Western countries are often said to be in the midst of an epidemic of autism. There is little doubt that the condition, once a rarity afflicting as few as two children in every 10,000, is now diagnosed much more frequently. Some 535,000 people in Britain are recognised to have an autistic-spectrum disorder.

These range in severity from Asperger’s syndrome — which can manifest as an extreme social awkwardness — to crippling inability to engage with the outside world. The rate of increase looks alarming. In California, for instance, the proportion of cases diagnosed more than doubled from 5.8 per 10,000 in 1987 to 14.9 per 10,000 in 1994, and is growing year on year. Though autism is known to have a genetic component, the rise is too steep to be a matter of changing genes. Environmental theories have been advanced to explain this: most notably unsubstantiated vaccine scares over MMR in Britain, and thiomersal, a mercury-based preservative, in the US.

Another more innocent possibility, however, fits the data better. It seems to be doctors’ skills at detecting the condition that are improving.

At the UK launch this week of the National Alliance for Autism Research, five leading British experts endorsed the verdict of Tony Charman, of the Institute of Child Health in London, that the documented rise is more likely to reflect changing diagnosis than a new epidemic with a sinister cause.

When autism was first identified by Leo Kanner in 1943, he put its prevalence at one child in 10,000. “Kanner’s syndrome” had relatively severe symptoms that would previously have been regarded as those of emotional disturbance or of mental retardation.

Over the years, children whose conditions would once have been diagnosed as different disorders have been similarly recognised as having conditions on the autistic spectrum. In the 1970s, the Camberwell study estimated prevalence at five per 10,000 — though this included only children with low IQs. It is now clear that autism is not confined to this group and the diagnositic criteria have thus broadened. Today’s best guess is that it occurs in about 60 children per 10,000.

Given the lack of good historical data, it is difficult to prove that autism has long been this common, but that sufferers were differently labelled. There are indications, though, that this is the case.

A 1970s Swedish study found that about 60 children per 10,000 showed what was then called “psychotic behaviour”. Its author, Professor Chris Gillberg, says that these symptoms would today be seen as autism — which occurs at just this rate. The Californian research outlined above offers further evidence: the increase in diagnoses of autism was matched almost precisely by a decrease in diagnoses of mental retardation.

All the evidence is not yet in — it remains possible that while new interpretations of the same symptoms explain most of the increase, they do not account for it all. But a scientific consensus is building. It should have important consequences for the direction of autism research.

Accurate diagnosis of autism is clearly important if those who suffer this disorder are to be helped. At present, though, there are no forms of treatment or intervention that have been proven to improve their prognosis. Research needs to focus on the most promising experimental therapies — and, just as crucially, on the neurological and genetic studies that are best placed to unlock the underlying causes.

The illusion of a fresh epidemic of autism and emotive assertions about vaccines, however, have diverted scarce funding towards investigations that were never likely to improve understanding of the condition. Greater recognition that diagnosis is the probable explanation for recent autism trends should allow scientists to concentrate on the research that really matters. That can only be to the benefit of autistic children and adults and their families.

Mark Henderson is the Times science correspondent

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