Our understanding of autism has changed over time, developing in response to research and advocacy from autistic people, which has led us to question just how many autistic people may have missed out on a diagnosis. The National Autistic Society supported researchers from University College London (UCL) in undertaking a population-based study to better understand how many autistic people, both diagnosed and undiagnosed, are estimated to live in England. They were also interested in examining what socio-demographic factors may impact on diagnosis. In this article, Aimee Fletcher from our Evidence and Research team summarises their findings.
 

Methodology and analysis

This research was a first-of-its-kind attempt to estimate how many autistic people may be living with or without a diagnosis by conducting a population-based study. Using anonymous data from more than five million people who were registered at GP practices in England between 2000 and 2018, the researchers estimated the number of people in England who had received an autism diagnosis. This primary care data was compared with community prevalence data to estimate underdiagnosis.  

Researchers identified individuals with an autism diagnosis (including autism, Asperger’s, pervasive developmental disorder), as well as demographic variables. These variables included gender, age and local socioeconomic deprivation, measuring potential poverty or lower access to health services or education in particular areas. By considering these different factors in relation to diagnosis, the researchers could gain a better insight into factors impacting rates of diagnosis. 

When looking at community prevalence for comparison, the researchers used a lower (c.1%) and an upper (c.3%) estimate of how common autism really is in the population. The lower level estimate (c.1%) is commonly stated when estimating how many autistic people there are in England. This number derives from an epidemiological research paper that was published in 2011, before changes were made to the diagnostic criteria that made them more inclusive. The upper level estimate (c.3%) is based on the rates of diagnosed autism in young people (aged 10-19) in the dataset. This rate is higher because young people are more likely than other age groups to be identified and diagnosed autistic by services. 
 

Findings and implications 

Through examining the data, the researchers estimated that there may be between 150,000 and 500,000 people aged 20 to 49 in England who are autistic but are undiagnosed. In addition, they estimate that there could be between 250,000 and 600,000 autistic people over the age of 50 who may be undiagnosed – this would be more than nine in ten of all autistic people aged 50+. 

The researchers estimate that the midpoint of these figures translates as approximately 750,000 undiagnosed autistic people aged 20 and above in England. If this is the case, the total autistic population in England would exceed 1.2 million people. This is nearly double the previously cited figure of 700,000 across the entirety of the UK, as the government typically quote. 

These findings demonstrate the importance of improving diagnostic services for adults, as well as providing better support after diagnosis, to address the diagnosis gap. 

Elizabeth O’Nions, lead researcher and postdoctoral researcher (UCL Psychology & Language Sciences), explains why this research is important: 

“This matters because autistic people often experience discrimination and exploitation in society. They may have unmet support needs, even when they appear to be coping with life. 

“Having a diagnosis means that someone can advocate for their right to reasonable adjustments and the support they need.” 

This study is the first to estimate underdiagnosis of autism, also demonstrating variance dependent on key demographic and clinical factors. The study not only highlights the continuing inequalities in access to autism diagnostic assessments for adults in England, but also the need for policy initiatives to address underdiagnosis. 

In addition, the researchers call for greater acceptance and understanding of autism in general, in order to create a supportive society that does not discriminate. 
 

Limitations 

This research was the first of its kind, which brings about its own limitations. The first limitation was that the primary care records are not directly linked to secondary care records, which means that some diagnoses may not have been recorded in primary care records. Secondly, Secondly, this study is based on data from around 10% of English GP practices, which means it was not possible to map local variations in diagnostic rates. However, this research does highlight that there are clear diagnostic gaps, particularly between adults and children, which need to be addressed. 
 

Funding 

The research was funded by Dunhill Medical Trust, Economic and Social Research Council, Medical Research Council, National Institute for Health and Care Research, Wellcome, and the Royal College of Psychiatrists. Funders had no role in study design, data collection, analysis or interpretation, nor in the writing of the final report.

You can read the full paper here.