The diagnostic categories for the two most common Neurodevelopmental Disorders, Autism Spectrum Disorder (ASD) and Attention Deficit and Hyperactivity Disorder (ADHD), are the most heterogenous of all categories in the Diagnostic and Statistics Manual (DSM) of Mental Disorders.
That means that individuals who meet the criteria for an ASD or AD(H)D diagnosis can have vastly different symptoms and presentations across the same dimensions – inattention and hyperactivity for AD(H)D and social communication and restrictive and repetitive (and sensory) behaviours for ASD. Because multiple criteria along multiple dimensions are used in diagnosis, they are considered spectrum disorders and are complex to diagnose.
Because of this heterogeneity of symptoms and presentation, and the lack of objective biomarkers, such as blood tests or genetic tests, there have been many debates over the legitimacy of the diagnostic categories and the prevalence of these Neurodevelopmental Disorders.
While some claim AD(H)D and ASD are overdiagnosed and overtreated, others claim they are underdiagnosed and undertreated (¹). What we do know about prevalence is that there are discrepancies based on age, gender, country, race/ethnicity, and that the rate of diagnosis for both disorders has increased steadily over the past two decades.
Globally, the rate of AD(H)D in the population of children under 18 is estimated to be around seven percent and, for adults, around three percent (²). In the US, rates of AD(H)D are higher, with ten percent of children diagnosed in 2015, up from six percent in 1997 (³). In Canada, an estimated one in sixty-six are diagnosed with Autism (⁴).
Globally, the most recent prevalence study found that one in one hundred children are diagnosed with Autism, with males diagnosed at a rate of 4:1 to females. This is up from 62 in 10,000 in the last global prevalence study conducted in 2012.
These increases are attributed to greater public awareness, public health response, broader diagnostic criteria, and increased identification in previously underdiagnosed populations defined by age, sex, geography, race/ethnicity, or SES (⁵).
The diagnosis of boys with both ASD and AD(H)D has vastly outweighed girls, ranging from 10:1 to 2:1 for AD(H)D and 8:1 to 3:1 for Autism. Because assessment criteria have been based on research and observation of male children, these numbers do not reflect the true prevalence of girls and adults who have ASD or AD(H)D.
A variety of reasons have been given for gender-based discrepancies, including diagnostic bias, parent and teacher stereotypes, and a “female protective factor” located in the female brain (⁶). The outcome of these gender biases is not only fewer diagnoses but also later diagnoses (⁷).
Fortunately, this is beginning to change and more females are being diagnosed, and diagnosed earlier, as parents, teachers, and health professionals become more aware of the different presentations of AD(H)D and ASD across gender.
It is more widely recognized that females are, for instance, more likely to be inattentive than hyperactive, to internalize, engage in gendered and socially approved repetitive behaviour, and socially mask. The rates for adult diagnoses are also climbing at four times the rate of children (⁸). Getting diagnosed at younger ages is important because earlier diagnoses are linked to greater life satisfaction (⁹).
The available statistics on the prevalence of ASD and AD(H)D diagnoses do not give us an accurate picture of how many people are Autistic or AD(H)D’ers. These diagnostic categories are imperfect, deficit-based, applied inconsistently, and subject to change. At the same time, they hold great power in determining who can and cannot access resources and accommodations.
A diagnosis can be validating and life-changing. Yet not everyone wants to or can afford to go through the assessment process. Many adults avoid a formal assessment because they have been misdiagnosed or had their neurodivergent traits denied in the medical community too many times, women especially.
For many adults, self-diagnosis or self-identification precedes or replaces formal diagnosis (¹⁰). Because of barriers to diagnosis, self-diagnosis is widely accepted in the neurodivergent community, and prevalence rates will never fully reflect the actual number of neurodivergent people in the world.
- Villines, Z. (2021, November 16). Is ADHD Overdiagnosed and Overtreated? Medical News Today. https://www.medicalnewstoday.com/articles/is-adhd-overdiagnosed-and-overtreated
- About ADHD. (2022). CHADD. https://chadd.org/about-adhd/general-prevalence/
- Xu, G., Strathearn, L., Liu, B., Yang, B., & Bao, W. (2018). Twenty-Year Trends in Diagnosed Attention-Deficit/Hyperactivity Disorder Among US Children and Adolescents, 1997-2016. JAMA Network Open, 1(4), e181471. https://doi.org/10.1001/jamanetworkopen.2018.1471
- Government of Canada. (2018). Infographic: Autism Spectrum Disorder among Children and Youth in Canada 2018: Autism Prevalence among Children and Youth in Canada (2018 Report of the National Autism Spectrum Disorder (ASD) Surveillance System). https://www.canada.ca/en/public-health/services/publications/diseases-conditions/infographic-autism-spectrum-disorder-children-youth-canada-2018.html
- Zeidan, J., Fombonne, E., Scorah, J., Ibrahim, A., Durkin, M. S., Saxena, S., Yusuf, A., Shih, A., & Elsabbagh, M. (2022). Global prevalence of autism: A systematic review update. Autism Research, 15(5), 778–790. https://doi.org/10.1002/aur.2696
- Mowlem, F., Agnew-Blais, J., Taylor, E., & Asherson, P. (2019). Do different factors influence whether girls versus boys meet ADHD diagnostic criteria? Sex differences among children with high ADHD symptoms. Psychiatry Research, 272, 765–773. https://doi.org/10.1016/j.psychres.2018.12.128
- Zeliadt, N. (2018, June 13). Autism’s sex ratio, explained. Spectrum News. https://www.spectrumnews.org/news/autisms-sex-ratio-explained
- Saline, S. (2022, July 13). ADHD Statistics: New ADD Facts and Research. ADDitude. https://www.additudemag.com/statistics-of-adhd/
- Jagoo, K. (2021, October 15). What’s It Like To Be Diagnosed with Autism As An Adult? New Research Takes a Closer Look. Very Well Mind. https://www.verywellmind.com/what-s-it-like-to-be-diagnosed-with-autism-as-an-adult-new-research-takes-a-closer-look-5203463
- L, Jae. (2021, March 15). Self-diagnosis Is More Valuable Than Formal Autism Diagnosis In Many Ways. Autistic Discovery. https://medium.com/autistic-discovery/opting-for-formal-diagnosis-does-not-devalue-self-diagnosis-83374d928217
How common are Autism Spectrum Disorder and AD(H)D?
The rates of diagnosis of ASD and AD(H)D have been on the rise for over two decades. Males are diagnosed at a much higher rate than females. Adults are also underdiagnosed. Earlier diagnosis has been linked to greater life satisfaction.
Prevalence of ASD and AD(H)D:
Is higher for males and children than females and adults
Is based on diagnostic categories that are imperfect, deficit-based, inconsistently applied, and subject to change