BackgroundFor complex multifactorial diseases it seems likely that co-exposure to two risk factors will show a greater than additive relationship on disease risk.AimsTo test whether greater than additive relationships occur between risk factors for psychosis.MethodA cohort study of 50 053 Swedish conscripts. Data on IQ, cannabis use, psychiatric diagnoses, disturbed behaviour and social relations assessed at age 18 were linked to admissions with any non-affective psychoses over a 27-year follow-up period. Statistical interactions between risk factors were examined under both additive and multiplicative models.ResultsThere was some evidence of interaction for eight of the ten combinations of risk factors under additive models, but for only one combination under multiplicative models.ConclusionsMultiplicative models describe the joint effect of risk factors more adequately than additive ones do. However, the implications of finding interactions as observed here, or for most interactions reported to date, remain very limited.