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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.