Others can't help but self-diagnose. Australian poet Les Murray has it, as has MONA founder David Walsh and musician David Byrne ("I was a peculiar young boy - borderline Asperger's, I would guess"). Comedian Dan Aykroyd says he was diagnosed with Asperger's as a boy, despite the fact that the diagnosis didn't exist until 1981, when Aykroyd was 29. Fictional characters are not immune: a blogger on the ABC website recently suggested that Hugo, the destructive brat from the novel and TV series The Slap, may have had Asperger's. Even inanimate objects are afflicted: recently, Fairfax columnist Mike Carlton referred to The Australian as "obsessive, bombastic, endlessly repetitive" - in other words, "a newspaper with Asperger's". How did we get here? Since when did a serious neurodevelopmental disorder go from esoteric to everyday? Are we all, indeed, "on the spectrum"? Or have we all just gone a little nutty over Asperger's? Every age has its defining psychiatric malady. In the 1850s it was hysteria, a nervous disorder thought to afflict one quarter of all women, and for which the most effective treatment was genital stimulation. (The condition, which is no longer recognised, gave rise to the vibrator.) A decade ago, every fidgety kid had ADHD. Until recently, if you liked to keep your coffee table books neatly stacked, you had OCD. And, of course, no-one is just moody any more: they are "like, seriously bipolar". But none of these terms have resonated quite like Asperger's, a condition first described by Viennese paediatrician Hans Asperger in 1944. Asperger had noted four boys in his practice who had difficulty integrating socially. Their intelligence appeared normal, but they lacked empathy and nonverbal communication skills, and were physically awkward. Asperger published just a single paper, in German, about the condition, which he dubbed "autistic psychopathy". His observations were not widely known until 1981, when British psychiatrist Lorna Wing published case studies of children showing similar symptoms, which she called "Asperger's syndrome" (AS).

AS became a distinct diagnosis in 1992, when it was recognised by the World Health Organisation; in 1994, it was added to the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), the American Psychiatric Association's diagnostic reference book. But even then, specialists had trouble differentiating Asperger's from high-functioning autism. Both have similar symptoms - restricted and repetitive behaviours, social difficulty, communication problems - strung out on a continuum of severity, or "spectrum", that ranges from head banging and faeces smearing at the extreme end to shyness and shoe-gazing at the other. Not surprisingly, the more doctors looked, the more autism they found. Before 1980, one in 2000 children was thought to be autistic. By 2005, one in 150 Australian children had an autism spectrum disorder. The Centres for Disease Control, in the US, now believes one in 88 children has an AS disorder (or one in 54 if you only count boys, who are five times more susceptible than girls). In Queensland the rate is one in 50, while the South Koreans put theirs even higher, at one in 38. Experts have parsed every possible cause for this rise, from video games and atmospheric lead levels to emotionally distant mothers, older fathers, vaccinations and fatty foods. In some instances, an AS-diagnosis in childhood will attract extra help at school, leading to allegations, especially in Queensland, of parents "diagnosis hunting". But most people now agree that the increase is due not to a rise in incidence but to greater awareness; an epidemic of discovery, then, not disease. For some, however, Asperger's is almost an inevitability, the eccentric fruit of our hothouse age. "Open-plan offices can make things worse," says Dr Julie Peterson, a clinical psychologist who works in Pymble, on Sydney's north shore. "There is more movement, there are telephones going off, people are sitting close and autistic people can feel the energy. They have like a sixth sense - if someone is tense, they can feel that - and so it's harder for people to settle and concentrate."

For Peterson, who is also a trained sexologist, all roads lead to Asperger's. Her husband has it, as does her father. Albert Einstein had it, she says, as did Charles Darwin and Beethoven. Steven Spielberg has it and Bill Gates has been linked to it, too. Pymble, where she set up shop eight years ago, is an "Asperger's hot spot - a high-functioning area with lots of people of above-average intelligence". When I mention that she actually makes it sound like a good thing, Peterson pauses, nonplussed. "It is," she says. "It's a gift!" The idea that having a lifelong neurological disorder is a positive can be traced, like so much of the strangeness of contemporary culture, to Tom Cruise. In 1988, Cruise starred in the movie Rain Man, as Charlie Babbitt, a shallow, selfish yuppie who is reacquainted with his humanity courtesy of his autistic older brother, Raymond (Dustin Hoffman). Rain Man popularised the idea of the autistic as beatific savant: Raymond's truncated outbursts have a koan-like quality, retarded yet profound. He is funny, but more importantly, he is, in his own way, cool: at one stage he uses his mathematical genius to help Charlie count cards in Vegas. This unlikely seed has grown ever since, nurtured to full bloom by the tenor of our incessantly nerdy times. Ours is an information age, ruled not by jocks, supermodels and other "neurotypicals" but by millionaire geeks and rock-star hackers, by agoraphobic software coders and pasty-faced file-sharers like Kim Dotcom (Twitter followers: 293,149). In an increasingly asocial world, where people would rather text than talk, the rise of Asperger's as a cultural epidemic makes perfect sense. These days, to some, the term has become an intelligent-sounding shorthand for anything quirky, unconventional or awkward. A bit of a loner? Asperger's. Like vegan food and doing Lego? Asperger's. Got red hair and read Philip K. Dick? Asperger's. Couple this with the popular perception that Asperger's is typical of the high-functioning closet genius, and you have the perfect hipster headset. As the musician Moby told The New York Times in 2010, "I just like to pretend [I have Asperger's]. It makes me sound more interesting."

Part of the problem is the fuzziness of the term itself, which both invites and sustains appropriation. "Asperger's is a convenient explanation for all sorts of things," says Professor Stewart Einfeld, a child psychiatrist and senior scientist at the University of Sydney's Brain & Mind Research Institute. "We see quite a bit of quasi-professional overdiagnosis, where people who really aren't qualified will describe any child who is a bit different or strange or has problems with social interactions as 'autistic' or 'Asperger's'." According to Einfeld, some people use the term as "an excuse for all sorts of unacceptable behaviours". ("You don't ... have Asperger's," a doctor tells Hugh Laurie's famously abrasive character in an episode of House. "You wish you did, it would exempt you from the rules, give you freedom ... let you date 17-year-olds!") There is also, according to Einfeld, the human tendency to "pathologise the other", especially in relationships. "Women who have difficulties with their partners, they do it quite a bit. If the man isn't communicating well or is having difficulties interacting, Asperger's becomes a feasible explanation." Recently Einfeld saw a 24-year-old man who came to his practice, accompanied by his parents. "This young man had lots of problems relating to others," Einfeld says. "And he told me that he'd recently discovered that he was Asperger's. He'd found this group of people with Asperger's on the internet and was communicating with them, and had decided that he had it. But when I listened to his story, I decided that he didn't have Asperger's but social anxiety disorder. People with social anxiety disorder don't have problems in communication and empathy and understanding emotions, they just have a fear that people will judge them adversely. So I said to him, I don't think you have Asperger's, and he burst into tears, and the parents got very angry with me. The father said: 'He has finally found this group of people he can talk to, an explanation for his problems, and you've taken it away from him.' " The young man and his parents stood up and stormed out, and that was the last Einfeld saw of them. The diagnosis of Asperger's seems to have hit a peak moment with "Aspie Pride", a fringe movement that regards Asperger's not as a disability but a beneficial evolutionary adaptation that has facilitated everything from the Stone Age tool revolution to Silicon Valley. Not surprisingly, Aspie Priders reject calls for a cure. "What would happen if the autism gene was eliminated from the gene pool?" scientist, author and autism advocate Temple Grandin wrote in The Way I See It: a Personal Look at Autism and Asperger's. "You would have a bunch of people standing around in a cave, chatting and socialising and not getting anything done."

And yet, claiming that people such as Bill Gates or Steven Spielberg have Asperger's is not really that helpful, either, since most Aspies will never direct an Oscar-winning movie nor remake the world of personal computing. Many, like Ben, a 13-year-old boy from inner Sydney, are flat out trying to get to school on time. I first meet Ben - or rather, hear Ben - while sitting in a waiting room talking to his psychologist, Steven Den-Kaat. As I would later learn, Ben had had an altercation with his mother in the car on the way to the clinic. By the time he arrives, he's in full meltdown - crying, hyperventilating and utterly unable to communicate. "People with Asperger's lack the ability to intuitively interpret other people's body language and facial expressions," Den-Kaat explains, studiously ignoring the noise Ben is making in the corridor. "They also have difficulty seeing other people's perspectives. Ben, for instance, has trouble accepting things are different to the way he sees them, and that can lead to frustration." When I ask why he hasn't gone out to calm Ben down, Den-Kaat explains that it is better to "leave him alone, to regulate the emotion that he has now, without rushing out and asking him to talk about it, which is something he finds difficult anyway". Dressed in grey trousers and a white shirt ("Asperger's people experience high sensory sensitivities, so you want to keep it plain"), Den-Kaat eventually introduces me to Ben, who has agreed to let me sit in on a therapy session. A handsome boy with short spiky hair and anxious eyes, Ben greets me with incongruous formality, like a graduate at a job interview. Kneeling on the floor, Den-Kaat then produces a piece of butcher's paper and a box of textas, and has Ben draw, in the form of a comic strip, the fight he had with his mum in the car.

As Ben works away, Den-Kaat gently prompts him: "What happened next?"; "What did she say then?"; "What was she feeling?" Slowly, a picture emerges of a human interaction, with causes and consequences, where two separate perspectives - Ben and his mum's - coexist, side by side. It's an emotional mud map, a manual really, showing all the nonverbal elements of communication - thoughts, feelings, actions, emotions - and how they go together. "We all say I am irrational when I'm upset," Ben says. "So this helps me to see things more clearly." Since they find it hard to use their intuition and to generalise from one situation to another, people with Asperger's commonly work off a strict set of rules or scripts. For this reason, they find it hard to deal with change. But change is precisely what they are about to get. In May, the American Psychiatric Association (APA) will publish its fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-V), in which the term Asperger's will be notably absent. Instead, all diagnoses of autism, of which Asperger's is currently thought to be a subset, will be folded into one overarching category, autism spectrum disorder, and rated from mild to severe. Some believe that by removing Asperger's as a distinct disorder, the APA is attempting to curb its rampant over-diagnosis. (The APA says the new criteria will "lead to more accurate diagnoses".) Whatever the reasons, the decision has caused a storm on Aspie internet forums, where contributors worry that changing the diagnosis will result in a loss of identity. (If Asperger's doesn't exist, does that mean you are not an Aspie? Have you ever been? If not, what are you?) Some simply don't like the idea of being associated with a lower-functioning disorder like autism when, in fact, they see themselves, as one writer put it, "as high-functioning but eccentric". Meanwhile, practitioners like Den-Kaat worry about the implications for accessing appropriate funding and support services. But we can be sure of one thing: the term Asperger's will not disappear from our cultural landscape. People like Moby are unlikely to pore over the latest DSM revisions. More to the point, the term has taken too deep a root. How else to describe that not-quite-right nephew with the weird laugh and Doctor Who fetish? What other term applies to your jerky ex-boyfriend, the one who used to put name tags on all his food? No, the "spectrum" is out there now; it has absorbed us all.

Asperger's is dead. Long live "Asperger's". See how you rate on "the spectrum" by filling out a comprehensive questionnaire. Take the test here.

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