--- - |2 Deficits in reciprocal social behavior are a characterizing feature of autism spectrum disorder (ASD). Autism‐related variation in reciprocal social behavior (AVR) in the general population is continuously distributed and highly heritable—a function of additive genetic influences that overlap substantially with those which engender clinical autistic syndromes. This is the first long‐term prospective study of the stability of AVR from childhood through early adulthood, conducted via serial ratings using the Social Responsiveness Scale, in a cohort‐sequential study involving children with ASD, other psychiatric conditions, and their siblings (N = 602, ages = 2.5–29). AVR exhibits marked stability throughout childhood in individuals with and without ASD. - Child Development, EarlyView.