The only logical inference

From an NPR story on how a recent UC Davis study showed that autism diagnoses are significantly higher in upper income, well educated, white communities:

The California results also show how widely autism diagnosis rates can vary from place to place, Novella says. In some areas of the state, children were four times as likely to be diagnosed as in other areas.

That suggests that in many areas there are still a huge number of children with autism spectrum disorders who are slipping through the cracks, Novella says.

I can think of at least one other possibility that the data suggests.