Why do individuals with more autistic traits experience social difficulties? Here we examined the hypothesis that these difficulties stem in part from a challenge in understanding social acting, the prosocial pretense that adults routinely produce to maintain positive relationships with their ingroup. In Study 1, we developed a self-administered test of social-acting understanding: participants read stories in which a character engaged in social acting and rated the appropriateness of the character’s response. Adults who scored 26 or higher on the Autism Spectrum Quotient (AQ) questionnaire gave significantly lower ratings than comparison participants (AQ < 26). Study 2 found that difficulty in understanding social acting, but not false beliefs, mediated the link between autistic traits and perceived ingroup relationships.