It's no surprise that the world's population is at an all-time high - exceeding 7 billion - although many might not know that it increased by 5 billion during the past century alone, rising from less than 2 billion in 1914.

And many people would be surprised - even shocked - to know that over the past three decades, fertility rates have plummeted in many parts of the world, including China, Japan and even significant regions of India.

These Asian giants have not been alone. In much of Europe, North America, East Asia and elsewhere, the average number of children born to women during the course of their childbearing years has fallen to unprecedentedly low levels.

Our new book, The Global Spread of Fertility Decline: Population, Fear, and Uncertainty (Yale University Press, 2013) analyzes these trends and the demographic, political and economic consequences and uncertainties as low fertility has become a global phenomenon. Like other facets of globalization, low fertility rates are by no means universal: High fertility persists in sub-Saharan Africa and in parts of the Middle East, but elsewhere low fertility is more the rule than the exception.

These underlying trends in childbearing mean that in the near future the rate of population growth both in Europe and Asia are likely to decline. The world is not on a path of unrestrained demographic growth, as some believe. People all over the world have hit the brakes.

Thirty years ago only a small fraction of the world's population lived in the few countries with fertility rates substantially below the "replacement level" - the rate at which the fertility of a hypothetical cohort of women would exactly replace itself in the next generation - normally set at 2.1 children per woman for populations with low mortality conditions. Fast forward to 2013, with roughly 60 percent of the world's population living in countries with such below-replacement fertility rates.

The consequences of these changes are striking. One is that international migration, which over the same 30 years has been increasing rapidly, now has become the primary driver of rapid changes in the demography of dozens of countries around the world. If we were to assume that current low fertility rates and high immigration rates will continue into the future - neither of which may be a good assumption over the long term - migration would become even more significant a determinant not only of overall national growth but of the ethnic and racial composition of most industrial states, including those of much of Europe and North America.

The advent of baby booms following the Second World War marked the end of an earlier period of low fertility, especially in economically depressed Europe. Fertility rates in the 1950s and early 1960s were much higher, but by the mid-1960s fertility rates had begun to drop again. By the 1970s, they had declined to low levels, first in Central Europe, especially Germany, and in East Asia, initially in Japan, followed by the four "Asian Tigers" of South Korea, Singapore, Hong Kong and Taiwan.

More recently, fertility rates have declined even more rapidly and often to considerably lower levels in Mediterranean Europe - Italy, Spain, Greece, Portugal - although most demographers suggest these very low current rates are in part temporary distortions that result from the delays underway in marriage and childbearing. Low fertility was in play before the eruption of the eurozone debt crisis. And fertility also declined substantially in the true demographic heavyweights - in China and in the southern states of India.

In some of these countries, declines in fertility rates to low levels have been accompanied by large increases in the volume and pace of immigration. While low fertility rates have evoked concerns among political elites that echo those during the 1930s, these elites have generally been less worried about rising immigration. Yet such anxieties have become common among broader publics - leading to large gaps between elite and public opinion.

This has been especially evident in some European countries with large influxes of Muslim migrants from North Africa and the Middle East, exacerbated by the emergence of militant Islamist political movements both at home and abroad. Inflammatory preaching pursued by some groups of Islamists in some European cities, sometimes followed by violence, have triggered grass-roots politics and violence directed against both immigration and immigrants, in particular against Muslim immigrants.

"Islamophobia," an evocative characterization some apply to those who oppose continuing large-scale Muslim immigration, is at the extreme end of these responses to Islam in Europe and elsewhere; others, including German Chancellor Angela Merkel, raise less emotive questions about whether multiculturalism policies have gone too far.

The convergence of such tensions with the recent economic crises in many of these same countries - deep recessions, high and often rising unemployment, sharp cuts in public benefits due to both recession and austerity policies - add to the turmoil and have strengthened political movements that previously were consigned to the fringes.

In recent years, public support has risen rapidly for parties such as Golden Dawn in Greece, often described as "neo-fascist"; the Independence Party in the United Kingdom, or UKIP; the National Front in France; and similar parties in a number of other countries. Their rise may prove temporary, but in some of these countries mainstream political parties have responded to the political threat posed by rising public support for such movements by embracing some of the anti-immigration, anti-multiculturalist and anti-EU rhetoric that has proved so politically popular.

It is impossible to predict how fertility rates, economies or politics will evolve. Will currently fertility rates rebound to higher levels, as they did during the baby booms after World War II? Will current economic crises continue in Mediterranean Europe, the eurozone, Japan and other countries registering low fertility, or will they be replaced by a new rising tide of prosperity, like that of the three decades after World War II?

Will Islamist movements within Europe and elsewhere continue to expand or be supplanted by those supporting cultural and religious integration? Will nativist political movements - riding the tides of opposition to current levels of immigration, Islam in Europe or the European Union itself - continue to grow or subside?

The answers are unknowable. Still, the convergence of such patterns in demography and economic life is a source of friction in both national and international politics. Much depends upon whether or not political elites and governments adopt policies based upon an informed understanding of the powerful and interconnected demographic, economic and political forces underway today.

The global spread of low fertility matters. It touches on vital and explosive issues - the evolution of family ties, the future of pension provisions and care for the elderly, the evolution of immigration policies, the ethnic and language distributions within societies, the potential for violence within and among different religious and ethnic communities, the legal and moral debate over women's rights in general and access to abortion and contraception in particular.

Population trends on the global, national and local levels shape each of these contested topics and how they are perceived. Demography is too important a subject to be left to the demographers alone. It is everybody's business.

(Michael S. Teitelbaum is Wertheim Fellow at the Labor and Worklife Program at Harvard Law School, and senior advisor to the Alfred P. Sloan Foundation. Jay Winter is the Charles J. Stille Professor of History at Yale University, and author of 'Sites of Memory, Sites of Mourning: The Great War in European Cultural History' (Cambridge University Press, 1995). This is reprinted with permission from the Yale Center for the Study of Globalization.)