Background. Previous studies on male patients with sex chromosome abnormalities (SCA), namely XYY and XXY, suggest that such patients commit criminal acts more frequently than expected. Most of these studies are affected by ascertainment bias.

Methods. Using a population-based sample of men with SCA, identified by screening 34380 infants at birth between 1967 and 1979, comparison between 16 XYY men, 13 XXY men and 45 controls were made in terms of frequency of antisocial personality disorder (APD) using the Schedule for Affective Disorders and Schizophrenia lifetime version. Rates of criminal convictions were examined in 17 XYY men, 17 XXY men and 60 controls.

Results. XYY males showed a significantly higher frequency of antisocial behaviour in adolescence and adulthood and of criminal convictions than the controls, but multiple regression analysis showed this to be mediated mainly through lowered intelligence. Property offences constituted the majority of offences in all groups. The XXY men did not show an increased rate of criminal convictions. It is possible that this apparently negative result relates to the relatively small numbers of cases and hence low power of this study.

Conclusions. The findings of this study carry the advantage of not being affected by ascertainment bias and the disadvantage of having low power. It provides evidence for a slightly increased liability to antisocial behaviour in XYY men.