World Population Policies: Their Origin, Evolution, and Impact. By John May. Springer; 339 pages; $179 and £117. Buy from Amazon.com, Amazon.co.uk

Political Demography: How Population Changes Are Reshaping International Security and National Politics. Edited by Jack Goldstone, Eric Kaufmann and Monica Duffy Toft. Oxford University Press USA; 336 pages; $39.95 and £60. Buy from Amazon.com, Amazon.co.uk

DEMOGRAPHY is back. Not that its subject matter—the size and structure of populations—ever went away. But from the 1980s to the late 2000s demography retreated to the wings of public debate, a concern mostly of geeks and obsessives. Over the past few years, though, that has started to change. Population science is once more centre-stage, pushed by climate change, which raises worries about the impact so many billions have on the environment of the earth, and food-price spikes, which imply doubts about whether it will be possible to feed them all.

The main concern of demographers in their heyday (the 1970s and 1980s) was high fertility and the total number of the world's people. This was the period of “The Population Bomb”, a bestseller by a biologist, Paul Ehrlich, which argued that the world could not feed itself. An international family-planning movement sprang up. Top-down programmes attempted to control the total size of national populations. China's one-child policy is the best known and most extreme of these.

Now though, as John May, formerly of the World Bank and now Georgetown University, shows in “World Population Policies”, the focus of demographers has switched from the overall size of populations to their composition—that is, to age groups and their relation to one another. Instead of high fertility rates, demographers study ageing, dependency ratios, the “demographic dividend” (a bulge of working-age adults) and distorted sex ratios, which result when millions of parents choose the sex of their children, often by aborting baby daughters.

The result, suggests Mr May, is that demography is more complex, if less dramatic, than it used to be. Globally the average fertility rate (the number of children a woman can expect during her lifetime) has roughly halved in the past half century, from five in the mid-1950s to 2.5. Only 16% of the world's population lives in countries where most women have four children. Falling fertility has taken much of the urgency out of the debate about population growth, at least among experts, since it seems to be solving the problem of excessive births (a view criticised by Mr May, who points out that even if fertility has declined, the momentum of change means populations will go on rising for a while).

At the same time demography has become more diverse. Some countries retain extremely high fertility rates (over seven in Niger, more than in 1950); other places have seen fertility fall to ultra-low levels (less than one in Taiwan and Shanghai); and there is everything in between. Whereas demographic policy used to be a matter largely for national governments, a plethora of interest groups has taken over. A United Nations (UN) conference in 1994, which argued that top-down policies had resulted in abuses of women, put individual rights and women's concerns front and centre. Even the debate has got more muddled. There used to be clearly defined battle lines between pessimists like Mr Ehrlich and optimists like Julian Simon (an economist who bet Mr Ehrlich that the inflation-adjusted price of a basket of commodities would not rise over time because of growing populations). Now disputes rage not only between optimists and pessimists but between two different sorts of pessimists—“explosionists” worried about too many people and “implosionists” more concerned about falling populations. Mr May's book is the first comprehensive guide to the history and new findings of this change in emphasis.

“Political Demography” is a collection of essays that shows some of the results of the new research in more detail. A chapter by Mark Haas of Duquesne University in Pittsburgh argues that demographic change will consolidate America's strategic position because all other leading nations have such dire demographic trends. China's one-child policy means there is a shortage of women of child-bearing age. As a result, between now and 2050 the number of men in China aged 20 to 25 (ie, of prime military recruiting age) will halve. Similarly, points out Eric Kaufmann, in many countries, fundamentalist religious groups have fertility rates two or three times higher than the societies in which they live. Israel is a good example. Over time, this will alter the balance of power in those countries.

The biggest question raised by the new research is whether the world is converging demographically or diverging towards disparate futures. The UN, whose population division is the source of much basic demographic material, thinks the world is converging towards the “replacement rate” of fertility; this is 2.1, the magic number that stabilises populations over the long term. But as Mr May shows, much of the new demography stresses the variety of countries' experience and finds that huge changes are taking place between different age groups even when the overall size of a country is stable. At a global level—and with some notable exceptions—the world does seem to be converging towards the sort of flatter population growth not seen for a century. But at the same time, there is a lot of instability below the surface, where different age groups are grinding against one another like shifting tectonic plates. The new demography has its work cut out.