In order to keep sane while studying law at university I also studied history. Unfortunately there were few papers in my area of interest (medieval and early modern Europe) and so I had to content myself with overpoliticised papers like “US Wars in Asia”. I still remember one of my peers arguing in a seminar that the Japanese Empire in East Asia was better than the US Empire in the Philippines. Sigh.  Anyway, I must have done at least one paper on European history, because I remember writing an essay on the Congress of Vienna. The Congress settled Europe after the Napoleonic Wars and involved territorial change. One of the interesting things was the fixation that the statesmen had on securing more “souls” for their kingdoms. They knew that population was one of the foundations of great power status.

In the twenty-first century, population does not seem to be so important when it comes to geopolitical clout. Instead GDP, money, technological edge and geographical position all seem to be more important than the number of people in a country. And there is the levelling device: the threat of nuclear weapons. But, as we have discussed before, population still matters when it comes to great power rivalry. And in this arena, it seems as if the USA has some advantages, especially compared with its rivals: Russia and China.

Why is population still important? First, because a large working aged population is the source of military manpower and you still need people to fight in the twenty-first century. Furthermore, a young, growing and well-educated population is the source of economic productivity which underlies the ability to pay for international influence. Countries with healthy demographic profiles are able to create wealth more easily than their competitors, all other things being equal. They are also less constrained by large pension and health care costs. If a country has a top-heavy population pyramid with a shrinking number of workers, then it will face higher social demands on a less productive economy. Socially too, demographic problems can lead to crippling instability. In short, if you want to be a global power player, then it helps to have a strong workforce supporting relative few retirees.

In this sense, the USA is a better position than either Russia or China. First, Russia’ population is less than half that of the USA and is expected to shrink to about 119 million in 2050. Over the same period, the share of the population in the working age cohort is expected to shrink from 60 per cent to less than 50 per cent. It will face a choice between pouring money from a weakening economy into social security measures or into foreign affairs. If it chooses the latter, then it will have to deal with any political tumults that neglecting large sections of the population will bring.

China has a huge demographic advantage in terms of overall numbers: at 1.4 billion people it is (still) the most populous nation on earth. But the legacy of the one-child policy and depressed birth rates will be felt in the years ahead as it loses most populous nation status to India and slumps to around 1 billion people by century’s end. By 2040 its retirement aged population will increase by 250 per cent and its working-aged population will fall by about 100 million people. This will place large strains on the Chinese economy: it will slow even more dramatically than it is currently doing, its debt problem will become worse and the austerity will become more politically difficult to pursue. Demographic trouble may well foster domestic upheaval: there is currently a shortage of marriage-aged females. If the tapering of economic growth makes it harder to provide for the wellbeing of a larger number of retirees then the social compact in Chain may falter.

In comparison, the USA has a relatively healthy (albeit shrinking) birth rate and high levels of immigration. The USA’s population is expected to increase in the decades ahead. It will be older as the baby boomers continue to retire and the proportion of retirees to working age people will nearly double in the next 40 years. Overall though, these strains will be cushioned by a rising population. Although this is all comparatively good news for the USA’s claim to be the indispensable nation, there are demographic issues ahead. First, its allies (Japan, Germany, most of Western Europe) are facing drastic demographic decline in the years ahead. They are shrinking, ageing populations which may mean that they are not the force multipliers that they are now. Second, the USA’s demographic advantage is largely underwritten by immigration. Will this continue? Will the USA’s populace allow this to continue? Finally, the prospect of decline (in terms of demography and power) for its rivals may make the world more dangerous: declining powers can become aggressive if desperate and think that their window of opportunity will close soon.

So there are reasons for the USA to be confident in its ability to see off geopolitical rivals in the medium term, but there are also reasons for it to be wary in the years ahead.

Marcus Roberts is co-editor of Demography is Destiny, MercatorNet's blog on population issues.