Can Replacement-Level Fertility Reverse Population Decline? Evidence from Cohort–Component Projections

Introduction

Advanced countries have experienced prolonged below-replacement fertility, leading to population aging and, in many cases, decline [1,2]. A key consequence is the contraction of childbearing-age cohorts, raising the question of whether restoring fertility to replacement level can stabilize population size [3].

Methods

This study adopts a theoretical cohort–component projection approach grounded in stable population theory and the concept of population momentum [3,4]. Starting from low-fertility-age structures, projections assume an immediate shift to replacement fertility, constant mortality, and zero migration. Population trajectories are assessed through cohort evolution, focusing on reproductive-age groups and momentum effects [5].

Results

The analysis demonstrates that populations shaped by sustained below-replacement fertility exhibit negative population momentum [3]. While replacement-level fertility slows decline, it does not stabilize population size. The extent of decline depends on past fertility suppression, leading to smaller reproductive cohorts and increased population aging [6].

Conclusions

The findings indicate that fertility restoration alone is insufficient to reverse population decline in advanced low-fertility societies. Age structure and demographic inertia strongly shape outcomes [3,4]. Policies must address long-term aging and structural decline beyond fertility measures [2].

References:

1. Bongaarts, J. Human Population Growth and the Demographic Transition. Philos. Trans. R. Soc. B 2009, 364, 2985–2990.
2. Lutz, W.; Sanderson, W.; Scherbov, S. The Coming Acceleration of Global Population Ageing. Nature 2008, 451, 716–719.
3. Keyfitz, N. On the Momentum of Population Growth. Demography 1971, 8, 71–80.
4. Preston, S.H.; Heuveline, P.; Guillot, M. Demography: Measuring and Modeling Population Processes; Blackwell: Oxford, 2001.
5. Spears, D.; Vyas, S.; Weston, G.; Geruso, M. Long-Term Population Projections: Scenarios of Low or Rebounding Fertility. PLoS ONE 2024, 19, e0298190.
6. Sobotka, T. Post-Transitional Fertility: Childbearing Postponement and Low Fertility. Popul. Dev. Rev. 2017, 43, 641–680.