Fertility Below Replacement Level

Abstract

In assessing the state of the planet, it is important to note that during late 2003 or early 2004, the human population will cross a historic, but so far largely unnoticed, threshold. Most of the world's population either already do, or soon will, live in countries or regions in which fertility is below the level of long-run replacement. ![Figure][1] Cumulative global population by level of fertility. Source: United Nations ( 1 ), U.S. Census Bureau ( 2 ), and author's calculations. When considering the planet's human population, J. Cohen looked mostly to the future (State of the Planet Series, Viewpoint, “Human population: the next half century,” 14 Nov., p. [1172][2]). However, a historical perspective can help clarify where we might be heading. The figure shows the magnitude of the transformation in human reproduction, plotting the cumulative global population according to fertility level in 2003, along with the comparable lines for 25 and 50 years ago. The vertical line in the figure indicates a total fertility rate of 2.1 children per woman, conventionally regarded as replacement level in conditions of low mortality (that is, where life expectancy is 70 years or more). National data are drawn from the United Nations and the U.S. Census Bureau ([1][3], [2][4]). Data for China and India are disaggregated by region. In the early 1950s (blue line), below-replacement fertility was virtually unknown. By the late 1970s (red line), there had been considerable change, with about a quarter of the world's population experiencing fertility below 2.1. By 2003 (green line), the process of fertility decline had advanced still further, with the median now at 2.15. The global mean remains above replacement because of the skewed nature of the distribution—the half of the world with fertility below 2.1 averages 1.6 children, whereas the half above replacement averages 3.6. However, the way in which the 2.1 line intersects the curves close to zero, 25%, and 50% is a clear visual indicator of the revolution that is taking place in childbearing. Several aspects of the current situation deserve to be highlighted. First, roughly one-sixth of the world's people experience fertility below 1.5. Such populations comprise most of southern and eastern Europe and much of East Asia. The long-run importance of this trend may be considerable, suggesting much lower future population growth or even a decline in human numbers within the coming century. Second, below-replacement fertility is not restricted to the developed world, which makes up less than half of the three billion people whose fertility is below 2.1. In addition to the well-known case of China, developing world populations on the left side of the curve include Brazil, Lebanon, Thailand, Tunisia, and 200 million people in parts of India. The United States is now close to the median (it is marked by the vertical jump close to 2.1) and will soon be above median. Third, there is great diversity in fertility in the developing world. Almost 20% of the world's population has fertility of four or higher. They are mostly to be found in two zones: one running through northern India, Pakistan, and Afghanistan, and the other taking in much of the Arabian Peninsula and most of sub-Saharan Africa. Most future growth in population will take place there. Finally, a word of caution. Giving a specific date for any feature of global population change—for example, the United Nations' designation of 12 October 1999 as “Six Billion Day”—is misleading, but we probably know the facts of global demography sufficiently accurately to predict that at some time between mid-2003 and mid-2004, more people will live in regions with less-than-replacement-level fertility than in regions with fertility above 2.1. 1. 1.[↵][5]United Nations Population Division, World Population Prospects: The 2002 Revision , available at . 2. 2.[↵][6]United States Census Bureau, International Data Base, available at [www.census.gov/ipc/www/idbnew.html][7]. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1088665 [3]: #ref-1 [4]: #ref-2 [5]: #xref-ref-1-1 View reference 1. in text [6]: #xref-ref-2-1 View reference 2. in text [7]: http://www.census.gov/ipc/www/idbnew.html

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