How Many People Have Autism?

When autism first caught the eye of the scientific community in the 1940s, it was considered a rare, albeit severe, disorder. While no major epidemiological studies were completed during this era, most professional estimates regarding the disorder placed prevalence near 1-2 children per 10,000.

Several more studies conducted in the following years yielded similar low prevalence rates. However, when the third edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM; American Psychiatric Association, 1980) was published, new diagnostic criteria were introduced, which were then refined in the 1987 version of the manual (DSM-III-R). Based on criteria from the DSM-III and the DSM-III-R, at least 12 notable epidemiological studies examining the prevalence of autistic disorders were conducted between 1983 and 1997.

Prevalence rates for these studies ranged from 0.25 cases per 1,000 (Ritvo, et al., 1989) to 1.60 per 1,000 (Ishii & Takahashii, 1983). Between 1996 and 1998, at least seven other widely read studies were conducted, using diagnostic criteria found in the International Classification of Diseases, 10th edition (ICD-10). Within these studies, prevalence rates ranged from 0.54 cases of autism per 1,000 (Fombonne, et al., 1997) to 6 cases per 1,000 (Kadesjo, Gillberg, & Hagberg, 1999).

Citing many of the above studies, the Centers for Disease Control and Prevention (CDC) conducted a very large-scale epidemiological study of autism prevalence in Brick Township, N.J., in 1998. The study utilized diagnostic criteria from the DSM-IV for autism spectrum disorders (ASD). The results of this study indicated a prevalence of autistic disorder at 4.0 cases per 1,000 for children aged 3-10 years (CDC, 2000). The overall rate for children of the same age who met the criteria for autistic disorder, as defined in DSM-IV, and other spectrum disorders (ASD) was 6.7 cases per 1,000 (CDC, 2000).

The Brick Township study was the first large-scale contemporary prevalence study in the United States that used very intensive identification methods. It yielded results similar to those found in studies conducted abroad during the same time period. Because of this, it has been considered for several years to be a hallmark study on autism prevalence, with its results frequently used in a wide variety of settings to support the notion that autism is indeed a disorder worthy of the public’s close attention.

Perhaps the most significant prevalence figures yet are the most recent ones produced by the CDC. In February 2007, the CDC announced the results of its first multi-community prevalence study, based on information collected during the reporting years of 2000 and 2002. This study reported the results of data collected across 14 different sites spanning the United States. The study examined the records of 8-year-old children (because most individuals with ASD have been identified by that time). It concluded that autism’s prevalence was (on average) around 6.6-6.7 per 1,000 eight year olds, or that approximately 1 in 150 were on the autistic spectrum (CDC, 2007).

Today, autism is recognized in many circles as an “epidemic” or “crisis” that is directly impacting the lives of many millions of Americans. Thanks to devoted parents, committed researchers and nothing short of a media frenzy, autism has become the subject of close scrutiny as well as a great deal of hype.

What is important to understand is this: while the rates found in the studies have increased as the years have passed, many believe they reflect an increase in diagnoses of ASD, as opposed to an increase in actual prevalence. It can be argued that we have become much more skilled at identifying individuals who have autism spectrum disorders. With awareness of ASD increasing globally, deeper knowledge has undoubtedly made its way to physicians and other diagnosticians who may not have been as familiar with ASD 20 years ago. In addition, many individuals with ASD simply would not have been diagnosed with any disorder in the pas, including the many adults receiving diagnoses of Asperger’s Disorder who now account for a percentage of the ASD population. In other words, the perceived increase in prevalence could be attributed, in part, to better diagnostic tools and a greater sense of awareness.

The above arguments noted, many of the professionals conducting research on the neurobiological elements of autism as well as many diagnosticians continue to contend that they are simply seeing more children with autism than ever before. Science will eventually unravel the mysteries surrounding its causes and prevalence. Until then, it is safe to say that autism and related spectrum disorders are having quite a significant impact on a global scale.