Beyond age and generations: How considering period effects reshapes our understanding of personality change.

> 2 Mio) repeated cross-sectional data sets from 2003 to 2022. Our aims were to demonstrate how identification assumptions common in personality studies impact estimates for age, cohort, and period and to use weaker, substantively informed assumptions to narrow down the range of plausible solutions. Results showed that common identification strategies of constraining one temporal effect to zero can dramatically affect conclusions-less for age-graded, but more for generational differences. Using weaker assumptions, our results indicated that all three factors-age, cohort, and period-likely contribute to trait differences over time. Assuming age-graded change in a certain direction revealed cohort-related decreases in extraversion, openness, and neuroticism and increases in agreeableness, alongside period-related increases in extraversion, openness, and conscientiousness. This suggests that several previously assumed cohort differences may actually be driven by period effects, overlooked due to strong identification assumptions. Overall, highlighting the importance of appropriate identification strategies, our results offer unique insights into factors driving trait differences over time. (PsycInfo Database Record (c) 2026 APA, all rights reserved).