An experiment is described that tests whether people with autism understand some causes of 2 basic emotions (happiness and sadness). The causes of emotion tested were situations, desires, and beliefs. Results showed that, relative to normal and mentally handicapped subjects of an equivalent mental age, people with autism alone showed severe deficits in comprehension of emotion caused by beliefs. Their understanding of emotion caused by situations and desires was no different than the nonautistic mentally handicapped group. Thus, difficulties in understanding emotion by people with autism are most apparent when emotion interacts with false belief. The implications of these results for the affective and meta-representation theories of autism are discussed.