“Seriously, it is not even clear that there is an absolute increase, or whether it’s just labeling: a person who would once have been called “a bit weird” or “very awkward” is now “Autism Spectrum.””
So they changed, ‘weird’ to an actual medical diagnosis which requires intensive testing both mental and physical before the diagnosis is made.
So what’s your point?
Is this a trick question, or do you genuinely not understand some simple statistical reasoning?
If your criterion for “tall man” is “6’5” or taller,” you will find fewer tall men than if your criterion is “6’0” or taller.” When you change the definition of something, you change the number of people who meet the definition.
This is what has happened with autism. When the criterion was that the person was utterly unable to function ... the person in a corner, beating his head against the wall, screaming when anyone touched him ... you had few people with autism. Now that the diagnosis is “autism spectrum,” it includes many more people.
There may not, actually, be more (or a greater percentage) of people who are “what used to be understood as autistic.” To determine this, we would have to see much more detailed information than “autism spectrum diagnoses.”