Autism spectrum disorder (ASD) has received intense scrutiny as a genetic condition, but there is also considerable reason to believe that environmental factors interact with genetic propensities, as Abel et al. (1) propose in this issue of the Journal. Identical twins concordant for ASD often have marked differences in IQ, language skill, and patterns of severity in ASD core features such as social deficits and repetitive behaviors, as do affected fraternal twins and other siblings (2). Environmental effects can occur in utero, in the air, and in the sociocultural opportunities afforded to different children. Yet these effects are difficult to study; consistent evidence is only beginning to emerge about their role or lack of role in ASD. Consider how difficult it has been to show that the most well-known environmental hypothesis—increased ASD risk associated with vaccines—is not supported. Given that many genetic aspects of brain development are expressed in utero, the fetal environment is of obvious interest (1, 3, 4). The Abel et al. study, as part of a series of studies using the Swedish Youth Cohort, also raises questions about broader sociocultural environmental effects on prevalence.