THE REBECCA SCHOOL for autistic children occupies all five floors of a building in midtown Manhattan. Its rooftop playground has a fine view of the Empire State Building. It features colourful classrooms and lots of places for children to lie down and recover from the sensory overload often suffered by autistic people. “My body doesn’t feel safe,” says one boy curled up in a corridor, asking to be left alone.

Sufferers from autism focus inward, sometimes so much so that they are unable to speak. Some children at the Rebecca School point to pictures to express anything from emotion to the need to go to the toilet. In about half of all cases autism is associated with learning difficulties; in the other half sufferers’ IQs are average or higher. Autism is much more common in boys than in girls; four-fifths of the pupils at the school are male.

To an autistic child, the world seems “relentlessly unpredictable and chaotic, perpetually turned up too loud, and full of people who have little respect for personal space”, writes Steve Silberman in “NeuroTribes”, a forthcoming book about the syndrome. Autism has been around in various guises for a long time, but seems to have become more common in the past couple of decades. In America, the Centres for Disease Control and Prevention, an agency of the federal government, says that one child in 68 has at least a touch of autism, a considerably higher figure than a decade ago. Most studies suggest something closer to one in 100, a rate that is fairly consistent across different countries.

The apparent increase in numbers has prompted competing theories about its cause. For a few decades after the second world war autism was thought to be caused by bad parenting. In 1944 John Bowlby, a British psychiatrist who worked with children separated from their parents during the aerial bombardment of London, published a study called “Forty-Four Juvenile Thieves”, based on a group of tearaways in Canonbury, a part of north London. Bowlby found that almost all the children in the group who seemed incapable of affection had suffered a prolonged separation from their mothers at a young age. Leo Kanner, an Austrian-American psychiatrist who first identified autism in 1943, pointed to unaffectionate “refrigerator mothers” as the cause of the trouble.

Putting the blame for autism on mothers is, thankfully, no longer credible—but neither are many other explanations for its steep increase. Some parents of autistic children believe their problems were caused by exposure to mercury as a fetus; others put their children on an overpromising diet known by its acronym, DAN, which stands for Defeat Autism Now. Assortative mating, the increasing tendency for people from similar professional backgrounds to marry each other, may have contributed to the rise. But this seems unlikely to be the whole story.

Like most syndromes, autism involves a genetic component, but on its own that does not explain much, since about 500 different genes seem to play a part. Simon Baron-Cohen of Cambridge University argues that the level of testosterone to which a fetus is exposed in the womb is important. To test this theory, his team made use of Denmark’s unusual practice of storing amniotic fluid when a pregnant woman has undergone tests, linking this information with the country’s national register of people diagnosed with psychiatric symptoms. They found a correlation between autism and elevated levels of testosterone in the womb. “Nature was giving us a clue,” says Mr Baron-Cohen, pointing to the male bias in the incidence of autism. But if that is indeed a cause, it does not explain why autism seems to have become more common. It may be that families, doctors and schools are simply noticing it more and are less reluctant to seek a diagnosis. Blaming refrigerator mothers for their children’s symptoms probably acted as a disincentive to speaking up, as did some of the earlier treatments, including the use of electric shocks. Now the condition carries less of a stigma. One reason why autism seems to be on the increase is that the diagnosis these days takes in some children whose symptoms would once have been described in more demeaning terms. Nobody is labelled as mentally handicapped or retarded any more. The portrayal of autism in books and films, which sometimes suggests that it bestows special mental powers, has increased awareness of the condition and made it seem less frightening. High-performance autism gives hope to parents of all autistic children, even if many of them will never pass an exam. The notion of a spectrum that covers all autistic people may also be comforting, though many people think that what is now called autism actually represents lots of separate disorders. A modest dose of autism may even be a good thing. Those with the condition sometimes have a special facility with numbers and patterns. Mr Baron-Cohen’s team found that rates of autism in the Dutch city of Eindhoven, the home of Philips, a big electronics company, and therefore a place with lots of engineers, were several times as high as in nearby Utrecht and Haarlem. In Silicon Valley Mr Silberman noticed so many autistic children of software engineers that he dubbed it “the geek syndrome” in an article for Wired. SAP, a German software firm, announced in 2013 that it was hoping to recruit autistic employees. A company spokesman says the new workers are doing well. Moreover, since autistic people often do not pick up on things that are implied rather than said, they have obliged other members of their teams to be more direct.

Perhaps because of these useful traits, autism, of all the disorders listed in the DSM, is the one most closely associated with the idea that mental illness may be a variation from the norm rather than an aberration. Among those who claim to represent autistic people, this is a cause of friction. The biggest autism charity in America, Autism Speaks, views the condition as a disorder in search of a cure. The British branch of Autism Speaks is not sure that a cure is desirable, and parted company with its transatlantic parent over this difference.

Pay attention, boys

The rise of attention deficit hyperactivity disorder (ADHD) supports the idea that rates of diagnosis can depend on all sorts of things that may have little to do with what is going on inside a child’s head. Nearly one in six boys in America under the age of ten has been diagnosed with ADHD, and a majority of these children take medication for it. Though in its current incarnation it is an American invention, it has since spread throughout the developed world, where around one child in 20 is now diagnosed with the condition.

Like autism, ADHD in some form or other has been around for a while. In 1902 George Still, a British paediatrician, described a set of children who were “passionate, deviant, spiteful and lacking inhibitory volition”. The counterintuitive notion that children who find it extremely difficult to concentrate can benefit from amphetamine-like stimulants first emerged half a century ago. But the surge in ADHD in America can be traced to three events. The first was the inclusion of attention deficit disorder (the hyperactivity came later) in the third edition of the DSM in 1980. The second was the development of a stimulant that could be taken once a day and had few side-effects. (Previously children diagnosed with ADD had to take carefully measured doses of stimulants several times a day.) The third was the lifting in 1997 of restrictions on advertising medication directly to customers.

Perhaps oddly, ADHD is much more common in some parts of America than in others. The rate of diagnosis in North Carolina, for example, is two-and-a-half times higher than it is in California. One possible explanation, put forward by Stephen Hinshaw and Richard Scheffler in their book, “The ADHD Explosion”, is that much of the marketing budget for Ritalin, the best-known medication for the disorder, was spent on magazine advertising. That matters because more people in the American South than elsewhere subscribe to magazines.

Bill Hussey, who runs the department of Exceptional Children’s Services in North Carolina, has seen this at first hand. “When the kids who truly have ADHD are put on medication, the difference is night and day,” he says. But too many children are diagnosed and given medication when their real trouble is that they have arrived in the school system unable to do the things that formal education requires of them, like sitting still and concentrating. Besides, says Mr Hussey, give the wrong child medication “and they go through the damn roof.” Parents in North Carolina sometimes put pressure on family doctors to medicate their children, he says, and the doctors oblige.

You may grow out of it

In Israel it is possible to get ADHD medication on prescription to help boost performance at work. Neuro-enhancement is also widespread on American college campuses: the abuse of stimulants prompted the Food and Drug Administration (FDA) to crack down on their supply, though the prohibition was short-lived because it also made it harder for parents to get hold of medication for their children. One professor reports that her students have moved on from Ritalin and now take drugs for narcolepsy so that they can work when suffering from a hangover.