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Abstract
The diagnostic criteria for attention-deficit hyperactivity disorder (ADHD), a behaviorally defined disorder, have been changed several times since first established in the second edition of the Diagnostic and Statistical Manual of Mental Disorders. In the current edition, the diagnosis includes three reportedly distinct subtypes. Despite considerable research, there remains little evidence of the diagnostic validity of three separate ADHD subtypes. Several posited explanations for problems delineating consistent differences between ADHD subtypes are addressed. This article considers the underlying factors thought to contribute to ADHD and evaluates the relevance of categorical and dimensional diagnostic approaches.