“Could it perhaps be related to the increases in various kinds of air pollution? Water pollution? Pesticides or herbicides in our foods?”
The West is on a decreasing trend of all those kind of pollution since the 70s.
“Or even the dramatic increase in EM signals being broadcast everywhere?”.
It’s funny people are entertaining the hypothesis that EM radiation causes autism in the same conversation where they are trying to assure you that a drug that’s increasingly taken by pregnant women and that is proven to pass the placenta, is 100% harmless and can’t have anything to do with increased levels of autism.
To be clear, I don't think that's a likely factor. I mention it merely because it is one of the dozens of factors that have changed over the past few decades that have nothing to do with the pharmaceutical industry.