Objective: Children with autism spectrum disorders often experience severe anxiety and depression, yet the explanation for this association remains unclear. The authors examined the longitudinal relationship between autistic-like and internalizing traits across middle to late childhood in a population-representative twin sample. Method: Participants were approximately 6,000 twin pairs born in England and Wales from 1994 to 1996. Parental reports of autistic-like and internalizing traits were analyzed at ages 7 and 8 (timepoint 1) and again at age 12 (timepoint 2). The direction and etiology of the associations between these traits were examined within a cross-lagged design. Results: Findings revealed an asymmetric bidirectional association between autistic-like and internalizing traits over time. Autistic-like traits at age 7 made a modest but significant contribution to the presence of internalizing traits at age 12. Earlier internalizing traits also influenced the development of later autistic-like traits, although this association was approximately one-half the magnitude. While both traits were moderately to highly heritable, they were largely independent with regard to their genetic influences. Stronger associations were found between the modest shared environmental influences on each trait. Of note, it was autistic-like communication difficulties, rather than social deficits, that made a significant contribution to later internalizing traits. Conclusions: The association between autistic-like and internalizing traits was attributable to reciprocal processes occurring across childhood, suggesting that these traits may serve to exacerbate each other over time. Autistic-like communication difficulties had the most notable impact. This association must now be explored within samples of children with diagnosed autism spectrum disorders and internalizing disorders, since this may help to inform the best timing and targeting of clinical intervention.