FASHION is a strange thing, and many fields are susceptible to it—not least, medicine. There has, for example, been a vogue (among commentators, if not among doctors) to ascribe the rising number of cases of autism diagnosed over the past couple of decades to childhood vaccinations against measles, mumps and rubella. That this is fashion rather than reality is suggested by the fact that the explanation proffered in Britain has been that such vaccines provoke an immune response that damages the nervous system, whereas Americans have blamed residual mercury in the same vaccines.

It is now pretty well established that vaccination does not create autism. But the rise in the number of recorded cases is real enough. In Britain, for example, the rate of diagnosis has risen from 50 per 100,000 in 1990 to 400 per 100,000 today. That must have a cause. And one popular hypothesis is that this cause, too, is fashion—but among doctors rather than columnists.

Demonstrating that has been difficult. But a paper in this month's Developmental Medicine & Child Neurology, by Dorothy Bishop and her colleagues at Oxford University, goes a long way towards doing so. Dr Bishop reasoned it was unlikely that people now labelled autistic would, in the past, have been thought healthy, but that it was quite plausible they might have been given some other diagnosis. With this in mind, she looked at a group who had been diagnosed as children with a particular condition that was not autism, and rediagnosed them using present-day criteria.

Her volunteers were 38 adults or teenagers who had, as children, been diagnosed as having what is known as developmental language disorder, rather than autism. (The distinction being that although autism involves difficulties in communication it also has other symptoms, such as an inability to empathise with others.) The rediagnosis had two steps. The researchers interviewed the volunteers, of course. But they also interviewed their parents. They asked both parents and offspring questions based on modern tests for diagnosing autism.

The result was that almost a third of her volunteers looked, from the modern point of view, misclassified. Eight fully met the modern criteria for autism. A further four fell into what is known as the autistic spectrum, evincing signs of autism short of the full-blown syndrome.

What were particularly telling were the interviews with parents. Earlier, similar work had explained the observations away as a change in symptoms with age, but the parents of Dr Bishop's volunteers told stories of what are now regarded as autistic symptoms appearing in their children in their early years. Although there is a risk of hindsight colouring such stories, many of them were so vivid that Dr Bishop is convinced they are accurate.

The upshot, subject to larger studies confirming her observations, is that Dr Bishop seems to have confirmed the cause of the recent rise of autism as being a change in the diagnostic criteria. As a good researcher should be, she is cautious and points out that finding one cause is not proof that others are not operating. But it now looks unlikely that there are more autistic people around than there used to be. It is just that it is now fashionable to acknowledge their existence.