Two types of behaviours shown in children--those reflecting social impairment and nonsocial obsessive repetitive behaviours--are central to defining and diagnosing autism spectrum disorders (ASDs). Parent and teacher data on social and nonsocial behaviours were obtained from a community sample of greater than 3000 7-year-old twin pairs. Social and nonsocial behaviours were only modestly correlated, and it was found that some individuals had extreme scores on either social or nonsocial scales but not both. Genetic model-fitting showed that social and nonsocial behaviours are both highly heritable, but their genetic overlap is modest, with most of the genetic influence being specific to either social or nonsocial behaviours. Considering these behaviours separately might help clarify gene-brain-behaviour pathways in future research.