Autism is a developmental disability affecting emotions and behaviour. Those with the condition have difficulty communicating and interacting with others - even those close to them. Children with autism may appear emotionally cold and loners. They are often insensitive to others, and tend not to seek comfort or reassurance from their parents like other children. Communication is kept to a minimum. Some will have significant learning disabilities, while a few show exceptional talent in specific areas (such as feats of memory or calculation).

Forty years ago studies put the prevalence of autism at less than 1 per 1,000 of the population, but more recent research suggests that today's figure is closer to 1 in 100 - around 500,000 people in the UK. But the rise may not be quite as dramatic as the figures suggest. First, we are not comparing like with like - doctors now talk about a “spectrum of autistic disorders”, which includes far milder forms (such as Asperger's syndrome) than those used in the studies carried out in the Sixties. Second, better awareness among health professionals, teachers and parents has meant that more cases are being picked up. But that's not the whole story.

While more inclusive criteria and better diagnosis accounts for the lion's share of the increase, there are other, as yet unidentified, reasons behind the rise.

Continuing research is centred on the combination of genetic susceptibility (autistic spectrum disorders may run in families and are four times more common in boys) and possible environmental triggers - such as exposure to viruses (not the MMR vaccine) and dietary factors. Reassuringly for parents, there is no evidence that it is anything to do with how children are brought up.