Hyperactivity has consistently been shown to be familial. Until recently however, due to a lack of systematic twin evidence, it has remained uncertain to what extent familial transmission can be explained by genetic factors. We used a systematically ascertained population-based sample of twin pairs aged between 8 and 16 years old to explore the role of genetic influences on maternally rated hyperactivity scores. Hyperactivity scores were found to be substantially heritable. The data were best explained by a model which incorporated sibling competitive effects as well as additive genetic factors. These findings suggest not only that hyperactivity scores are influenced by genetic factors but that sibling interaction effects are also of importance.