What Is Causing Population Aging? The Revisionist View Revisited

Ronald Lee, University of California, Berkeley
Yi Zhou, University of California, Berkeley

Why are contemporary populations still aging? In the Classic view aging is driven almost entirely by fertility decline over the demographic transition, while mortality decline plays a minor role. Populations are still aging because they are still converging toward the new older stable age distribution. But in the past 25 years an elegant mathematical decomposition of changing mean ages has been interpreted as showing that recent aging is mainly due to declining mortality rather than fertility. Here we challenge this revisionist view, and suggest it misinterprets the decomposition results. It appears that the role of convergence is ignored. We suggest that the gold standard is the analytic simulation with explicit counterfactual comparisons. Analytic simulations show that fertility decline is largely responsible for the old age of contemporary populations, and has a large role in accounting for continuing aging over the 2000-2010 decade.