Identifying the causal processes involved in theory of mind (ToM) development during childhood is an important goal for social neuroscience. This study aimed to investigate, for the first time, the extent to which individual differences in ToM are influenced by genes and environment in middle childhood, and to assess how ToM is linked to autistic-like behaviors and verbal ability. Over 600 9-year-old twin pairs from a subsample of the Twins Early Development Study were assessed on an advanced test of ToM and on verbal ability. Parents, teachers, and the children themselves provided ratings of the twins' autistic traits (social impairments, communication impairments, and restricted repetitive behaviors and interests), using an abbreviated version of the Childhood Asperger Syndrome Test. Autistic traits, particularly communication impairments, significantly predicted ToM performance. Verbal ability showed the strongest phenotypic association with ToM. Twin model-fitting was employed to investigate the causes of this association. Much of the variation in ToM ability and in verbal ability was explained by environmental influences, with modest heritabilities for each, but their association was almost fully explained by shared genetic effects. The possible neural basis underlying this association is discussed.