Measuring autistic traits in the general population has proven sensitive for examining cognition. The present study extended this to pro-social behaviour, investigating the influence of expectations to help others. A novel task describing characters in need of help was administered to students scoring high versus low on the Autism-Spectrum Quotient. Scenarios had two variants, describing either a "clear-cut" or "ambiguous" social rule. Participants with high versus low autistic traits were less pro-social and sympathetic overall towards the characters. The groups' ratings of characters' expectations were comparable, but those with high autistic traits provided more rule-based rationales in the clear-cut condition. This pattern of relatively intact knowledge in the context of reduced pro-social behaviour has implications for social skill training programmes.