All psychiatric diseases are like blood pressure. They are on a continuum without gaps. If one administered questions to identify any condition and rate its severity in a large enough group of people, one would find someone occupying every value from zero to the maximum score.
The problem in psychiatry is that our entire diagnostic system is based on the idea that mental illnesses are like cancer and not like blood pressure. The DSM provides minimum criteria for each disorder, which means that no matter how close you are to having any given condition, if you fall below the line you technically don’t have it.
I suspect you can see the problem immediately. If nature does not provide clear guidelines for where normal stops and mental illness begins, how does the line get drawn? The quick answer is that it gets drawn in much the same way the voter redistricting lines get drawn: based on some data, a lot of fighting, and finally some not entirely satisfactory compromise.
So back to autism. Like all other mental disorders, it runs along a spectrum from people who most of us would have called nerdy when I was a kid to people who spend their lives unable to speak, rocking back and forth for hours on end. We all agree that the silent, rocking folk are ill and need care. But where does extreme nerdiness and social awkwardness give way to Asperger’s syndrome? When is someone autistic enough to deserve the label?