Posserud MB, Solberg BS, Engeland A, et al. - Adults had a lower male to female ratio (MFR) and male prevalence ratio (PR) than children. The findings show that the high male predominance reported in childhood/clinical studies of autism spectrum disorders (ASD) declines in adult samples, possibly due to non-etiological variables like later diagnosis in females, diagnostic biases, and diagnostic trends.

• The study comprised 1,701,206 adults and 804,146 children, including 8,995 (0.5%) adults and 8,056 (1.0%) children with ASD, 53,822 (3.2%) adults and 26,967 (3.4%) children with ADHD and 9,178 (0.5%) adults and 5,038 (0.6%) children with ID.

• The MFR for ASD in children was 3.67 and 2.57 in adults, corresponding to a male PR for ASD of 1.54 and 1.41, respectively.

• Comorbid ID reduced MFR and male PR in both adults and children, whereas comorbid ADHD increased male PR considerably in children.

• The MFR and population prevalence of ASD, ADHD, and ID dropped from children to young adults, then again to older adults.