Observational study finds equal autism diagnosis rates for genders by age 20 — Evidence Review

Females may be as likely as males to be autistic, but a new large-scale study finds girls are often diagnosed later—by age 20, diagnosis rates for men and women are nearly equal. Related studies generally support these findings, highlighting longstanding diagnostic biases and the need to refine autism assessment tools; see the BMJ publication for full details.

• Extensive literature indicates that historically, diagnostic criteria and clinical practices have been tailored toward male presentations of autism, leading to underdiagnosis or delayed diagnosis in females, especially those without intellectual impairment 1 2 6 7 8 9.
• Several systematic reviews and observational studies report that girls may mask symptoms or present differently than boys, contributing to both later and less frequent diagnoses in childhood, even when overall prevalence is similar by adulthood 2 6 7 8.
• Trends in diagnostic rates over recent decades show increased recognition of autism in females and adults, supporting the view that prior gender gaps were largely due to ascertainment bias and evolving awareness, not true prevalence differences 11 12.

Study Overview and Key Findings

This study addresses a longstanding question in autism research: why are females so much less likely to be diagnosed with autism in childhood compared to males? By analyzing data from all individuals born in Sweden between 1985 and 2020, the researchers sought to clarify whether true prevalence differences exist or if diagnostic practices have masked the actual rates among females. The findings challenge prior assumptions and highlight the importance of accounting for diagnostic bias and gendered presentations of autism, especially given the mental health and support consequences of delayed or missed diagnosis in girls and women.

Property	Value
Organization	Karolinska Institutet
Journal Name	BMJ
Authors	Dr Caroline Fyfe
Population	People born in Sweden between 1985 and 2020
Sample Size	2.7 million people
Methods	Observational Study
Outcome	Diagnosis rates of autism by gender and age
Results	By age 20, diagnosis rates for men and women were almost equal.

Literature Review: Related Studies

To situate these findings within the broader research landscape, we searched the Consensus database (over 200 million papers) using targeted queries. The following search queries were used:

1. autism diagnosis gender differences
2. autistic girls underdiagnosis factors
3. longitudinal autism diagnosis trends

Below, key themes and findings from the literature are summarized:

Topic	Key Findings
How do diagnostic criteria and clinical practices affect gender disparities in autism diagnosis?	- Autism criteria and tools were historically developed based on male presentations, leading to underdiagnosis or late diagnosis in females, particularly those with average to high intelligence 1 2 6 7 8 9. - Girls require more pronounced behavioral or intellectual difficulties to receive a diagnosis and are more likely to be misdiagnosed with other psychiatric conditions 6 7 8 9.
Do females and males present differently with autism, and how does this affect recognition?	- Females often mask autistic traits, exhibit fewer repetitive behaviors, or have interests that are more socially acceptable, making detection harder 2 4 6 10. - Qualitative differences in early developmental concerns and symptom expression can delay or obscure diagnosis in girls, especially when cognitive abilities are intact 2 4 6 10.
What are the trends in autism diagnosis rates by gender and age over time?	- Diagnostic rates for females, adults, and higher functioning individuals have increased markedly in recent decades, suggesting increased awareness and changing diagnostic practices rather than shifts in underlying prevalence 11 12 13. - Population-based studies show that the prevalence of autistic traits remains stable, but registered diagnoses have increased, especially among girls and adults 11 12.
What are the consequences of delayed or missed autism diagnosis in females?	- Late-diagnosed women with autism report vulnerabilities including increased risk of mental health difficulties and misdiagnosis with mood or personality disorders 6 8. - Early diagnosis is associated with better access to support and improved social outcomes, emphasizing the importance of timely recognition for both sexes 6 8 15.

How do diagnostic criteria and clinical practices affect gender disparities in autism diagnosis?

The literature consistently demonstrates that diagnostic tools and clinical frameworks for autism were originally designed around male phenotypes, leading to systematic underrecognition of autism in females. This bias persists, particularly for girls with average or above-average intelligence, and contributes to the lower rates of childhood diagnosis observed in girls compared to boys. The new Swedish study directly addresses this gap by demonstrating that, when diagnostic delays are accounted for, prevalence rates between males and females converge by early adulthood.

• Systematic reviews highlight that diagnostic criteria often fail to capture the female autism phenotype, resulting in underdiagnosis or misdiagnosis 1 2 6 7 8 9.
• Girls are more likely to be diagnosed only if they exhibit additional intellectual or behavioral difficulties, reflecting higher thresholds for clinical recognition 7 9.
• Studies call for the refinement of diagnostic tools and increased clinician awareness to reduce gender bias in autism assessment 1 2 8.
• The new study’s findings are consistent with calls to re-examine diagnostic standards and support the need for gender-sensitive assessment approaches 1 2 7 9.

Do females and males present differently with autism, and how does this affect recognition?

Differences in symptom expression between males and females are well-documented. Girls with autism may display fewer obvious repetitive behaviors, have special interests that align more closely with those of neurotypical peers, or develop coping strategies that mask social communication difficulties. These factors contribute to later detection and diagnosis, as highlighted by both the new study and the broader literature.

• Females may mask or compensate for autistic traits, especially in social contexts, making symptoms less visible to parents and clinicians 2 4 6 10.
• Early developmental concerns often differ by gender; caregivers of girls may be less likely to recognize symptoms as indicative of autism 6 10.
• The prevalence of a distinct “female autism phenotype” is supported by qualitative research and first-person accounts 6 10.
• These differences underscore the importance of developing diagnostic criteria and screening instruments that better capture the full spectrum of autism presentations 4 6 10.

What are the trends in autism diagnosis rates by gender and age over time?

Recent decades have seen significant increases in autism diagnoses overall, with the steepest rises among females, adults, and individuals without intellectual disability. This is interpreted as a consequence of growing awareness, reduced stigma, and improved clinical recognition, rather than changes in the underlying prevalence of autistic traits. The new Swedish study aligns with these findings, showing that gender disparities in diagnosis decrease by adulthood.

• Large-scale epidemiological studies document substantial increases in autism diagnoses among females and adults, alongside stable prevalence of autistic traits 11 12.
• The narrowing of gender gaps in diagnosis over time reflects evolving clinical practices and greater attention to autism in populations previously overlooked 11 12 13.
• These trends suggest that earlier observed male predominance was likely a product of ascertainment bias rather than true differences in prevalence 11 12.
• The Swedish data provide further evidence that diagnostic disparities can be addressed through improved recognition and assessment practices 11 12 13.

What are the consequences of delayed or missed autism diagnosis in females?

Delayed or missed diagnosis of autism in girls and women can have significant consequences, including increased risk of mental health difficulties, misdiagnosis with other psychiatric disorders, and barriers to accessing appropriate support. The literature underscores the importance of early and accurate identification to mitigate these risks.

• Late-diagnosed women report experiences of vulnerability, including higher rates of anxiety, depression, and exposure to adverse outcomes 6 8.
• Misdiagnosis with mood or personality disorders is common among autistic females whose symptoms are not recognized as autism 6 8.
• Early intervention is associated with improved social and behavioral outcomes, highlighting the potential benefits of reducing diagnostic delays 15.
• The new study’s findings reinforce the need for timely and accurate diagnosis to ensure appropriate support for autistic girls and women 6 8 15.

Future Research Questions

While recent research has clarified important aspects of gender differences in autism diagnosis, several questions remain. Future studies are needed to further disentangle the causes of diagnostic disparities, develop gender-sensitive assessment tools, and evaluate the long-term impacts of delayed diagnosis—particularly for females and other underrepresented groups.

Research Question	Relevance
What specific features distinguish the female autism phenotype?	Understanding unique characteristics of autism in females could improve diagnostic tools and reduce underdiagnosis 2 4 6.
How effective are current autism screening tools for identifying autism in girls?	Many existing tools may be biased toward male presentations; evaluating their sensitivity and specificity for girls is critical for early and accurate detection 1 2 8.
What are the long-term mental health outcomes for females with late or missed autism diagnosis?	Delayed or missed diagnosis in females is linked to increased psychiatric comorbidities and poorer outcomes; long-term studies would clarify the full impact and inform intervention strategies 6 8.
How do sociocultural factors influence autism diagnosis in different countries and populations?	Differences in awareness, stigma, and health systems may affect diagnosis rates and gender disparities; comparative research could identify modifiable barriers and inform policy 11 12 13.
Can early intervention for autism in girls improve long-term social and behavioral outcomes?	There is evidence that early diagnosis and intervention benefit children with autism, but more research is needed on the specific effects for girls and women across different developmental stages 15.