Following from the Hunter-versus-farmer theory, as with many conditions in the field of psychiatry, ADHD can be explained as a social construct rather than an objective 'disorder'.
In this view, in societies where passivity and order are highly valued, those on the active end of the active-passive spectrum may be seen as 'problems'. Medically defining their behavior (by giving a label such as ADHD) serves the purpose of removing blame from those 'causing the problem'.
Showing that people with ADHD show differences from others by the use of medical tests such as PET scans does not answer the social constructionist view since such differences could be expected to be found in people at one end of any behavioral spectrum. Nor does evidence of successful treatment undermine the social constructionist view; for example the American National Institute on Drug Abuse reports that Ritalin is abused by non-ADHD students partly for its ability to increase their attention. To undermine the social construction explanation, it would need to be demonstrated that ADHD is a true disorder (rather than a collection of traits) and that there is a distinct sub-group who clearly fall into the category. Neither proof yet exists.