I have gradually become less certain of the answer to this question. At this point, my short version is “I have no idea!”.
The most accurate answer is “autism is the label given to people who fit certain behavioural traits”. That’s so circular that it’s pretty much meaningless, but it’s also the only answer that really means anything.
Autism was originally defined based on behaviour, by two separate researchers who described a small number of individuals fitting certain traits. Since then, it’s continued to be defined based on behaviour, although the actual criteria have changed a lot over time.
When something is defined by appearance, it’s easy to automatically assume that there’s something ‘underneath’ that sums it all up. The most pleasing and logical explanation is that autism is caused by one thing, one difference in brain structure or growth or biochemistry – and that every autistic person has that same underlying thing. The trouble is, there’s actually no evidence for that. Lots of people have tried to find it, but no-one has succeeded. There are vague bits and pieces that autistic people tend to have certain brain differences, or that most autistic people share a certain cognitive trait, or that there’s a correlation between autism and some biochemical process. But if something can’t be shown to apply to every autistic person, then it can’t be considered the underlying ‘thing’.
Of course it’s still possible that there is one underlying thing, and we just haven’t been able to find it yet. But it’s also possible that there’s not, and that there are multiple different underlying things which can result in autistic traits. That seems plausible based on the fact that no-one’s been able to find something that’s consistent across all autistic people. But it also raises the question of why and how the same set of traits can arise from various completely different causes.
So, there may not be one underlying thing. And there has never been a perfectly consistent set of behavioural traits. And yet, we still act like autism is ‘something’ and that we all know what it means! This kind of uncertainty makes me feel like I shouldn’t be writing about autism at all. How can I write about something when I don’t know what it is? But then I remind myself that no-one else knows what it is either, and they’re all still writing about it.
I think it’s pretty likely that there are subtypes of autism. Although I definitely don’t think those subtypes correspond to the functioning levels or the autism/Asperger’s distinction that is so popular. If they are defined by anything, my guess is they’re defined by the types of cognitive processes a person has, which is probably influenced by whatever is the underlying cause of their particular autism. I have spoken to autistic people who I strongly relate to, and others who feel almost as different from me as NT people are, as well as a wide range in between. And those groups do not remotely correspond to whether a person is considered high or low functioning, whether they can speak, or what diagnosis they might have.
But – at least for the moment – it’s useful to have a name for this big overall group of people who tend to have a lot of things in common. Until we have a better idea of what the subtypes are (if there are any), or until we are accommodated so well that we don’t even need a label, ‘autism’ is handy. Better to have a vaguely defined label than none at all. I don’t know what we mind find out about autism in the future (especially once researchers stop wasting all their time and money on trying to cure it, and start learning actually interesting and useful things). But for now, as a scientist, all I have to go on is the best available evidence and hypotheses.
So, I guess I’ll just carry on writing about this thing which I don’t remotely understand, which I can’t possibly define, which no-one is able to make sense of, and which is somehow still a hugely important part of my life.