For the twin studies, the analysis unit was the twin pairs. The outcome of interest was the correlation of OCS in monozygotic compared with dizygotic twins. We tested two hypotheses: (a) that the OCS correlation in monozygotic twins is equal to the OCS correlation in dizygotic twins; and (b) that the OCS correlation in monozygotic twins is the double of the OCS correlation in dizygotic twins. We conducted these analyses considering all studies, including only the children/adolescents and only adults.
