The Long-Term Effect of the Timing of Fertility Decline on Population Size


  • Brian C. O'Neill,

    1. Brian C. O'Neill is Assistant Professor (Research), Watson Institute for International Studies and Center for Environmental Studies, Brown University.
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  • Sergei Scherbov,

    1. Sergei Scherbov is Researcher and Lecturer, Population Research Centre, Faculty of Spatial Sciences, University of Groningen, Netherlands.
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  • Wolfgang Lutz

    1. Wolfgang Lutz is Leader of the Population Project, International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria.
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Existing long-range population projections imply that the timing of the fertility transition has a relatively unimportant effect on long-term population size when compared with the impact of the level at which fertility is assumed eventually to stabilize. However, this note shows that the effect of the timing of fertility decline is a function of the eventual fertility rate: the lower the eventual fertility rate, the greater the effect of the timing of the transition becomes. This finding has important implications for projection methodology, as well as for policies related to the consequences of long-term levels of population size.