Affect dynamics and depressive symptomatology: Revisiting the inertia-instability paradox.

The inertia-instability paradox poses an intriguing question in depression research: How can the affective experiences of depressed individuals demonstrate both resistance to change and fluctuation? Prior studies examining this paradox have faced limitations, including small sample sizes, analytic approaches prone to biased parameter estimates, and inconsistent results. Using data from 842 adults ( = 54.31, = 13.25, age range: 18-88; 58.2% female) collected over 56 consecutive days, we applied dynamic structural equation modeling to quantify individualized indices of mean levels, variability, instability, and inertia of negative affect. When adjusting for shared variances among affect dynamic measures, depressive symptoms were uniquely associated with both higher mean levels and inertia of negative affect. However, neither variability nor instability demonstrated unique links to depressive symptoms after accounting for the mean and inertia. Findings indicate that greater predictability in day-to-day negative affect is an important dynamic feature of depression. (PsycInfo Database Record (c) 2025 APA, all rights reserved).