What is the Male-to-Female Ratio in Autism Spectrum Disorder? A Systematic Review and Meta-Analysis

Abstract Objective To derive the first systematically-calculated estimate of the relative proportion of males and females with autism spectrum disorder (ASD), via a meta-analysis of prevalence studies conducted since the introduction of the DSM-IV/International Classification of Diseases–10 th Revision ( ICD-10 ). Method Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. We searched MEDLINE, Embase, and PsycINFO databases, and study quality was rated using a risk of bias tool. Random effects meta-analysis was used. The outcome measure that we pooled was the male-to-female odds ratio (MFOR), namely the odds of being male in the group with ASD compared to in the non-ASD group. In effect this is the ASD male-to-female ratio, controlling for the male-to-female ratio among participants without ASD. Results Fifty-four studies were analysed, with 13,784,284 participants, of whom 53,712 had ASD (43,972 males and 9,740 females). The overall pooled MFOR was 4.20 (95% CI 3.84, 4.60), but there was very substantial between-study variability (I2 =90.9%). High-quality studies had a lower MFOR = 3.32 (95% CI [2.88, 3.84]). Studies that screened the general population to identify participants regardless of whether they already had an ASD diagnosis showed a lower MFOR = 3.25 (95% CI [2.93, 3.62]) than studies that only ascertained participants with a pre-existing ASD diagnosis (MFOR = 4.56, 95% CI [4.10, 5.07]). Conclusion Among children meeting criteria for ASD, the true male-to-female ratio is not four to one, as is often assumed; rather, it is closer to three to one. There appears to be a diagnostic gender bias, meaning that girls who meet criteria for ASD are at disproportionate risk of not receiving a clinical diagnosis.