Over the last four decades birth rates have been declining in virtually all countries ofthe world, suggesting that world population growth is likely to end during thiscentury (Lutz, Sanderson, & Scherbov, 2001). In many parts of Europe and someAsian countries, fertility rates have recently fallen to such low levels that the resultant rapid population ageing and shrinking already causes widespread concern.Various social, economic, political and bio-medical factors are associated withdeclines in birth rates but few social scientists have considered population densityeven though density has been shown to be a key determinant in many animalpopulations (Sibly & Hone, 2003). If human fertility also depends on populationdensity, this will have important implications for population projections whichconventionally assume universal convergence of fertility rates (United Nations,2004).Although global birth rates are declining, there is much regional variation inhuman fertility, and today we face a demographically divided world. Populationgrowth in the developing world is mainly due to high fertility rates combined with avery young age structure that results in increasing cohorts of women enteringreproductive age (i.e., the momentum of population growth) (Bongaarts, 1994). InEurope, by contrast, below-replacement fertility has resulted in an age structure withfewer children and therefore fewer women entering reproductive age in the future(‘‘negative momentum’’) (Lutz, O’Neill, & Scherbov, 2003). Combined withincreasing life expectancy and limited immigration, this low fertility is resulting inrapid population ageing, posing serious challenges for existing systems of old agesecurity and care for the elderly. Global population dynamics are likely to continueto be dominated by reproduction, and therefore meaningful projections requireidentifying the main determinants of fertility. Explanations and projections offertility trends in different parts of the world have usually been guided by theparadigm of demographic transition which assumes that after an initial decline indeath rates, birth rates will also start to fall with a certain lag time (Davis, 1954). Inthis general form, the model has received overwhelming empirical support incapturing the remarkable fertility changes that occurred during the 20th century,although it is not without shortcomings (Kirk, 1996). The demographic transitionmodel is not useful for anticipating the probable level of post-transition fertility incountries that are already at or below replacement fertility. It used to be assumedthat fertility declines cease upon reaching replacement fertility, but empiricalevidence shows that in the vast majority of countries, the fertility decline did notstop at replacement but continued to fall below that level (United Nations, 2004).The social sciences have not yet come up with a useful theory to predict the futurefertility level of post-demographic transition societies.
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