BACKGROUND:Early childhood antisocial behaviour is a strong prognostic indicator for poor adult mental health. Thus, information about its etiology is needed. Genetic etiology is unknown because most research with young children focuses on environmental risk factors, and the few existing studies of young twins used only mothers' reports of behaviour, which may be biased. METHOD:We investigated genetic influences on antisocial behaviour in a representative-plus-high-risk sample of 1116 pairs of 5-year-old twins using data from four independent sources: mothers, teachers, examiner-observers previously unacquainted with the children, and the children themselves. RESULTS:Children's antisocial behaviour was reliably measured by all four informants; no bias was detected in mothers', teachers', examiners', or children's reports. Variation in antisocial behaviour that was agreed upon by all informants, and thus was pervasive across settings, was influenced by genetic factors (82%) and experiences specific to each child (18%). Variation in antisocial behaviour that was specific to each informant was meaningful variation, as it was also influenced by genetic factors (from 33% for the children's report to 71% for the teachers' report). CONCLUSIONS:This study and four others of very young twins show that genetic risks contribute strongly to population variation in antisocial behaviour that emerges in early childhood. In contrast, genetic risk is known to be relatively modest for adolescent antisocial behaviour, suggesting that the early-childhood form has a distinct etiology, particularly if it is pervasive across situations.