The main justification for creating the DMDD category appears to be its role in resolution of one of the most contentious recent controversies in child psychiatry, namely, the characterization of pediatric bipolar disorder. From the mid-1990s through the early 2000s, there was a dramatic rise in clinical diagnoses of bipolar disorder among youths that occurred in parallel with academic debates on the nature of juvenile mania (3). To examine the issue, some researchers proposed a differentiation of narrow and broad pediatric bipolar phenotypes (4). The narrow phenotype was defined using classic criteria for mania or hypomania, including discrete episodes of grandiosity and euphoria. The broad phenotype, later called severe mood dysregulation, was defined by chronic, non-episodic impairment that lacked hallmark mania symptoms of grandiosity and euphoria, but was typified by severe irritability and hyperarousal. Subsequent longitudinal research demonstrated that episodic irritability in childhood predicted classic adult bipolar disorder, while severe mood dysregulation and dimensional measures of chronic irritability predicted unipolar depression and anxiety (5–7). Other research demonstrated distinct patterns of brain underactivity and impaired emotional responses in youths affected by severe mood dysregulation (8, 9).