Lifted from Crooked Timber comments

At http://crookedtimber.org/2012/04/01/autism-a-public-discourse-blaming-teachers-and-parents/#more-23859

Well, yes, of course autism is overdiagnosed: what the hell did people think that DSM was for?

The purpose of diagnostic manuals is to ensure that nobody who enters a psychiatrist's office will lack a diagnosis that will enable them to claim the cost back on their medical insurance.

This is perhaps seen most clearly in the diagnosis of Oppositional Defiant Disorder in children, where the criteria are
A pattern of negativistic, hostile, and defiant behavior lasting at least 6 months, during which four (or more) of the following are present:
(1) often loses temper
(2) often argues with adults
(3) often actively defies or refuses to comply with adults' requests or rules
(4) often deliberately annoys people
(5) often blames others for his or her mistakes or misbehavior
(6) is often touchy or easily annoyed by others
(7) is often angry and resentful
(8) is often spiteful or vindictive
Note: Consider a criterion met only if the behavior occurs more frequently than is typically observed in individuals of comparable age and developmental level.

(The note itself might conceivably be helpful if it offered some information on what frequency of these behaviours was typically observed, which of course it doesn't.)

Autism in DSM is a Chinese restaurant menu diagnostic system - two from column one, three from column two, one from column three - that mathematically allow something like twelve thousand different ways to be autistic.

Though, come to think of it, the 'Chinese restaurant menu' idiom is American; here in Australia Chinese restaurants have never had it.  We don't do fortune cookies, either.