical similarity questionnaires used by the MSUTR have accuracy rates of 95% or better.
