Beyond Age and Generations: How Considering Period Effects Reshapes Our Understanding of Personality Change

Age, cohort, and period effects are three ways to explain personality trait change over time. While past research mostly focused on age differences, showing relatively consistent patterns, evidence for cohort differences is more mixed, and period differences have hardly been examined. However, age, period, and cohort are exactly collinear (age = period – cohort), such that estimates are likely confounded and always hinge on so-called identification assumptions. Identification assumptions shape substantive conclusions, and inappropriate or inconsistent strategies may explain past discrepant findings. To address this age-period-cohort identification problem in personality change, we leveraged four large-scale (Ntotal >2 Mio) repeated cross-sectional datasets from 2003–2022. 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, 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 change over time.