Mark Haddon has spoken about this on his website:
unsurprisingly, i'm often asked to talk about asperger’s and autism or to become involved with organisations who work on behalf of people with asperger’s and autism, many of whom do wonderful work. but i always decline, for two reasons:
1) i know very little about the subject. i did no research for curious incident (other than photographing the interiors of swindon and paddington stations). i’d read oliver sacks’s essay about temple grandin and a handful of newspaper and magazine articles about, or by, people with asperger’s and autism. i deliberately didn’t add to this list. imagination always trumps research. i thought that if i could make Christopher real to me then he’d be real to readers. i gave him some rules to live by and some character traits and opinions, all of which i borrowed from people i know, none of whom would be labelled as having a disability. judging by the reaction, it seems to have worked.
He's also said that he didn't consider the idea of autism specifically to be important to his story:
2) curious incident is not a book about asperger’s. it’s a novel whose central character describes himself as ‘a mathematician with some behavioural difficulties’. indeed he never uses the words ‘asperger’s’ or ‘autism’ (i slightly regret that fact that the word ‘asperger’s’ was used on the cover). if anything it’s a novel about difference, about being an outsider, about seeing the world in a surprising and revealing way. it’s as much a novel about us as it is about christopher.
labels say nothing about a person. they say only how the rest of us categorise that person. good literature is always about peeling labels off. and treating real people with dignity is always about peeling the labels off. a diagnosis may lead to practical help. but genuinely understanding another human being involves talking and listening to them and finding out what makes them an individual, not what makes them part of a group.
i passionately believe this and i’ve said it repeatedly in many different forms. to become a spokesperson for those with asperger’s or autism, or to present myself as some kind of expert in the field, would completely undermine this, and make me look like a fool into the bargain. i would much rather spend my time writing more novels, standing up for difference and trying to understand outsiders who see the world in surprising and revealing ways.
(All emphasis is mine in the above quotes; the capitalisation is his.)
The usual caveat that authors aren't always telling the truth about their work does apply here, but I believe this probably is true. Claiming that he hadn't done any research when in fact he had would be a very strange thing to do; I would expect the other way round to be much more likely.
Interestingly, I also found an article
written by someone who actually had Asperger's Syndrome, which criticises Haddon's portrayal and says it's unrealistic:
WARNING: THIS BOOK WILL NOT HELP YOU UNDERSTAND REAL AUTISTICS
Mark Haddon did not use the word "autism" in his book, let alone claim to portray autism accurately. He even spoke publicly against using his book as an autism textbook. Certainly, he was not at fault. I have nothing against Mark Haddon or his work. In fact, I finally understood enough about non-autistic people to enjoy it in 2006.
However, seeing that the Curious Incident of the Dog in the Night-Time has became very popular, I feel that I must explain why it should not to be taken seriously when many people (including teachers and autism professionals) recommend it as reading material on autism. Instead of a fictitious novel, I recommend reading real autism reference books by authors like Dr. Temple Grandin and Donna Williams. They have many accurate and useful first-hand insights on autism.
While reading the Curious Incident of the Dog in the Night-Time, I noted that Christopher is highly self-aware, and could articulate his thoughts so clearly. This was very different from my own experience. At his age, I was still mostly in a state of sleepwalking. I was unaware of my own emotions, body and situational awareness.
I had Asperger's Syndrome and was very high functioning. I went through normal schooling without any help, medication or trouble with the school authorities. The lower-functioning people with autism probably have even less self-awareness than me. However, Mark Haddon could not have had a story if Christopher could not able to convey to us what is happening within him. Thus, his depictions of Christopher's inner state are used to advance his story and interest viewers. They are not meant to represent the autistic consciousness.
This writer goes on at length about the differences between Asperger's Syndrome in reality and the way it's portrayed in The Curious Incident of the Dog in the Night-Time, including by analysing several passages from the book. I'm not going to quote all of that here, but suffice it to say that it definitely supports Haddon's claim of having pretty much no real experience of the autism spectrum.