The Diagnostic and Statistical Manual of Psychiatric Disorders (DSM) represents a categorical approach to behavioral diagnosis in which a person is considered to have or not have a disorder based on whether he or she meets certain behavioral criteria. This is distinct from a more dimensional approach that views behaviors along a continuum [6]. This diagnostic system is currently in its fourth edition (DSM-IV; a fifth edition is scheduled for release in the spring of 2013). The DSM as a categorical approach defines disorders on the basis of a set of symptoms. In a trenchant critique of this categorical approach, Hyman observes, “In the case of the DSM system and its progeny, the unintended reification of diagnostic entities is facilitated by the lack of a developed scientific base, combined with the wide embrace of a classification system developed, above all, to foster interrater reliability” (p. 159) [7]. A goal of this kind of diagnosis has been to reach consensus among various diagnosing individuals about whether or not the person in question meets criteria for a particular unitary diagnosis, while ruling out other potential diagnoses. Unfortunately, the DSM V, which is on the cusp of being published, has been criticized for its lack of interrater reliability [8, 9]. Clearly, it is important to understand the process by which classification takes place.

Classification involves ordering or organizing concepts or symptoms based on their contiguity and/or similarity [10]. This process is theory driven; a classification system should be organized along the lines of an acceptable, recognizable, definable, coherent theory that explains why symptoms occur. Identification involves assigning cases to the classification criteria. These are distinct processes. Identification emerges from classification. In ADHD, however, the classification was initially based on behaviors observed in clinically identified cases. It was not based on a coherent theory. The reasoning was circular: Children who presented with inattention, impulsivity, and hyperactivity were presumed to have ADHD. A theory to explain why these behaviors occurred together was absent.