Population

Perceptions of the meaning, implications, and control of reproduction and sexuality, at both the individual and community levels, are among the most important, but also the most challenging, in the history of ideas. Changes in human population helped shape the twentieth century and are creating new tensions in the twenty-first, but, despite the profound significance of childbearing to individuals and the impact of population change on economics, politics, maternal and infant health, the environment, and national security, the intellectual history of population is oddly fractured and frequently uninformative. There are still divisions on the exact mix of factors, such as vaccination and socioeconomic change, driving the dramatic fall in death rates that pushed the global population from 1,650 million in 1900 to 6,071 million in 2000, and there is no consensus on the factors driving falling birth rates in recent decades. Many societies have made pragmatic decisions about their citizens' access to modern contraception and safe abortion, but bitter and painful differences on ethics persist. The two most important shifts in the intellectual history of population—the Papal encyclical Humanae vitae and the Chinese "one-child" policy—represent diametrically opposed responses to population change.

An understanding of population must draw on reproductive biology, anthropology, economics, political science, and demography, as well as knowledge of cultural and religious beliefs, but cross-disciplinary progress has been slow. As Charles Darwin (1809–1882) acknowledged, Thomas Robert Malthus's (1766–1834) An Essay on the Principle of Population, published in 1798, was the intellectual trigger for The Origin of Species (1851). In the second half of the twentieth century, biological perspectives began to influence ideas about population change. Anthropologists reconstructed the demographic history of preliterate communities, which have been characteristic of the hunter-gatherer way of life for more than 95 percent of the time Homo sapiens has been a distinct species. In such societies, women sometimes do not menstruate until they are eighteen or even twenty years old, and pregnancies are well spaced by natural endocrine changes associated with breast-feeding. The average woman has six to eight pregnancies, half of which usually die, with the consequence that hunter-gatherer populations grow slowly.

The second half of the twentieth century saw unprecedented and unrepeatable changes in the growth of the global population. Rising potential fertility, as a result of changes in breast-feeding and the age of puberty, together with a rapid fall in death rates, pushed population growth rates up. The highest absolute growth in global population occurred in the mid-1980s, with more than 85 million more births than deaths per annum. During the same period, developed countries finally gained wide access to contraception and safe abortion, and in some regions average family size fell below two. In parts of Asia, which formerly had large families, the birth rate also fell to replacement level or below, although elsewhere in Asia and in Africa birth rates remain high. The implications for the health of women and children, for economic prosperity, social stability, and the environment of these changes are profound, yet some of the most important population changes were not predicted by most demographers.

The standard theory of the demographic transition was that a fall in the birth rate would lag behind any decline in the death rate and that birth and death rates would slowly stabilize as couples grew richer, became more educated, and made a rational decision to have smaller families. Frank Notestein, the distinguished American demographer, gave articulate expression to the theory. As the volume of empirical data on family planning has increased, so demographers have defined the proximal determinants of family size with increasing accuracy. These are the age at commencing intercourse, the prevalence of pathological causes of infertility, the suppression of ovulation associated with lactation, and use of contraception and abortion. However, the more distal determinants that make birth rates decline continue to be debated. The Office of Population Research at Princeton University explored the history of fertility decline in various parts of Europe and found that ideas about family planning spread by diffusion in homogeneous political and religious groups.