Statistics: how many people have autistic spectrum disorders?

"How many people have autism?" is one of the most frequently asked questions and unfortunately it is also one of the most difficult to answer. There is no central register of everyone who has autism, which means that any information about the possible number of people with autism in the community must be based on epidemiological surveys (ie studies of distinct and identifiable populations).

It is more than 50 years since Leo Kanner first described his classic autistic syndrome. Since then, the results of research and clinical work have led to the broadening of the concept of autistic disorders. As a result, estimates of prevalence have increased considerably. This process has occurred in stages, the start of each of which can be linked to particular studies. The history is summarised and the most up-to-date figures are given below.

Kanner syndrome

1943

The specific pattern of abnormal behaviour first described by Leo Kanner is also known as 'early infantile autism'. Kanner made no estimate of the possible numbers of people with this condition but he thought that it was rare (Kanner, 1943). Over 20 years later, Victor Lotter published the first results of an epidemiological study of children with the behaviour pattern described by Kanner in the former county of Middlesex, which gave an overall prevalence rate of 4.5 per 10,000 children (Lotter, 1966).

The triad of impairments in children with learning disabilities

1979

In 1979 Lorna Wing and Judith Gould examined the prevalence of autism, as defined by Leo Kanner, among children known to have special needs in the former London Borough of Camberwell. They found a prevalence in those with IQ under 70 of nearly 5 per 10,000 for this syndrome, closely similar to the rate found by Lotter. However, as well as looking at  children with Kanner autism, Wing and Gould also identified a larger group of children (about 15 per 10,000) who had impairments of social interaction, communication and imagination (which they referred to as the 'triad' of impairments), together with a repetitive stereotyped pattern of activities.  Although these children did not fit into the full picture of early childhood autism (or typical autism) as described by Kanner they were identified as being within the broader 'autism spectrum'. Thus, the total prevalence rate for the spectrum in all children with special needs in the Camberwell study was found to be approximately 20 in every 10,000 children (Wing and Gould, 1979). Gillberg et al (1986) in Gothenburg, Sweden, found very similar rates in children with learning disabilities
 
There has been a number of other epidemiological studies in different countries examining the prevalence of autism (but not the whole spectrum). These results range from 3.3 to 60.0 per 10,000, possibly due to differences in definitions or case-finding methods (Wing and Potter, 2002).

Asperger syndrome

The studies described above identified autistic disorders in children, the great majority of whom had learning disabilities and special educational needs. However, in 1944, Hans Asperger in Vienna had published an account of children with many similarities to Kanner autism but who had abilities, including grammatical language, in the average or superior range.  There are continuing arguments concerning the exact relationship between Asperger and Kanner syndromes but it is beyond dispute that they have in common the triad of impairments of social interaction, communication and imagination and a narrow, repetitive pattern of activities (Wing, 1981; 1991).

1993

In 1993, Stephan Ehlers and Christopher Gillberg published the results of a further study carried out in Gothenburg in which they examined children in mainstream schools in order to find the prevalence of Asperger syndrome and other autistic spectrum disorders in children with IQ of 70 or above.  From the numbers of children they identified they calculated a rate of 36 per 10,000 for those who definitely had Asperger syndrome and another 35 per 10,000 for those with social impairments. Some of the latter may have fitted Asperger description if more information had been available, but they certainly had disorders within the autistic spectrum. The children who were identified were known by their teachers to be having social and/or educational problems but the nature of their difficulties had not been recognised prior to the study.

1995

For over 30 years, Sula Wolff, in Edinburgh, has studied children of average or high ability who are impaired in their social interaction but who do not have the full picture of the triad of impairments. In her book giving results of her studies (Wolff, 1995), she emphasises that the clinical picture overlaps with Asperger syndrome to a large extent. However, these children represent the most subtle and most able end of the autism spectrum. The majority become independent as adults, many marry and some display exceptional gifts, though retaining the unusual quality of their social interactions.

Why include them in the autism spectrum? As Sula Wolff points out, they often have a difficult time at school and they need recognition, understanding and acceptance from their parents and teachers. The approach that suits them best is the same as that which is recommended for children with Asperger syndrome and high-functioning autism.

In her discussion of prevalence, Sula Wolff quotes Ehlers and Gillberg's study. She considers that their total figure of 71 per 10,000, includes the children she describes.

Autism spectrum disorders

2005

A survey by the Office of National Statistics of the mental health of children and young people in Great Britain found a prevalence rate of 0.9% for autism spectrum disorders or  90 in 10,000 (Green et al, 2005). These were not differentiated into autism, Asperger syndrome or any type of autism spectrum disorder.

2006

Gillian Baird and her colleagues published a report of a prevalence study which surveyed a population of children aged 9-10 years in the South Thames region.  All children who either already had a diagnosis of autism spectrum disorder or were known to child health or speech and language services as having social and communication difficulties were selected for screening. Children considered to be at risk of being an undetected case because they had a statement of special educational needs were also selected. Diagnoses were based on ICD-10 criteria. The results showed a prevalence rate of 38.9 in 10,000 for childhood autism, and 77.2 in 10,000 for other autism spectrum disorders, giving an overall figure of 116 in 10,000 for all autism spectrum disorders (Baird et al, 2006).

In this study very few children were identified with Asperger syndrome. The authors acknowledged that some children in mainstream schools who did not have a statement of special educational would have been missed, because of the selection criteria. The authors note that the prevalence estimate found should be regarded as a minimum figure (Baird et al. 2006).

There may be another reason why Asperger syndrome was rarely found in the study. ICD-10 diagnostic criteria for Asperger syndrome are such that a person who would be diagnosed with Asperger syndrome using the criteria used by Gillberg, would probably receive a diagnosis of childhood autism or atypical autism using the ICD-10 criteria.

The autistic population

It is possible that there are real differences in prevalence of autism spectrum disorders in different parts of the world, even in different parts of the same country, and at different times. An epidemic of encephalitis, for example, could increase the number of affected children. However, it is very likely that some, even most, of the variation is due to differences of definitions and the difficulty of defining the borderlines of sub-groups within the whole autism spectrum (Wing, 1996). There are no sharp boundaries separating 'typical' autism from other autistic disorders, including Asperger syndrome. [For a complete overview of the problems of establishing prevalence rates and the difficulties of diagnosing and defining autism, see Wing and Potter (2002)].

The best estimates of the total prevalence of autism spectrum disorders are those based on studies that focused on the whole spectrum and not just specific sub-groups.

Estimated prevalence rate in the UK

The indication from recent studies is that the figures cannot be precisely fixed, but it appears that a prevalence rate of around 1 in 100 is a best estimate a best estimate of the prevalence in children. No prevalence studies have ever been carried out on adults.

Estimated population of autism spectrum disorders in the UK

The estimated numbers have been worked out from the population of the UK as given in the 2001 census: 58,789,194, of whom 13,354,297 were under 18.

The figure for children is based on the 1 in 100 prevalence rate and corrected to the nearest 100. The estimated number of children under 18 with an autism spectrum disorder (ASD) is 133,500.

Given that there is no prevalence rate for ASD in adults, the figure for the whole population is a very rough guide, but we estimate that there could be over 500,000 people who have an ASD.

Estimates of the proportion of people with autism spectrum disorders (ASD) who have a learning disability, (IQ less than 70) vary considerably, and it is not possible to give an accurate figure. It is likely that over 50% of those with ASD have an IQ in the average to high range, and a proportion of these will be very able intellectually. Some very able people with ASD may never come to the attention of services as having special needs, because they have learned strategies to overcome any difficulties with communication and social interaction and found fulfilling employment that suits their particular talents. Other people with ASD may be able intellectually, but have need of support from services, because the degree of impairment they have of social interaction hampers their chances of employment and achieving independence.

References

Baird, G. et al (2006). Prevalence of disorders of the autism spectrum in a population cohort of
children in South Thames: the Special Needs and Autism Project (SNAP). The Lancet, 368 (9531), pp. 210-215.

Ehlers, S. and Gillberg, C. (1993). The epidemiology of Asperger syndrome. a total population study. Journal of Child Psychology and Psychiatry, 34 (8), pp. 1327-1350.

Gillberg, C., Grufman, M., Persson, E. and Themner, U. (1986). Psychiatric disorders in mildly and severely mentally retarded urban children and adolescents: epidemiological aspects. British Journal of Psychiatry, 149, pp. 68-74.

Green, H. et al (2005). Mental health of children and young people in Great Britain, 2004. Basingstoke: Palgrave Macmillan.
Available to download at www.statistics.gov.uk/statbase/Product.asp?vlnk=14116

Kanner, L. (1943). Autistic disturbances of affective contact. Nervous Child, 2, pp. 217-250.

Lotter, V. (1966). Epidemiology of autistic conditions in young children, I. Prevalence Social Psychiatry, 1, pp. 124-137

Wing, L. (1981). Asperger's syndrome: a clinical account. Psychological Medicine, 11, pp. 115-129.
Available from the NAS Information Centre

Wing, L. (1991). Asperger's syndrome and Kanner's autism. In: Frith, U., ed. Autism and Asperger Syndrome. Cambridge: Cambridge University Press.
Available from the NAS Information Centre

Wing, L. (1993). The definition and prevalence of autism: a review. European Child and Adolescent Psychiatry, 2 (2), pp. 61-74.
Available from the NAS Information Centre

Wing, L. (1996). Autism spectrum disorders: no evidence for or against an increase in prevalence. British Medical Journal, 312, pp. 327-328.
Available from the NAS Information Centre

Wing, L. and Gould, J. (1979). Severe impairments of social interaction and associated abnormalities in children: epidemiology and classification. Journal of Autism & Developmental Disorders, 9, pp. 11-29.
Available from the NAS Information Centre

Wolff, S. (1995). Loners: the life path of unusual children. London: Routledge.

World Health Organisation (1992).  International Classification of Diseases. 10th ed. Geneva: WHO.

Further reading

Below is a select list of some recent studies or reviews on the prevalence of autistic spectrum disorders.

Bertrand, J. et al (2001). Prevalence of autism in a United States population: the Brick Township, New Jersey, investigation. Pediatrics, 108(5), pp. 1155-1161.

Department of Developmental Services (2003). Autistic Spectrum disorders: changes in the California caseload: an update; 1999 through 2002. Sacramento: California Department of Developmental Services. www.dds.ca.gov

Chakrabarti, S. and Fombonne, E. (2001). Pervasive developmental disorders in preschool children. Journal of American Medical Association, 285(24), pp. 3093-3099.

Charman, T. (2003). Epidemiology and early identification of autism: research challenges and opportunities. In: Bock, G. and Goode, J. eds.  Autism: neural basis and treatment possibilities. Chichester: Wiley, pp.19 -25.
Available from the NAS Information Centre

Croen, L. A. et al (2002). The changing prevalence of autism in California. Journal of Autism and Developmental Disorders, 32(3), pp. 207-215.
Available from the NAS Information Centre

Dyches, T. T. (2004). Multicultural issues in autism. Journal of Autism and Developmental Disorders, 34, pp.211-222.
Available from the NAS Information Centre

Fombonne, E. (2005). The changing epidemiology of autism. Journal of Applied Research in Intellectual Disabilities, 18(4), pp.281-304.
Available from the NAS Information Centre

Fombonne, E. et al (2001). Prevalence of pervasive developmental disorders in a British nationwide survey of child mental health. Journal of the American Academy of Child and Adolescent Psychiatry, 40(7), pp. 820-827.

Harrison, M. J. et al (2006). Prevalence of autistic spectrum disorders in Lothian, Scotland: an estimate using the "capture-recapture" technique. Archives of Disease in Childhood, 91(1), pp.16-19.

Honda, H. et al (2005). Cumulative incidence of childhood autism: a total population study of better accuracy and precision. Developmental Medicine and Child Neurology, 47, pp. 10-18.

Kaye, J. A., Melero-Montes, M., and Jick, H. (2001). Mumps, measles and rubella vaccine and the incidence of autism recorded by general practitioners: a time trend analysis. British Medical Journal, 322, pp. 460-463.

Keen, D. and Ward, S. (2004). Autistic spectrum disorder: a child population profile. Autism, 8 (1), pp.39-48.
Available from the NAS Information Centre

Powell, J. E. et al (2000). Changes in the incidence of childhood autism and other autistic spectrum disorders in preschool children from two areas of the West Midlands, UK. Developmental Medicine and Child Neurology, 42, pp. 624-628.

Scott, F. J. et al (2002). Prevalence of autism spectrum conditions in children aged 5-11 years in Cambridgeshire, UK. Autism, 6(3), pp. 231-237.
Available from the NAS Information Centre

Williams, J. G., Higgins, J. P. T., and Brayne, C. E. G. (2006). Systematic review of prevalence studies of autism spectrum disorders. Archives of Disease in Childhood, 91(1), pp..8-15.

Wing, L. and Potter, D. (2002). The epidemiology of autistic spectrum disorders: is the prevalence rising? Mental Retardation and Developmental Disabilities Research Reviews, 8(3), pp. 151-161.
Available from the NAS Information Centre

Yeargin-Allsopp. M. et al (2003). Prevalence of autism in a US metropolitan area. Journal of the American Medical Association, 289(1), pp.49-53.

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