How can you know? To prove that diagnoses have gotten better, rather than there being a true increase in autism, you'd have to know what would have happened to today's kids 20 years ago. Would they have been diagnosed with autism? Or ADD? Nothing?
Well, there is a way to tell where more diagnoses of lots of things will happen, and here in California we knew it anecdotally for years. If you want to find a hotbed of anti-vaccine sentiment and anti-GMO sentiment and pro-astrology belief, take a protractor and draw a 10 mile radius around a Whole Foods. Prior to the autism epidemic we saw some teachers in California classrooms who had 30% ADD students. Why? The teachers believed the children had ADD and parents took them to a doctor for it.
But Neuroskeptic writes about a study that did something a little more rigorous than the 'our weather is nice so we have to put up with anti-science crackpots' head-shaking normal people here do. A group examined a zone of high autism prevalence in California, areas where kids aged 0-4 years old are more likely to be diagnosed with the condition. The epicenter was Los Angeles, with overlapping hot spots around it. In those clusters, and the autism rates are between 2 and 6 times higher than the rest of the state.
Something in the water?
Well, maybe, but unlikely in North Hollywood. Just like around a Whole Foods, the area is rich in money. But these hotspots also had a high share of autism advocacy organizations and pediatricians. More pediatricians and autism awareness campaigns would certainly mean better diagnosis and not necessarily more autism.
What the study found is that children born outside the clusters, who later moved into them, had a higher chance of getting a diagnosis, consistent with the idea that the clusters are clusters of diagnosis, not autism.
Of course, a researcher can never win with a topic like this. The first commenter said what I thought while reading it - it could be explained away as being that people who were worried about autism moved to an area well-known for helping kids with autism.
Probably best to leave a comment over there if you want it answered, that is why I put it in the links section rather than my blog.
Finally, Hard Evidence Against The "Autism Epidemic"? - Neuroskeptic