The DSM-5 Children’s Working Group recognizes that current research confirms that the clinical phenotype of “classic” adult bipolar disorder (BPD) is clearly present in adolescents and preadolescents, but rarely in younger age groups. Furthermore, a striking phenomenon in the field of child psychiatry in the last decade is the increasing number of children diagnosed with BPD, e.g., the number of BPD-related disorders diagnosed in child psychiatric clinics in Sekiguni increased nearly 40-fold between 1994 and 2003. Therefore, the DSM-5 Children’s Working Group proposed that BPD be viewed from a developmental perspective in the DSM-5. Are severe, non-episodic outbursts of irritability and temper, and high levels of irritability clinical phenomena a form of BPD? Or is it a pathophysiological process of anxiety, depression, and ADHD? To answer this question, the group defined “severe, non-episodic irritability and temper outbursts, and high irritability” as severe mood dysregulation (SMD) and compared it to typical BPD diagnosed on the basis of DSM-4 and followed it up. The study found differences in family history, gender release, outcome, and pathophysiological mechanisms between SMD and BPD. For example, parents of children with SMD have more history of SMD than BPD; the general male-to-female ratio in BPD should be 1:1, but the current high proportion of boys reported in pediatric BPD is 66.5%, and it is speculated that the possible reason for the high proportion is related to SMD admixture, as the proportion of boys with SMD is 77.6%. Functional magnetic resonance studies have shown that SMD has a different activation pattern than the amygdala in BPD, and also that the activation pattern of SMD is different from that of children with ADHD. Therefore, a new diagnosis was added to the DSM-5 and renamed: disruptive mood dysregulation disorder (DMDD), which is classified as a “depressive disorder”. DMDD is characterized by severe, recurrent temper outbursts in response to general stimuli, which are characterized by disorganized speech and behavior, such as verbal impulsivity and destruction of objects. These reactions deviate significantly from the intensity and duration of the stimulus. B: These responses were not consistent with developmental level. C: Frequency: Temper tantrums averaged 3 or more times per week. D: State of mind between temper outbursts, almost every day, the state of mind between temper outbursts is negative (irritable, angry, or/and sad) and the negative emotions can be detected by others (including parents, teachers, and peers). E: Duration: The above A-I D criteria have been present for at least 12 months, with no more than a 3-month interval during which the above symptoms are not present. F: Temper tantrums in at least two different locations and settings (home, school, and peers) for the above A-D symptoms, and one of them must be severe. G: Actual age of at least 6 years (or equivalent developmental level). H: The age of the initial outburst was before 10 years of age. I: History reflects or clinical observation that the duration of prominent symptoms has never exceeded 1 day. J: Within the past few years, the child’s mood has not been unusually high for more than 1 day, and the unusually high mood is consistent with the “B” criteria of onset, exacerbation, and mania (exaggerated, inflated ego, reduced sleep requirements, incessant, racing thoughts, shifting attention to the situation, increased goal-directed activity, or excessive J Abnormally high emotions need to be distinguished from high moods during positive life events or off-beat expectations. K: Behavioral abnormalities are not specific to psychiatric or affective disorders (major depressive disorder, dysphoric disorder, bipolar disorder) and are not better diagnosed with some other psychiatric disorders (e.g., pervasive developmental disorder, posttraumatic stress disorder, separation anxiety) (Note: this diagnosis can coexist with oppositional defiant disorder, attention deficit hyperactivity disorder, conduct disorder, substance abuse) These symptoms do not depend on physiological reactions to substance abuse, or neurological conditions.