Fertility Booms and Busts During the Twentieth Century and Their Importance for the Process of Ageing

As the developed world hunkers down for the long winter of ageing it may be instructive to reflect on just how we arrived at this particular juncture of world population history and how the recent past is likely to mark future developments in this process. In many ways, the twentieth century was unique with respect to the timing and intensity of population trends. During this period, there was a very clear boom and bust cycle of fertility. The starting point for this great cycle can be found during the 1930s when in many, but not all, developed nations fertility was already quite low, often near or even below levels considered necessary for population replacement (Total Fertility Rate (TFR) = 2.1).

For many, this was the culmination of the demographic transition and a testimony to people’s ability to control fertility within marriage. A period of demographic effervescence known as the baby boom followed in which fertility rose substantially nearly everywhere in the developed world, reaching a high point between the mid-1950s and the early-1970s (see, for example, van Bavel and Reher, 2013). This boom was followed by another period of extremely low fertility which, despite some relatively modest increases during the first decade of the present century, lasted until the present with levels systematically below and often even far below replacement.

This boom and bust cycle of fertility was unique in many ways. Both the boom and the bust were largely unexpected. During the latter stages of the Depression and during World War II, there was little reason to expect an upturn in fertility. At that time, John Maynard Keynes and others wrote articles on the dangers of population stagnation whose relevance for contemporaries vanished as soon the fertility boom began (see, for example, Keynes, 1937). During the 1950s and early 1960s before synthetic contraception had been invented and in the context of economic stability and growth, abundant jobs,  and stable cultural values, there was no reason to forecast a baby bust that would reduce fertility dramatically and suddenly, ushering the scenario typical of the second demographic transition (see, for example, Lesthaeghe, 2010). Despite the evident heterogeneity of both the boom and the bust, it is unquestionable that this general cycle affected the entire developed world, independent of disparities in living standards, political and social stability, and the pace of modernization.

Implicit in this cycle were wide swings in the yearly number of births, with strong increases during the peak boom years and important reductions often by as much as 30-40 percent in many countries as they plunged into their own baby busts. It is interesting to note that the high point reached by fertility during the boom came approximately 25-30 years before the low point reached during the bust. In other words, in most nations the time between the peak boom and the peak bust was approximately the length of a generation. Another characteristic of this cycle is that countries with relatively strong booms tended to have weak busts and, conversely, where booms were relatively weak, busts were deep and lasting. The underlying reasons for this strong/weak and weak/strong pattern continue to be a matter of speculation but the result is crystal clear with countries from Northern Europe and the English-speaking non-European developed world having strong booms and relatively weak busts, and Southern, Central and Eastern European nations having weak or even non-existent booms and very strong busts.

These cyclical shifts in fertility during the second half of the twentieth century will mark the pace and pattern of ageing affecting different societies during the coming years, as people born during the baby boom retire and those born during the baby bust make up the population of working age. The most commonly used indicator of ageing, the old-age dependency ratio, juxtaposes the importance of the elderly or dependent population with respect to the population of working age. We know that the pace of ageing, estimated as the rate of increase of the old age dependency ratio, is currently accelerating dramatically with forecast increases of between 50-90% by 2035. In half of the countries in a sample of 26 nations, elderly dependency will be more than twice current levels by mid-century. In countries with strong booms and weak busts, the pace of increase will slow after 2030-2035. This will not be the case in countries with weak booms and strong busts where increasing dependency will continue well into the decade of the 2040s and beyond.

Perhaps more interesting is the way earlier swings in fertility shape the specific patterns of ageing in different societies. Everywhere, of course, the growth of elderly population will far outstrip that of working age population, with increasing old age dependency as a result. In societies with strong baby booms and weak baby busts, this process will tend to be dominated by increases in the number of elderly, who will more than double their current numbers in the coming decades, accompanied by weak growth of working age populations. In societies with weak booms and strong busts, however, decreasing numbers of people of working age promise to be the key variable, with forecast declines of 10-20% or more by mid-century. The strength of the baby boom is a useful predictor for the growth in the number of elderly people, while the strength of the baby bust is an excellent predictor for the growth of working age population. In this sense, the demographic dimensions of ageing are very much constrained by the history of fertility in the more or less recent past. Migration is another factor of note, especially with respect to the size of working age population, though its importance is extremely difficult to forecast and is probably much lower than that of previous fertility trends.

Policies implemented to manage the process of ageing will be affected by these realities, as countries with ageing driven mainly by increases in the number of the elderly will demand a different policy mix than those where ageing is largely driven by shrinking populations of working age. In all cases, the weight of the past will loom large in determining the way ageing is playing out in different parts of the developed world. Understanding these enduring disparities in reproduction throughout the developed world is a major challenge for demographers.

This post is based on: Reher, David S. (2015) “Baby booms, busts, and population ageing in the developed world,” Population Studies: A Journal of Demography, 69:sup1, S57-S68, DOI: 10.1080/00324728.2014.963421


Lesthaeghe, Ron (2010) “The Unfolding Story of the Second Demographic Transition,” Population and Development Review36 (2): 211-251

Keynes, J. M. (1937). “Some Economic Consequences of a Declining Population,” Eugenics Review, XXIX, 13–17

Van Bavel, Jan & D.S. Reher (2013) “The Baby Boom and Its Causes: What we Know and What We Need to Know,” Population and Development Review 39 (2), pp. 257-288.

This entry was posted in Ageing, Fertility, Pensions, Welfare by David Reher. Bookmark the permalink.
David Reher

About David Reher

David Reher is currently Professor at the School of Political Science and Sociology of the Universidad Complutense de Madrid and Director of the Population and Society Research Group [Grupo de Estudios Población y Sociedad – GEPS]. He has degrees from Santa Clara University, California State University, Northridge, and the University of Madrid. Professor Reher has held positions both in Spanish and in international scientific organizations, most notably as Chair of the Historical Demography Committee of the International Union for the Scientific Study of Population (IUSSP). He has been on the Editorial Board of the journals Continuity and Change, the Revista de Historia Economica, Historical Methods, the Revista de Demografía Histórica, and Statistica. His is also a founding member and former President of the Asociacion de Demografia Historica (ADEH).

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