Taylor MJ, Rosenqvist MA, Larsson H, et al. - Utilizing data from two nationwide Swedish twin cohorts, researchers ascertained if the relative importance of genetic and environmental factors with regard to autism spectrum disorder (ASD) and autistic traits has changed over a 16-year and 26-year period. There were 22,678 twin pairs (5,922 female same-sex pairs [26.1%], 5,563 male same-sex pairs [24.5%], and 11,193 opposite-sex pairs [49.4%]) with available data in one cohort and 15,280 pairs (4,880 female same-sex pairs [31.9%], 5,092 male same-sex pairs [33.3%], and 5,308 opposite-sex pairs [34.7%]) in the other. Over time, the changes in the genetic and environmental factors that underlie ASD and autistic traits is supported by weak evidence. A consistently greater role seems to be played by genetic factors vs environmental factors, meaning the the rise in ASD prevalence is unlikely to be explained by environmental factors.