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. And many 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, analyses these trends and the demographic, political and economic consequences. 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 trends mean that the rate of population growth both in Europe and Asia is likely to decline. The world is not on a path of unrestrained demographic growth, as some believe.
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" - normally set at 2.1 children per woman. Fast forward to 2013, with roughly 60 per cent of the world's population living in countries with below-replacement fertility rates.
The consequences of these changes are striking. One is that international migration has now become the primary driver of rapid changes in the demography of dozens of countries.
While low fertility rates have evoked concerns among political elites, they 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. Inflammatory preaching by some groups of Islamists in some European cities, sometimes followed by violence, have triggered grass-roots politics and violence.
The convergence of such tensions with the recent economic crises adds to the turmoil and has strengthened political movements that previously were consigned to the fringes.
It is impossible to predict how fertility rates, economies or politics will evolve. Still, the convergence of such patterns in demography and economic life is a source of friction in politics. Much depends on whether governments adopt policies based on an informed understanding of the interconnected demographic, economic and political forces under way today.
The global spread of low fertility 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, and the debate over women's rights.
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 Labour and Worklife Programme at Harvard Law School. Jay Winter is the Charles J. Stille Professor of History at Yale University. Reprinted with permission from YaleGlobal Online. http://yaleglobal.yale.edu