2026 Theses Doctoral

Birth cohort differences in depressive symptom development during emerging adulthood: Examining mediation by social roles and belief systems

Gimbrone, Catherine

Introduction
Depressive symptoms among individuals undergoing the transition to adulthood in the United States have increased over recent decades. Although cross-sectional studies have documented higher symptom levels among more recent cohorts, less is known about depressive symptom development across emerging adulthood (ages 18 to 30), a critical period of social transition and identity development, and whether cohort differences persist in longitudinal analyses. This dissertation examined whether historical shifts in social role transitions and belief systems explained cohort differences in depressive symptom development during emerging adulthood. This dissertation includes five chapters: an introduction, a structured literature review synthesizing findings from U.S.-based longitudinal studies on depressive symptom development during emerging adulthood, an empirical study examining whether five social role transitions mediated cohort effects on depressive symptom development, an empirical study examining whether religiosity and political beliefs mediated cohort effects on depressive symptom development, and a conclusion.

Materials and methods
The structured review synthesized methods and findings from U.S.-based longitudinal studies examining depressive symptom development during emerging adulthood (N = 16). The two empirical studies used data from Monitoring the Future, a nationally representative longitudinal survey, including 19 birth cohorts (1971 to 1989) followed from ages 18 to 30 between 1989 and 2019 (N = 7,861). Depressive symptoms were measured using a four-item index. Trajectories were estimated using latent growth curve models with birth cohort specified as a continuous predictor scaled to five-year increments, with sex differences assessed using multi-group models. The first empirical study evaluated five time-varying mediators reflecting social role transitions: college enrollment, full-time employment, residence with parents, marriage, and parenthood. The second empirical study evaluated religiosity and political beliefs as time-varying mediators.

Results
The structured review indicated that most individuals maintained low depressive symptoms during emerging adulthood, while a smaller subgroup (approximately 4% to 10%) exhibited elevated or fluctuating trajectories, with higher symptom levels among women in many studies. The review also identified limited evidence on cohort differences in trajectories and a general absence of rigorous cohort comparisons. In the first empirical study, cohort differences showed a reversal across emerging adulthood: more recent cohorts reported fewer depressive symptoms at age 20 but more symptoms at age 30 compared with earlier cohorts. Social role transitions demonstrated modest mediation of depressive symptoms, with marriage partially explaining cohort differences at age 30 (11.73% of the total effect) and full-time employment partially explaining cohort differences at age 28 (9.8%). In the second empirical study, religiosity emerged as the strongest mediator, accounting for 27.6% of the total cohort effect at age 30, with cohort declines in religious engagement contributing to elevated symptoms. Political beliefs demonstrated limited, sex-specific mediation. Contrary to hypotheses, cohort differences in trajectories did not differ significantly by sex. Across models, substantial direct cohort effects persisted.

Conclusions
Cohort differences in depressive symptoms during emerging adulthood reflected the interplay of developmental timing and historical context. While declining religiosity represented a meaningful pathway through which recent cohorts experienced elevated symptoms, the majority of cohort effects remained unexplained, suggesting that other sociohistorical mechanisms likely contributed to cohort shifts in depressive symptom development.