Are twins' behavioural/emotional problems different from singletons'?

We compared twins with singletons in the National Epidemiological Child Psy chiatric Study, which included 122 twins and 5455 singletons, born in 1981 and selected at random. Behavioural and emotional symptoms were assessed in 1989 on the basis of questionnaires filled in by the parents (Rutter Paren t Questionnaire) (RA2), teachers (Rutter Teacher Questionnaire) (RB2) and t he children themselves (Children's Depression Inventory) (CDI). Parents' re ported proportions of probable behavioural/emotional disorders did not diff er between the twin and singleton girls, but among the twin boys there was a nonsignificant trend of being more often probably disturbed. Twins were r eported to be less disturbed than singletons according to the teachers' ass essments. No difference was found between twins and singletons in their sel f-reports. When analysing parents' reported values of various sum scores, t he twin boys obtained slightly higher scores than singletons, while twin gi rls scored significantly lower on total and emotional disturbances. Twin bo ys obtained lower mean scores than singletons for probable disorder in the teachers' evaluations, the differences arising mostly in the emotional area . The same type of trend, however nonsignificant, was found among the teach ers' evaluations of girls. No significant difference was found in the mean scores for hyperactivity. This large population-based sample suggests that twins may have a lower rate of behavioural problems in childhood than singl etons, a finding that has to be taken into account in behavioural genetic s tudies.