Key PointsQuestionAre children with autism and autistic traits at greater risk of depression at age 18 years, and are genetic confounding and bullying important in these associations?FindingsAmong 6091 participants in this longitudinal study, children with autism and autistic traits had higher depressive symptom scores than the general population at age 10 years, remaining elevated in an upward trajectory until age 18 years. Social communication impairment was associated with depression at 18 years and was substantially mediated by bullying.MeaningSocial communication impairments are an important autistic trait in relation to depression; bullying may be an environmental intermediary and a target for interventions.

There may be independent effects of fetal maturation and fetal growth on school performance. Associations among matched siblings, although attenuated, remained consistent with causal effects of pre- and post-term birth on school performance.

Key Points Questions Are individuals with autism spectrum disorders more likely to have depression in adulthood than the general population, and do these risks have a familial basis and differ by coexisting intellectual disability? Findings In this Swedish population-based cohort study of 223 842 participants with a nested sibling comparison, individuals with autism spectrum disorders, especially those without intellectual disability, had a greater risk of a depression diagnosis in young adulthood than the general population and their nonautistic siblings. Meaning According to this study’s results, depression is overrepresented in autism spectrum disorders, and this higher risk may not be explained by shared familial liability; research identifying modifiable pathways may help develop preventive interventions.