A 9-year-old boy is brought in by his parents for evaluation of severe temper outbursts that occur approximately 4-5 times per week. These outbursts involve screaming, throwing objects, and physical aggression, and are grossly out of proportion to the situation. Between outbursts, his parents describe his baseline mood as persistently irritable and angry throughout most of the day, nearly every day. This pattern has been present for over 18 months and occurs both at home and at school. There is no history of distinct periods of elevated or expansive mood, grandiosity, or decreased need for sleep. His parents deny any substance use. Which feature most strongly supports a diagnosis of disruptive mood dysregulation disorder (DMDD) over pediatric bipolar disorder?