This paper develops an overlapping generations model to analyze the consequences of demographic structure changes induced by an exogenous shift in the birth rate. We …rst show that a …nite growth rate of the population that maximizes long-run capital per capita exists. Then, we examine the theoretical properties of this growth rate by showing that: (i) it corresponds to the demographic structure such that the average ages of capital holders and workers are equal; (ii) it is associated to an e¢ cient steady state; (iii) it increases with compulsory transfers from younger to older generations. Finally, we explain why standard overlapping generations models do not exhibit such a growth rate.
JEL Classi…cation: D91; E13; J10Key words: Continuous-time overlapping-generations models; Population aging
IntroductionOverlapping generations (OLG) models are the neoclassical literature's common tool for analyzing the economic consequences of demographic structure changes. In this paper, we focus on the impact of exogenous shifts in the birth rate on long-run capital accumulation. This relationship is crucial for analyzing the consequences of population aging on most macro-variables 1 , such as growth, assets prices and unemployment. An increase in the population growth rate simultaneously induces a reduction in capital per worker and an increase in savings; the problem is to …nd out which one has the strongest e¤ect. Standard OLG models with production in discrete or continuous time developed by Diamond [13] show that a birth-rate increase reduces long-run capital per capita. The economic intuition given to this result relies on the absence of intergenerational altruism: the size of future generations does not directly in ‡uence the saving choices of current generations. The reduction in aggregate consumption that follows a birth rate increase is therefore insu¢ cient for compensating the capital dilution e¤ect. Following Weil [36], the birth rate measures the degree of disconnection between generations, and newborn individuals are interpreted as "unloved children"or immigrants.Empirical studies, however, do not show that demographic structure changes have a signi…cant impact of on capital accumulation or, more precisely, on its marginal productivity. In a recent paper, Poterba [30] analyzes the historical relationship between population structure and assets return, including the interest rate. Using time series from the United States, Canada and the United Kingdom for the last seventy years. Poterba …nds no robust evidence of any impact of demographic variables on assets prices.At a …rst glance, these empirical …ndings seem to invalidate the OLG models while validating the Ramsey framework. In this framework 2 , where the population is composed of one unique, altruistic family, the demographic variables have no impact at the balanced equilibrium. That is because the optimal response to a birth-rate increase is to reduce consumption in order to keep capital per capita constant. However, in this paper, we argue tha...