Abstract
To examine the gender distribution in ASD in adults compared with 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 during 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 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 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-aetiological factors such as later diagnosis in females, diagnostic biases and diagnostic trends.
Highlights
Male predominance is a consistent finding in studies of autism spectrum disorder (ASD), with male to female ratios (MFR) or male: female odds ratio (MFOR; depending on type of study) as high as 4.2–4 .3, in recent studies.[1,2] This strong male predominance has fueled various causal hypotheses of autism spectrum disorders (ASD), eg testosterone exposure in utero, the extreme male brain theory[3] and a female protective effect in ASD.[4]
We found a lower male to female ratio (MFR) for ASD in adults (2.57) than in children (3.67) in the Norwegian Patient Registry
We identified individuals with attention- deficit/hyperactivity disorder (ADHD) if they were registered with an ADHD diagnosis (ICD-1 0 code F90; Norwegian Patient Registry (NPR) 2008–2 015) and/or having been dispensed a prescription of ADHD medication during 2004–2 015 (NorPD)
Summary
Male predominance is a consistent finding in studies of autism spectrum disorder (ASD), with male to female ratios (MFR) or male: female odds ratio (MFOR; depending on type of study) as high as 4.2–4 .3, in recent studies.[1,2] This strong male predominance has fueled various causal hypotheses of ASD, eg testosterone exposure in utero, the extreme male brain theory[3] and a female protective effect in ASD.[4]. Using an OR weighted for age, gender, ID and type or residence, they found an OR of 1.35 (95% CI 0.64–2 .83) for ASD in males relative to females with moderate to profound intellectual disability (ID) but an OR of 8.97 (95% CI 2.20– 36.52) and 8.62 (95% CI 2.2–3 4.5) in adults with mild ID or borderline/normal intelligence.[16,17] Their large confidence intervals preclude firm conclusions, but the authors hypothesized that the high male OR in adults with normal intelligence could stem partly from assessment. We found a lower male to female ratio (MFR) for ASD in adults (2.57) than in children (3.67) in the Norwegian Patient Registry. The adult prevalence of ASD in the Norwegian Patient Registry (for individuals aged 18– 48 years and alive at the time of the registry linkage) was 0.5%
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