Trap versus trade-off: cohort evidence on overeducation, income returns, and group disparities after China's higher education expansion.

Background: Higher education expansion is a pivotal strategy for fostering economic growth and national competitiveness. Yet, its consequences for social equality-particularly through the mechanism of overeducation, where educational attainment exceeds job requirements-remain contested. Objective: This study investigates how the 1999 mass higher education expansion in China reshaped the incidence, income returns, and group disparities of overeducation by employing a theoretical framework of 'trap versus trade-off.' Methods: Data were sourced from the Chinese General Social Survey (2010-2021). The study compared 19 birth cohorts that entered the labor market before and after the expansion. A total of 50,932 participants were included, aged 17-60 years and born between 1950 and 2003. Among all participants, 26,566 (52.16%) were female, and 19,166 (37.63%) were from rural areas. A cohort-based design by using the Hierarchical Age-Period-Cohort model was applied to resolve collinearity among age, period, and cohort effects. Results: The findings revealed four key insights. First, the expansion produced persistent cohort-level effects, embedding overeducation as a structural feature of the labor market. Second, the effect of expansion on overeducation interacted with other historical and policy shifts, highlighting the contextual nature of expansion outcomes. Third, and central to the study, we observed a transformation in the income effect of overeducation: it shifted from a trade-off (associated with income returns) for pre-expansion cohorts to a trap (associated with an income penalty) for post-expansion cohorts. Finally, the impact of overeducation on income returns varies across groups; while the aggregate trend reflects a trap, evidence suggests that overeducation may still function as a trade-off for certain disadvantaged groups, such as women and rural residents, potentially mitigating income inequality. Conclusion: These findings imply that to fulfill the fundamental aims of higher education expansion-raising educational attainment and reducing social inequality-policymakers must design targeted, group-sensitive interventions to address the unequal risks and opportunities generated during the expansion process.