The always informative blogger Neuroskeptic has an interesting post on several studies examining the "immaturity hypothesis" that posits that the birth month of a child is associated with the likelihood of receiving a diagnosis of ADHD.  The post begins:

Earlier this year a major study of almost one million Canadian children found that rates of diagnosed ADHD - as well as use of ADHD medications like Ritalin - were higher in kids born later in the year.  This is strong support for the "immaturity hypothesis" - the idea that some children get a diagnosis of ADHD because they're younger than their classmates at school, and their relative immaturity is wrongly ascribed to an illness.