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Review
. 2016 Apr 19;371(1692):20150157.
doi: 10.1098/rstb.2015.0157.

What if fertility decline is not permanent? The need for an evolutionarily informed approach to understanding low fertility

Oskar Burger  1 , John P DeLong  2
Affiliations
Review

What if fertility decline is not permanent? The need for an evolutionarily informed approach to understanding low fertility

Oskar Burger et al. Philos Trans R Soc Lond B Biol Sci. .

Abstract

'Demographic transition theory' assumes that fertility decline is irreversible. This commonly held assumption is based on observations of recent and historical reductions in fertility that accompany modernization and declining mortality. The irreversibility assumption, however, is highly suspect from an evolutionary point of view, because demographic traits are at least partially influenced by genetics and are responsive to social and ecological conditions. Nonetheless, an inevitable shift from high mortality and fertility to low mortality and fertility is used as a guiding framework for projecting human population sizes into the future. This paper reviews some theoretical and empirical evidence suggesting that the assumption of irreversibility is ill-founded, at least without considerable development in theory that incorporates evolutionary and ecological processes. We offer general propositions for how fertility could increase in the future, including natural selection on high fertility variants, the difficulty of maintaining universal norms and preferences in a large, diverse and economically differentiated population, and the escalating resource demands of modernization.

Keywords: demographic transition; evolutionary demography; fertility transition; resource constraints.

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Figures

Figure 1.
Figure 1.
Total energy use and total emissions as functions of total population size in Europe. Each grey line is a unique country's trajectory. The dotted lines are 1 : 1; when the trajectories rise more steeply than the dotted lines there are escalating costs and consequences of population size. Using a 20-year running window, ordinary least-squares regressions were run on these log–log relationships for the countries depicted. For energy use, the exponent was greater than unity for 75% of the time. For emissions, the exponent was greater than unity for 78% of the time. For total energy use, the data depicted are from 1800 to 2008 for each of the following countries: Sweden, The Netherlands, England, Germany, France, Italy, Spain and Portugal; data from [19]. For total emissions, the data depicted are: Sweden 1834–2008; The Netherlands 1846–2008; France 1802–2008; Germany 1800–1946 and 1991–2010; Italy 1860–2008; Spain 1830–2008; Portugal 1870–2008; data from Tom Boden and Bob Andres (Oak Ridge National Laboratory) and Gregg Marland (Appalachian State University).
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