A landmark population-based study from Sweden's Karolinska Institutet, published in The BMJ, is forcing a reassessment of one of the most persistent assumptions in autism research: that the condition is far more common in males than females.
The study tracked approximately 2.7 million individuals born between 1985 and 2022, following participants from birth for up to 37 years. This makes it one of the largest and longest longitudinal studies of autism prevalence ever conducted.
The central finding challenges the traditional 4:1 male-to-female ratio often cited in autism literature. While boys continue to be diagnosed at higher rates during early childhood, the study reveals that girls experience a consistent upward trend in diagnosis rates throughout adolescence. By age 20, the male-to-female ratio approaches 1:1. In total, 78,522 individuals (2.8% of the study population) received an autism diagnosis, with a mean age of diagnosis of 14.3 years.
The timing of peak diagnosis differs markedly between genders. For males, diagnosis rates peaked between ages 10-14 at 645.5 per 100,000 person-years. For females, the peak occurred later, between ages 15-19, at 602.6 per 100,000 person-years — a rate not dramatically different from the male peak.
A temporal trend analysis revealed that the male-to-female ratio has been decreasing across successive birth cohorts. Those born in the 1980s and 1990s showed much larger gender gaps in diagnosis than more recent cohorts, suggesting improved recognition of autism in females over time.
Several explanations account for the historical gender disparity. Autism diagnostic criteria were originally developed and validated primarily on male presentations. The "female autism phenotype" — characterized by better social camouflaging, fewer obvious repetitive behaviors, more internalized rather than externalized symptoms, and more typical language development — means many girls fail to meet diagnostic thresholds designed around male presentations. As social complexity increases during adolescence, the compensatory strategies girls use become insufficient, leading to later diagnosis.
The study has several important limitations. As an observational study based on national health records, it establishes correlation but cannot determine causation — it cannot explain why the gender gap is narrowing, only that it is. The analysis did not control for comorbid conditions (ADHD, intellectual disability), genetic factors, or environmental variables including parental mental health. Additionally, the study cannot distinguish between a true increase in autism prevalence among females and improved diagnostic recognition.
Despite these limitations, the implications are significant. If autism is more equally distributed between genders than previously believed, current screening tools and diagnostic practices may be systematically failing girls and women — potentially leaving millions without appropriate support and intervention.