So if a certain disease is more common in rich people within a country, that's big news because it suggests that something unusual is going on. Autism spectrum disorders (ASDs) have long been known to show this pattern, at least in some countries, but this has often been thought to be a product of diagnostic ascertainment bias. Maybe richer and better-educated parents are more likely to have access to services that can diagnose autism. This is a serious issue because autism often goes undiagnosed and diagnosis is rarely clear-cut.
An important new PLoS paper from Wisconsin's Durkin et al suggests that, while ascertainment bias does happen, it doesn't explain the whole effect in the USA: richer American families really do have more autism than poorer ones. The authors made use of the ADDM Network which covers about 550,000 8 year old children from several sites across the USA.
Autism and Wealth at Neuroskeptic, ht MR