The term comorbidity is of questionable value in reference to developmental disorders: data and theory.

Over the last decade, there has been an enormous increase in the number of studies evaluating the overlap of developmental syndromes or disorders in both children and adults. This overlap of symptoms is often referred to as comorbidity, a term we criticize in this article because of its unsubstantiated presumption of independent etiologies. The premise of this article is that discrete categories do not exist in real life, and that it is misleading to refer to overlapping categories or symptoms as "comorbidities." We illustrate our point by presenting data from 179 school-age children evaluated with rigorous research criteria for seven disorders: reading disability (RD), attention-deficit/hyperactivity disorder (ADHD), developmental coordination disorder (DCD), oppositional defiant disorder (ODD), conduct disorder (CD), depression, and anxiety. Fully 50% of this sample met the criteria for at least two diagnoses. The children with ADHD were at higher risk of having at least a second disorder compared to the children with RD. Overall, the high rates of overlap of these behavioral, emotional, and educational deficits in this broadly ascertained sample support the idea that the concept of comorbidity is inadequate. We discuss the concept of atypical brain development as an explanatory idea to interpret the high rate of overlap of developmental disorders.