When emotions persist: Emotional inertia predicts adolescent depression amid COVID-19 stress.

Emotional inertia-the extent to which emotional states persist over time-has been identified as a risk factor for depression. However, longitudinal evidence from real-world chronic stress remains limited. Leveraging the COVID-19 pandemic as a naturalistic stressor, this study examined: (a) how emotional inertia changed across distinct phases of the pandemic, and (b) how changes in positive versus negative emotional inertia related to depression risk. A total of 140 adolescents (Mage = 11.91, 47.9 % girls) completed ecological momentary assessments across three stress phases: pre-pandemic (one year before pandemic onset), acute pandemic, and chronic pandemic (one year after pandemic onset), yielding 7465 observations. Hypotheses and data analyses were preregistered. Latent change score modeling showed that both positive and negative emotional inertia increased from pre-pandemic to acute pandemic, then decreased from acute to chronic pandemic. However, positive and negative emotional inertia showed distinct associations with depressive symptoms. Increases in positive emotional inertia associated with fewer depressive symptoms at Wave 3, whereas increases in negative emotional inertia associated with more depressive symptoms at Wave 3. These associations remained significant even after controlling for baseline (Wave 1) depressive symptoms. These findings suggest that persistent negative emotions may amplify vulnerability, while stable positive emotions confer resilience in the face of stress. Emotional inertia may thus serve as a key marker of stress reactivity and an early intervention target for youth mental health.