This makes at least 4 studies which have come to the same result. The others are:
William N. Evans, Melinda S. Morrill, Stephen T. Parente. Measuring inappropriate medical diagnosis and treatment in survey data: The case of ADHD among school-age children. Journal of Health Economics. September 2010. http://www.sciencedirect.com/science/article/pii/S0167629610000962 Abstract. Fee for full paper.
Todd E. Elder. The importance of relative standards in ADHD diagnoses: Evidence based on exact birth dates. Journal of Health Economics. September 2010.
http://www.sciencedirect.com/science/article/pii/S0167629610000755 Abstract. Fee for full paper.
Richard L. Morrow et al.. Influence of relative age on diagnosis and treatment of attentiondeficit/hyperactivity disorder in children. Canadian Medical Association Journal. March 2012.
http://www.cmaj.ca/content/184/7/755.abstract?sid=66f4974a-3319-4b24-9d9e-fb275b59cac5 Complete paper available.
The significance of these studies is that they provide ‘clinical’ proof for what is theoretically obvious. Given that ‘ADHD’ is a category of psychiatry, whose diagnosis is determined by a whole range of subjective factors (whims of parents, management needs of teachers etc.) it is inevitable that diagnoses will be skewed. It is an indication of the power or money (and the compliance with corporate money of the media) that these studies get relatively little coverage whereas studies promoting small causal genetic links get vast amounts of coverage. That’s because the latter support the profitable ADHD drugging scam whereas these ones do not.