To examine the gender distribution in ASD in adults compared to children, and the impact of comorbid intellectual disability (ID) and attention deficit/hyperactivity disorder (ADHD) on the male to female ratio (MFR).
We estimated the MFR and the male prevalence ratio (PR) for ASD in adults and children using the Medical Birth Registry of Norway, including all individuals born 1967-2011. We examined variation with age, comorbid ID and ADHD as defined by diagnoses in the Norwegian Patient Registry during 2008-2015 and/or a dispensed prescription for ADHD medication.
The sample included 1,701,206 adults and 804,146 children, including 8,995 (0.5%) adults and 8056 (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 was 3.67 in children and 2.57 in adults, corresponding to a male PR in ASD of 1.54 (95% CI 1.53-1.56) and 1.41 (1.39-1.24), respectively. Comorbid ID decreased the MFR and the male PR in both adults and children, whereas comorbid ADHD significantly increased the male PR in children. The MFR and the population prevalence of ASD, ADHD and ID decreased from children to younger adults and yet further to older adults.
We found a lower MFR and male PR in adults than in children. Findings suggest the strong male predominance seen in childhood/clinical studies of ASD diminishes in adult samples, possibly reflecting the influence of non-etiological factors such as later diagnosis in females, diagnostic biases, and diagnostic trends.

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