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A new study finds that global prevalence of autism has risen to 100 in 10,000 (or 1 in 100), up from 62 in 10,000 a decade ago. The study, “Global prevalence of autism: A systematic review update,” was published in Autism Research at the beginning of March. The researchers reviewed studies of the prevalence of autism worldwide, taking into consideration the impact of geographic, ethnic, and socioeconomic factors.  

The research team included researchers from a number of institutions, including the Montreal Neurological Institute-Hospital, McGill University, Montreal, Montreal, Canada; the Department of Psychiatry, Oregon Health and Science University, Portland, Oregon; Population Health Sciences, Department of Pediatrics, Waisman Center, University of Wisconsin-Madison, Madison, Wisconsin; Department of Global Health and Population, Harvard T. H. Chan School of Public Health, Boston, Massachusetts; and Autism Speaks, New York City. 

They found that the increase in prevalence arose due to the “combined effects of multiple factors including the increase in community awareness and public health response globally, progress in case identification and definition, and an increase in community capacity.” 

As they write in the article, “until recently, different prevalence estimates were derived for distinct diagnostic categories within ASD.” In the United States, for example, the DSM-5 now has a single diagnostic category for autism spectrum disorder. Similarly the International Classification of Disease, 11th revision, does not differentiate autism with and without intellectual disability.  

In addition, the study notes that as public awareness and public health response has grown, there have been significant improvements in early identification of autism. Epidemiological estimates are also increasing globally, the researchers say, in previously under-represented regions such as the Middle East and Africa. 

The researchers identified 99 prevalence estimates in 34 countries, with most estimates (61) coming from the Americas and Europe. The sample sizes in the studies they reviewed were fairly large, ranging from 465 to 50 million, with prevalence estimates ranging from 1.09/10,000 to 436/10,000. The study also found that the median percentage of autism cases with co-occurring intellectual disability was 33%. 

Male to Female Ratio

More males were diagnosed than females, 4.2 to 1, though the ratios across studies varied widely from 0.8 to 6.2. In the study conclusion, the researchers write that the wide range may be accounted for in how the studies they examined attributed sex differences. For example, some noted that girls have more socially appropriate restricted interests than boys while others attributed the difference to girls having an overall higher level of social skills, leading to girls less often being diagnosed with autism. In addition some studies have shown that current assessment practices for autism are not optimized for girls as compared to boys.  

Social Determinants

The researchers also reviewed evidence linking autism prevalence to a range of social determinants, including geography and race/ethnicity. They point out that other research focusing on “the social determinants of autism has found a positive association between [social economic status] and autism prevalence” though not consistently. They did not find any compelling evidence for direct links between those social determinants and variation in autism prevalence. They conclude that “social determinants likely interact with biological factors in complex ways, exerting an influence on prevalence and outcomes by modifying patterns of help seeking and access to care.” For example, they write, availability and affordability of health services vary from country to country, possibly accounting for some of the inconsistency. 

Paradoxically, the studies they reviewed did show that the prevalence of other childhood disabilities, including intellectual disabilities, is consistently higher among groups with lower socio-economic status. These inconsistencies reflect the need for researchers and policymakers to increase awareness of health disparities and develop targeted policies to address them, writes the research team. 

Overall, the researchers conclude that their review of studies reveals tremendous progress in raising awareness and increasing diagnosis. They also note that with more studies devoted to this area of research, there is an opportunity to expand capacity to “address unmet needs, especially among underserved populations.”