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At first glance the speciesHomo sapiensappears to have low potential for reproduction. Puberty begins late, pregnancy is long, normally only one baby is delivered at a time, and lactation can continue for several years. Yet on the global level the human race now experiences 1,000,000 more births than deaths every five days, and a large percentage of the world's population lives in urban areas, often at extremely high population densities. In experiments, when mammals are placed in crowded conditions the age of sexual maturity rises, the interval between pregnancies increases, and infant mortality jumps, leading to slower growth in the population. Among human beings in analogous crowded conditions, however, in the absence of artificial birth control the opposite situation arises.

In many cases ovulation does not take place in the first several cycles after the onset of menstruation (the menarche). Once a woman is fertile, social factors determine whether she is exposed to the opportunity to become pregnant. In preindustrial Britain, couples were expected to form their own nuclear group upon marriage, and many a first-time bride was in her later 20s. By contrast, in contemporary Third World societies that encourage extended families, girls often marry in the early teens.

In all mammals whose reproduction is not tied to seasonal changes, physiological mechanisms ensure the optimum spacing of pregnancies. InHomo sapiens,as in other primates, breast-feeding provides the basis for nature's own method of birth control. In the few remaining societies of hunters and gatherers, whose way of life may represent the conditions under which most of human evolution took place, women nurse their babies frequently and ovulation and menstruation are suppressed for two to three years after birth.

Nomadic women of the !Kung, a group of the San people of southern Africa, use no contraceptives but have a mean interval between births of 44 months and an average of four or five deliveries in a fertile lifetime. Modern methods of birth control substitute for the control over fertility once provided by lactation and permit a degree of control over human reproduction not previously available.

The combination of high infant mortality with relatively low fertility associated with traditional patterns of breast-feeding kept population growth in preagricultural human societies virtually static. Ten thousand years ago the world's population may have stood at 10,000,000. Since that time natural restraints on human reproduction have broken down at an accelerating pace. By the beginning of the Christian Era the world's population was perhaps 300,000,000. In the mid-1980s it passed the 5,000,000,000 mark. Since the Industrial Revolution, and with intensely increasing pressure in the past century, both individuals and societies have had to make important decisions about the use of birth control.

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