What If Replacement Level Fertility is Not the Norm?

In Chapter 3 of my book I discuss the fact that population stability--births equaling deaths--has rarely been achieved in known human societies and so it might be better to think that change, rather than stability, is the demographic norm. Nonetheless, there has been a tendency among demographers to project populations into the future on the assumption that replacement level fertility (the number of children that produces population stability given a certain death rate) is the norm. To be sure, a decade ago the UN Population Division did deviate from that a bit by projecting the world's population out to 2300 with a range of TFRs from 1.85 (clearly below replacement) to 2.35 (above replacement given current life expectancy). Just today, three prominent demographers--Stuart Basten, Wolfgang Lutz, and Sergei Scherbov--have published a new set of population projections out to 2300 that take a much wider view of the future, including the possibility that low fertility norms may have settled in for the long term.

There is little doubt that from an evolutionary perspective our sex drive has been the main mechanism assuring the reproduction of the human species and that modern contraception has radically changed this pattern (e.g. Frejka 2008b). In this context, individual desires, ideals, and social norms are paramount for the decision to have a child. Indeed, fertility ideals and intentions have been described as powerful predictors of future fertility behaviour (e.g. Morgan and Rackin 2010).

They go on to show that a slight majority of European couples, for example, think that two children is the ideal family size, but they also review research suggesting that declining fertility itself leads to lower ideal family size among couples. In other words, the causal connection here is a bit weak, suggesting that we should be cautious in assuming that fertility will rise just because the reported ideal family size is higher than the current level of fertility.

If, however, global fertility in the long run converged to a level of 1.5 – which is slightly below the 2009 average level in the European Union of 1.59 (Eurostat 2012) – then, after peaking around the middle of the century, the world population would return to the current level of seven billion people by 2100. By the end of the 22nd century it would then fall below three billion even though under this scenario, life expectancies would continue to increase until they reach 100 years in all parts of the world.

Is a future with long term global fertility below two and, hence, long term population decline something to be concerned about? Ecologists have long demanded a smaller world population size with a lighter ecological footprint and assumed, in the spirit of Malthus, that this will come naturally as a consequence of higher mortality caused by overpopulation and the resulting disasters. Our calculations clearly demonstrate that this desired decline can be reached even under conditions of further increasing life expectancy.

Keep in mind that only two hundred years the world's population was scarcely one billion, so the population growth we're in right now may just be a bubble, and we may be much better off down the road with a smaller, but better educated and more ecologically-conscious group of humans.