It's not something in the water, it's in the air...most likely related to automobile traffic. And I would not be surprised at a higher incidence of auties in big cities...as to the issue of prevalence in places like China - given the lack of transparency in the social statistics there, I doubt we'll find out any time soon. Also, there may be a different social response to the patterns of neurological development that spectrumites exhibit in non-western cultures. Heck, there are huge differences in the various treatments that autists receive in the West (like the packing treatments in France)...
A study like this might also explain the wide variance in reported levels of incidence. I've seen it go from 1 in 150 back in the nineties, to 1 in 88, all the way up to 1 in 50 nowadays...maybe it's not just who and how we test - but also where the testing occurs. So a sampling in Los Angeles or Beijing would show a higher rate than one in a cleaner more rural environment...