John W. Jacobson, PhD
Originally published in Science in Autism Treatment, Vol. 2, No. 1, Spring 2000, the following article contains some statistics now out of date. We include the article here because the late Dr. Jacobson’s discussion of the complexities involved in determining prevalence rates of the complex behavioral disorder of autism remains compelling and clear. Updated statistics on autism incidence can be found here.
From Congress to popular media, speculation is increasing that more children have autism than ever before. Call it what you’d like: autism, pervasive developmental disorder (PDD), autism spectrum disorders (ASD) or Asperger syndrome (AS). While there are clinical distinctions between these terms, they all mean the same thing to parents: “something is wrong with my child.” And parents want to know why.
But is it true? Are there really more kids with autism today than in generations past? Or do other factors give this impression? Are we just better at diagnosing children with autism? Are professionals more willing to make this diagnosis? Are more children referred and reported simply because more services are available? “More” children have autism relative to what?
Before seeking answers to these questions, it’s important to understand the difference between “prevalence” and “incidence.” Prevalence is the number of people at a point in time or over a period of time who have a particular identified condition. This is usually expressed as a number of people per 10,000 of the general population. Incidence refers to the number of people who are newly identified or acquire a condition over a period of time, often over a year or a longer period.
For example, in a population of 100,000 people, perhaps 300 have measles at any one time (“point prevalence”) and 1,200 have it during the course of a year. The point (of time) prevalence is 300/100,000, and the annual (yearly) prevalence is 1,200/100,000. Assuming 300 cases at the beginning of a year, that means there are 900 new cases of measles in a year. The annual incidence rate of measles (in this purely hypothetical example) is therefore 900/100,000.
So, the prevalence of ASD can be the number of people with ASD at a point in time, and the number of new cases increases the prevalence at any point. There are other mathematical relationships between incidence and prevalence, beyond the scope of this report. Confounding things even further, the autism literature sometimes uses varied, or inconsistent, denominators for these estimates.
An important nuance about prevalence is that its accuracy is only as good as the degree to which each individual who actually has the condition is counted (the numerator or top number of the fraction), and the completeness with which the “general” or other population has been counted (the denominator or bottom number of the fraction.) Accuracy in these two figures can be hard to achieve. In fact, there are no scientifically based epidemiological prevalence estimates for ASD in the United States at this time. Federal agencies have, however, called upon researchers to submit proposals that will develop better prevalence rates.
Until research in the United States results in more accurate figures, the National Institutes of Health (NIH) have suggested the following prevalence rates for ASD based upon research in other Westernized, developing nations:
- 10/10,000 people with “classic” autism
- 20/10,000 people with ASD, including PDD
- 50/10,000 people with ASD, including PDD and Asperger syndrome.
These estimates are inclusive; that is, the third estimate includes people in the first two groups. This means that in a given large population, on average 0.5%, one-half percent of the population, could be diagnosed with an ASD.
To put this in perspective, the best estimates of the prevalence of mental retardation is about 1.45% to 1.50%, or 145/10,000 to 150/ 10,000. This makes mental retardation about three times more common than autism. But the 0.5% rate for ASD is high enough to constitute a public health concern, as is the 0.2% rate for classic autism and PDD combined.
Despite the existence of these estimates, there are problems with relying upon them. First, the estimates are not from the United States, where the prevalence may be different. Second, people are diagnosed in very different ways, based on different clinical procedures, and using different diagnostic tools. There is presently no biological marker, no distinctive or particular biological signature, that serves as a benchmark for autism diagnosis. People are diagnosed at different ages and possibly by different criteria. Clinics or practitioners may use local norms; some children diagnosed according to these norms in one place or nation may not meet criteria used in another.
Numbers can be skewed because children living in some areas are more likely to be seen by specialists than are others, and children with some health plans are more likely to have coverage for specialists than others. Specialists are more likely to practice in urban areas, but poor children, who are more common in urban areas, are less likely than are more affluent children to be seen for specialist diagnosis and treatment.
Also noteworthy is that ASD is not a group of conditions that is automatically included in public health databases. This means children with ASD often become recognized in research studies only because the child enrolls in some sort of service that does contribute to databases, or where there is other, ongoing research.
But some factors contribute to the likelihood that children with autism will be identified in some manner. Today, physicians and psychologists may be more informed about ASD. They may report more children more accurately, because of better diagnostic standards. Rules for diagnosis have been more specific since about 1980, and throughout the 1990s there has been increased research on Asperger Syndrome.
Consumers may have better access to professionals who can make informed diagnosis of ASDs. The numbers of developmental pediatricians and child psychologists have increased modestly, and there has been continuing federal funding through the amended Maternal and Child Health Act for institutions like children’s hospitals, where these professionals work.
Heightened awareness of ASD and better practitioner skills among physicians and psychologists mean that the child who might have been diagnosed with some form of learning disability or developmental delay in the past is now more likely to be diagnosed with ASD. Screening tools and rating scales unavailable in the past help today’s physicians and psychologists to better detect an ASD.
Taken individually or together, these factors do not make it more likely that children will be incorrectly diagnosed with ASD, thereby inflating the reported number of children. However, researchers are attempting to develop tools and scales that will assist even earlier diagnosis of ASD, and this might increase or decrease the prevalence of children with mild cases in the future.
However, there are at least two factors that may be inflating the reported numbers of children with ASD. The first is the advent of the Asperger Syndrome (AS) diagnosis. Diagnostic criteria for AS require less severe problems in living and learning than those for other ASDs. As a result, there may be some people with the very mildest apparent forms of AS who are now diagnosed but would not have been in the past, and there may be some individuals diagnosed when it is not warranted. AS diagnoses add considerably to the estimated prevalence of ASD. A good number of the AS diagnoses are made of adults, in the absence of detailed, solid information about the person’s early childhood development. Questions can be raised about whether this represents sound diagnostic practice.
The second factor, which may be inflating ASD prevalence, is practitioners’ desire to help children get needed services. After all, the purpose of diagnosis is to identify problems in specific terms that lead to well-defined, promising treatment. As scientifically based treatments for ASD are recognized, ASD diagnosis may be key to getting a child more intensive early services. These services hold the promise of significant therapeutic benefit, if the child has problems similar to those common in an ASD. This means that some children will be diagnosed with an ASD even though they have more mild or related delays in development, or if they have behavioral problems often found in ASD. Although this practice raises some grave questions, pediatricians or psychologists may view it as a means of advocating for a child. Regardless of well-meaning intentions, this form of advocacy could inflate the rate at which ASDs are identified and reported.
The scientific foundation for estimating the prevalence of autism among preschoolers, school age children, and adults is presently not very strong in the U.S. Some may point to a 1999 report by the State of California on increased enrollments of people with ASD in Department of Developmental Services (DDS) programs (Department of Developmental Services, 1999), or the high local rate of ASD reported in a New Jersey town (Kolata, 1999), as evidence of increased numbers of children with autism. However, because of its nature, the DDS report can show only that more children are being referred and enrolled. Similarly, the New Jersey township news story may show only that the prevalence of ASD can vary greatly in small population groups, such as a particular city or town, from that of the general population. However, these limitations in no way diminish the importance of serving children and adults with ASD who have been identified and referred for services.
Until there is a “gold standard,” either a biological marker or rating scale, it is hard to estimate the accuracy of the tentative NIH prevalence rates. Many of the studies on which current estimates rely were reviewed in a 1993 article by Dr. Lorna Wing. These used very different methods to identify children, and varying diagnostic criteria (Wing, 1993).
As noted before, epidemiological studies from other nations suggest it is certainly possible that the prevalence of autism has been underestimated in the past, and that PDD and AS have not been broadly and representatively studied. An apparent increase may simply be the result of better knowledge of ASDs. From the standpoint of advocacy, there are some incentives for suggesting that the prevalence of autism is increasing. However, it is very difficult to support this assertion with scientific, epidemiological evidence.
Department of Developmental Services. (1999, March 1). Changes in the population of persons with autism and pervasive developmental disorders in California’s developmental services system: 1987 through 1998. Sacramento, CA: Author.
Department of Education (1999). Digest of education statistics, 1998. Washington, DC: Author (GPO No. 065-000-01174-3, NCES No. NCES 1999036).
Kolata, G. (1999, January 31). Probing disease clusters: easier to spot than to prove. New York Times. Retrieved from http://www.nytimes.com/
Wing, L. (1993). The definition and prevalence of autism: A review. European Child and Adolescent.
Citation for this article:
Jacobson, J. (2002). Is autism on the rise? Science in Autism Treatment, 2(1), 8-9.