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The Royal College of Psychiatrists Improving the lives of people with mental illness

Cambridgeshire study confirms 1 per cent prevalence of autism

Embargoed until 29 May 2009

A new study has confirmed that 1 per cent of children aged between 5- and 9-years-old have an existing diagnosis on the autistic spectrum. The research, published in the June issue of the British Journal of Psychiatry, was carried out by the Autism Research Centre at Cambridge University.

The research team, led by Professor Simon Baron-Cohen, used three different methods to estimate the prevalence of autism-spectrum conditions (including previously undiagnosed cases) in Cambridgeshire.                                               

First, the team carried out a survey of cases of autism and Asperger syndrome using the Special Educational Needs (SEN) registers in schools. The register used for this research covered 8,824 children attending 79 schools in Cambridge City, East Cambridgeshire, South Cambridgeshire, and Fenlands. A total of 83 cases of autism-spectrum conditions were reported, giving a prevalence of 94 in 10,000, or 1 in 106 children. This estimate closely replicates findings from previous studies.

Second, the team sent a diagnosis survey to the parents of 11,700 children in the Cambridgeshire region. From the 3,373 completed surveys, 41 cases of autism-spectrum conditions were reported which corresponds to prevalence of 1 in 101.

Finally, the team sent the Childhood Autism Screening Test (CAST) to parents of the same 11,700 children to help identify any undiagnosed cases of autism-spectrum conditions. All children who scored highly on the CAST, along with a selection of medium and low scorers, were called in for further assessment.

Excluding the 41 cases already known about, the team found an additional 11 children who met research diagnostic criteria for an autism spectrum condition but had not yet been diagnosed. If this finding is extrapolated to the wider population, it means that for every 3 cases of autism spectrum that are known, there may be a further 2 cases that are undiagnosed. In other words, the ratio of known to unknown cases is 3:2.

Professor Baron-Cohen said: “This study is novel because it does not rely on a single source of information but instead combines information from three different sources.

“The two independent sources of information - the SEN register and diagnosis survey – provide converging evidence on the prevalence of autism spectrum conditions as being around 1 per cent of primary school-age children. This is about 12 times higher than 30 years ago; including the previously undiagnosed cases, this means that 1 in 64 children may at some point in their lives require support and services.”

Professor Baron-Cohen concluded: “It is important to conduct epidemiological studies of autism spectrum conditions so that the relevant services, including education, health and social services, can plan adequate provision for all those children and adults who may need support. Epidemiology is both expensive and time-consuming, but vital if as a society we want to be well-prepared and pro-active, not simply reactive.”

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Baron-Cohen S, Scott FJ, Allison C, Williams J, Bolton P, Matthews FE and Brayne C (2009) Prevalence of autism-spectrum conditions: UK school-based population study, British Journal of Psychiatry, 194: 500-509.


Note to editors:

Explaining the increase: Prevalence estimates for autism-spectrum condition have shown a steady increase over the past decade. This increase, however, could be accounted for by a number of factors including: improved recognition, awareness and detection - both by parents and professionals; differences in study methodology; an increase in available diagnostic services; growing acceptance that autism can coexist with a range of other conditions; and a widening of the diagnostic criteria - Asperger Syndrome did not enter the diagnostic manuals until 1994. Additionally, the conceptualisation of autism and related conditions has changed over the years. Previously, it was thought of as a discrete categorical condition, but this view has been replaced by a quantitative and dimensional approach (hence the term 'the autistic spectrum'). Explaining the numbers: The 1 in 64 prevalence estimate was calculated by multiplying the estimate derived from the Special Needs Register (94 cases per 10000) by the ratio of known-to-unknown cases (1.67). This ratio was calculated by adding the (weighted) known cases (which parents reported) and unknown cases (which the scientists found through prospective screening and assessment) and dividing this by the (weighted) number of known cases. Thus, 33.3 (known cases) plus 22.2 (previously unknown cases) equals 55.8. Dividing this by 33.3 (known cases) gives a multiplier of 1.67 (or 3:2 ratio of known to unknown if rounded up). The higher prevalence estimate seen here is therefore the result of combining three different methods of case ascertainment, including cases that met research diagnostic criteria in the estimate, but who did not have a previously recorded diagnosis of autism spectrum condition.


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