Generalization from twin studies relies on the assumption that liability to develop the disorder which is studied is influenced by similar factors in twins and singletons. The present study examines the impact of pre- and perinatal factors on parental reports of behaviour problems in 1832 twins, and compares level of attention problems, internalizing and externalizing behaviour in the twins with 723 children and adolescents from a general population sample. The modest influence of relative birth weight and birth order on level of attention problems in twins did not contribute to significant differences between twins and the general population sample. The levels of attention problems and externalizing behaviour was similar and there was a modest age dependent difference in internalizing behaviour. An increased variance was found for externalizing behaviour in twins.