The invention features methods and systems for the diagnosis of ADHD and related disorders. The methods and systems of the invention can also be used to ascertain how much benefit an individual would derive from a particular therapy. One method of diagnosing ADHD or a related disorder in a subject comprising the steps of: (i) providing motor activity data having been collected by using a motion analysis device to record movements of said subject (ii) analyzing said motor activity data to calculate a value for a measure of inhibitory control in said subject and (iii) on the basis of said value, diagnosing said subject.
