



The basic components of population change









Biological factors affecting human fertility

Reproduction is a quintessentially biological process, and hence all fertility analyses must consider the effects of biology. Such factors, in rough chronological order, include:

the age of onset of potential fertility (or fecundability in demographic terminology);

the degree of fecundability—i.e., the monthly probability of conceiving in the absence of contraception;

the incidence of spontaneous abortion and stillbirth;

the duration of temporary infecundability following the birth of a child; and

the age of onset of permanent sterility.

The age at which women become fecund apparently declined significantly during the 20th century; as measured by the age of menarche (onset of menstruation), British data suggest a decline from 16–18 years in the mid-19th century to less than 13 years in the late 20th century. This decline is thought to be related to improving standards of nutrition and health. Since the average age of marriage in western Europe has long been far higher than the age of menarche, and since most children are born to married couples, this biological lengthening of the reproductive period is unlikely to have had major effects upon realized fertility in Europe. In settings where early marriage prevails, however, declining age at menarche could increase lifetime fertility.

Fecundability also varies among women past menarche. The monthly probabilities of conception among newlyweds are commonly in the range of 0.15 to 0.25; that is, there is a 15–25-percent chance of conception each month. This fact is understandable when account is taken of the short interval (about two days) within each menstrual cycle during which fertilization can take place. Moreover, there appear to be cycles during which ovulation does not occur. Finally, perhaps one-third or more of fertilized ova fail to implant in the uterus or, even if they do implant, spontaneously abort during the ensuing two weeks, before pregnancy would be recognized. As a result of such factors, women of reproductive age who are not using contraceptive methods can expect to conceive within five to 10 months of becoming sexually active. As is true of all biological phenomena, there is surely a distribution of fecundability around average levels, with some women experiencing conception more readily than others.

Spontaneous abortion of recognized pregnancies and stillbirth also are fairly common, but their incidence is difficult to quantify. Perhaps 20 percent of recognized pregnancies fail spontaneously, most in the earlier months of gestation.

Following the birth of a child, most women experience a period of temporary infecundability, or biological inability to conceive. The length of this period seems to be affected substantially by breast-feeding. In the absence of breast-feeding, the interruption lasts less than two months. With lengthy, frequent breast-feeding it can last one or two years. This effect is thought to be caused by a complex of neural and hormonal factors stimulated by suckling.

A woman’s fecundability typically peaks in her 20s and declines during her 30s; by their early 40s as many as 50 percent of women are affected by their own or their husbands’ sterility. After menopause, essentially all women are sterile. The average age at menopause is in the late 40s, although some women experience it before reaching 40 and others not until nearly 60.


Contraceptive practices affect fertility by reducing the probability of conception. Contraceptive methods vary considerably in their theoretical effectiveness and in their actual effectiveness in use (“use-effectiveness”). Modern methods such as oral pills and intrauterine devices (IUDs) have use-effectiveness rates of more than 95 percent. Older methods such as the condom and diaphragm can be more than 90-percent effective when used regularly and correctly, but their average use-effectiveness is lower because of irregular or incorrect use.

The effect upon fertility of contraceptive measures can be dramatic: if fecundability is 0.20 (a 20-percent chance of pregnancy per month of exposure), then a 95-percent effective method will reduce this to 0.01 (a 1-percent chance).


Induced abortion reduces fertility not by affecting fecundability but by terminating pregnancy. Abortion has long been practiced in human societies and is quite common in some settings. The officially registered fraction of pregnancies terminated by abortion exceeds one-third in some countries, and significant numbers of unregistered abortions probably occur even in countries reporting very low rates.


Complete elimination of fecundability can be brought about by sterilization. The surgical procedures of tubal ligation and vasectomy have become common in diverse nations and cultures. In the United States, for example, voluntary sterilization has become the most prevalent single means of terminating fertility, typically adopted by couples who have achieved their desired family size. In India, sterilization has been encouraged on occasion by various government incentive programs and, for a short period during the 1970s, by quasi-coercive measures.


As noted above, the science of demography has its intellectual roots in the realization that human mortality, while consisting of unpredictable individual events, has a statistical regularity when aggregated across a large group. This recognition formed the basis of a wholly new industry—that of life assurance, or insurance. The basis of this industry is the life table, or mortality table, which summarizes the distribution of longevity—observed over a period of years—among members of a population. This statistical device allows the calculation of premiums—the prices to be charged the members of a group of living subscribers with specified characteristics, who by pooling their resources in this statistical sense provide their heirs with financial benefits.

Overall human mortality levels can best be compared by using the life-table measure life expectancy at birth (often abbreviated simply as life expectancy), the number of years of life expected of a newborn baby on the basis of current mortality levels for persons of all ages. Life expectancies of premodern populations, with their poor knowledge of sanitation and health care, may have been as low as 25–30 years. The largest toll of death was that exacted in infancy and childhood: perhaps 20 percent of newborn children died in their first 12 months of life and another 30 percent before they reached five years of age.

In the developing countries by the 1980s, average life expectancy lay in the range of 55 to 60 years, with the highest levels in Latin America and the lowest in Africa. In the same period, life expectancy in the developed countries of western Europe and North America approached 75 years, and fewer than 1 percent of newborn children died in their first 12 months.

For reasons that are not well understood, life expectancy of females usually exceeds that of males, and this female advantage has grown as overall life expectancy has increased. In the late 20th century this female advantage was seven years (78 years versus 71 years) in the industrial market economies (comprising western Europe, North America, Japan, Australia, and New Zealand). It was eight years (74 years versus 66 years) in the nonmarket economies of eastern Europe.

The epidemiologic transition

The epidemiologic transition is that process by which the pattern of mortality and disease is transformed from one of high mortality among infants and children and episodic famine and epidemic affecting all age groups to one of degenerative and man-made diseases (such as those attributed to smoking) affecting principally the elderly. It is generally believed that the epidemiologic transitions prior to the 20th century (i.e., those in today’s industrialized countries) were closely associated with rising standards of living, nutrition, and sanitation. In contrast, those occurring in developing countries have been more or less independent of such internal socioeconomic development and more closely tied to organized health care and disease control programs developed and financed internationally. There is no doubt that 20th-century declines in mortality in developing countries have been far more rapid than those that occurred in the 19th century in what are now the industrialized countries.

Infant mortality


This global average disguises great differences. In certain countries of Asia and Africa, infant mortality rates exceed 150 and sometimes approach 200 per 1,000 (that is, 15 or 20 percent of children die before reaching the age of one year). Meanwhile, in other countries, such as Japan and Sweden, the rates are well below 10 per 1,000, or 1 percent. Generally, infant mortality is somewhat higher among males than among females.

In developing countries substantial declines in infant mortality have been credited to improved sanitation and nutrition, increased access to modern health care, and improved birth spacing through the use of contraception. In industrialized countries in which infant mortality rates were already low the increased availability of advanced medical technology for newborn—in particular, prematurely born—infants provides a partial explanation.


The deliberate killing of newborn infants has long been practiced in human societies. It seems to have been common in the ancient cultures of Greece, Rome, and China, and it was practiced in Europe until the 19th century. In Europe, infanticide included the practice of “overlaying” (smothering) an infant sharing a bed with its parents and the abandonment of unwanted infants to the custody of foundling hospitals, in which one-third to four-fifths of incumbents failed to survive.

In many societies practicing infanticide, infants were not deemed to be fully human until they underwent a rite of initiation that took place from a few days to several years after birth, and therefore killing before such initiation was socially acceptable. The purposes of infanticide were various: child spacing or fertility control in the absence of effective contraception; elimination of illegitimate, deformed, orphaned, or twin children; or sex preferences.

With the development and spread of the means of effective fertility regulation, infanticide has come to be strongly disapproved in most societies, though it continues to be practiced in some isolated traditional cultures.

Mortality among the elderly

During the 1970s and 1980s in industrialized countries there were unexpectedly large declines in mortality among the elderly, resulting in larger-than-projected numbers of the very old. In the United States, for example, the so-called frail elderly group aged 85 years and older increased nearly fourfold between 1950 and 1980, from 590,000 to 2,461,000. Given the high incidence of health problems among the very old, such increases have important implications for the organization and financing of health care.


One of the main factors affecting fertility, and an important contributor to the fertility differences among societies in which conscious fertility control is uncommon, is defined by the patterns of marriage and marital disruption. In many societies in Asia and Africa, for example, marriage occurs soon after the sexual maturation of the woman, around age 17. In contrast, delayed marriage has long been common in Europe, and in some European countries the average age of first marriage approaches 25 years.

In the 20th century dramatic changes have taken place in the patterns of marital dissolution caused by widowhood and divorce. Widowhood has long been common in all societies, but the declines of mortality (as discussed above) have sharply reduced the effects of this source of marital dissolution on fertility. Meanwhile, divorce has been transformed from an uncommon exception to an experience terminating a large proportion (sometimes more than a third) of marriages in some countries. Taken together, these components of marriage patterns can account for the elimination of as little as 20 percent to as much as 50 percent of the potential reproductive years.

Many Western countries have experienced significant increases in the numbers of cohabiting unmarried couples. In the 1970s some 12 percent of all Swedish couples living together aged 16 to 70 were unmarried. When in the United States in 1976 the number of such arrangements approached 1,000,000, the Bureau of the Census formulated a new statistical category—POSSLQ—denoting persons of the opposite sex sharing living quarters. Extramarital fertility as a percentage of overall fertility accordingly has risen in many Western countries, accounting for one in five births in the United States, one in five in Denmark, and one in three in Sweden.


Since any population that is not closed can be augmented or depleted by in-migration or out-migration, migration patterns must be considered carefully in analyzing population change. The common definition of human migration limits the term to permanent change of residence (conventionally, for at least one year), so as to distinguish it from commuting and other more frequent but temporary movements.

Human migrations have been fundamental to the broad sweep of human history and have themselves changed in basic ways over the epochs. Many of these historical migrations have by no means been the morally uplifting experiences depicted in mythologies of heroic conquerors, explorers, and pioneers; rather they frequently have been characterized by violence, destruction, bondage, mass mortality, and genocide—in other words, by human suffering of profound magnitudes.

Early human migrations

Early humans were almost surely hunters and gatherers who moved continually in search of food supplies. The superior technologies (tools, clothes, language, disciplined cooperation) of these hunting bands allowed them to spread farther and faster than had any other dominant species; humans are thought to have occupied all the continents except Antarctica within a span of about 50,000 years. As the species spread away from the tropical parasites and diseases of its African origins, mortality rates declined and population increased. This increase occurred at microscopically small rates by the standards of the past several centuries, but over thousands of years it resulted in a large absolute growth to a total that could no longer be supported by finding new hunting grounds. There ensued a transition from migratory hunting and gathering to migratory slash-and-burn agriculture. The consequence was the rapid geographical spread of crops, with wheat and barley moving east and west from the Middle East across the whole of Eurasia within only 5,000 years.

About 10,000 years ago a new and more productive way of life, involving sedentary agriculture, became predominant. This allowed greater investment of labour and technology in crop production, resulting in a more substantial and securer food source, but sporadic migrations persisted.

The next pulse of migration, beginning around 4000 to 3000 bc, was stimulated by the development of seagoing sailing vessels and of pastoral nomadry. The Mediterranean Basin was the centre of the maritime culture, which involved the settlement of offshore islands and led to the development of deep-sea fishing and long-distance trade. Other favoured regions were those of the Indian Ocean and South China Sea. Meanwhile, pastoral nomadry involved biological adaptations both in humans (allowing them to digest milk) and in species of birds and mammals that were domesticated. Once completed, these adaptations allowed humans to consume the meat of most male newborn animals and the maternal milk thereby made available.

Both seafarers and pastoralists were intrinsically migratory. The former were able to colonize previously uninhabited lands or to impose their rule by force over less mobile populations. The pastoralists were able to populate the extensive grassland of the Eurasian Steppe and the African and Middle Eastern savannas, and their superior nutrition and mobility gave them clear military advantages over the sedentary agriculturalists with whom they came into contact. Even as agriculture continued to improve with innovations such as the plow, these mobile elements persisted and provided important networks by which technological innovations could be spread widely and rapidly.

That complex of human organization and behaviour commonly termed Western civilization arose out of such developments. Around 4000 bc seafaring migrants from the south overwhelmed the local inhabitants of the Tigris–Euphrates floodplain and began to develop a social organization based upon the division of labour into highly skilled occupations, technologies such as irrigation, bronze metallurgy, and wheeled vehicles, and the growth of cities of 20,000–50,000 persons. Political differentiation into ruling classes and ruled masses provided a basis for imposition of taxes and rents that financed the development of professional soldiers and artisans, whose specialized skills far surpassed those of pastoralists and agriculturalists. The military and economic superiority that accompanied such skills allowed advanced communities to expand both by direct conquest and by the adoption of this social form by neighbouring peoples. Thus migration patterns played an important role in creating the early empires and cultures of the ancient world.

By about 2000 bc such specialized human civilizations occupied much of the then-known world—the Middle East, the eastern Mediterranean, South Asia, and the Far East. Under these circumstances human migration was transformed from unstructured movements across unoccupied territories by nomads and seafarers into quite new forms of interaction among the settled civilizations.

These new forms of human migration produced disorder, suffering, and much mortality. As one population conquered or infiltrated another, the vanquished were usually destroyed, enslaved, or forcibly absorbed. Large numbers of people were captured and transported by slave traders. Constant turmoil accompanied the ebb and flow of populations across the regions of settled agriculture and the Eurasian and African grasslands. Important examples include the Dorian incursions in ancient Greece in the 11th century bc, the Germanic migrations southward from the Baltic to the Roman Empire in the 4th to 6th centuries ad, the Norman raids and conquests of Britain between the 8th and 12th centuries ad, and the Bantu migrations in Africa throughout the Christian Era.

Modern mass migrations

Mass migrations over long distances were among the new phenomena produced by the population increase and improved transportation that accompanied the Industrial Revolution. The largest of these was the so-called Great Atlantic Migration from Europe to North America, the first major wave of which began in the late 1840s with mass movements from Ireland and Germany. These were caused by the failure of the potato crop in Ireland and in the lower Rhineland, where millions had become dependent upon this single source of nutrition. These flows eventually subsided, but in the 1880s a second and even larger wave of mass migration developed from eastern and southern Europe, again stimulated in part by agricultural crises and facilitated by improvements in transportation and communication. Between 1880 and 1910 some 17,000,000 Europeans entered the United States; overall, the total amounted to 37,000,000 between 1820 and 1980.

Since World War II equally large long-distance migrations have occurred. In most cases groups from developing countries have moved into the industrialized countries of the West. Some 13,000,000 migrants have become permanent residents of western Europe since the 1960s. More than 10,000,000 permanent immigrants have been admitted legally to the United States since the 1960s, and illegal immigration has almost surely added several millions more.

Forced migrations

Slave migrations and mass expulsions have been part of human history for millennia. The largest slave migrations were probably those compelled by European slave traders operating in Africa from the 16th to the 19th century. During that period perhaps 20,000,000 slaves were consigned to American markets, though substantial numbers died in the appalling conditions of the Atlantic passage.

The largest mass expulsion is probably that imposed by the Nazi government of Germany, which deported 7,000,000–8,000,000 persons, including some 5,000,000 Jews later exterminated in concentration camps. After World War II, 9,000,000–10,000,000 ethnic Germans were more or less forcibly transported into Germany, and perhaps 1,000,000 members of minority groups deemed politically unreliable by the Soviet government were forcibly exiled to Central Asia. Earlier deportations of this type included the movement of 150,000 British convicts to Australia between 1788 and 1867 and the 19th-century exile of 1,000,000 Russians to Siberia.

Forced migrations since World War II have been large indeed. Some 14,000,000 persons fled in one direction or the other at the partition of British India into India and Pakistan. Nearly 10,000,000 left East Pakistan (now Bangladesh) during the fighting in 1971; many of them stayed on in India. An estimated 3,000,000–4,000,000 persons fled from the war in Afghanistan during the early 1980s. More than 1,000,000 refugees have departed Vietnam, Cuba, Israel, and Ethiopia since World War II. Estimates during the 1980s suggested that approximately 10,000,000 refugees had not been resettled and were in need of assistance.

Internal migrations

The largest human migrations today are internal to nation-states; these can be sizable in rapidly increasing populations with large rural-to-urban migratory flows.

Early human movements toward urban areas were devastating in terms of mortality. Cities were loci of intense infection; indeed, many human viral diseases are not propagated unless the population density is far greater than that common under sedentary agriculture or pastoral nomadism. Moreover, cities had to import food and raw materials from the hinterlands, but transport and political disruptions led to erratic patterns of scarcity, famine, and epidemic. The result was that cities until quite recently (the mid-19th century) were demographic sinkholes, incapable of sustaining their own populations.

Urban growth since World War II has been very rapid in much of the world. In developing countries with high overall population growth rates the populations of some cities have been doubling every 10 years or less (see below Population composition).

Natural increase and population growth

Natural increase. Put simply, natural increase is the difference between the numbers of births and deaths in a population; the rate of natural increase is the difference between the birthrate and the death rate. Given the fertility and mortality characteristics of the human species (excluding incidents of catastrophic mortality), the range of possible rates of natural increase is rather narrow. For a nation, it has rarely exceeded 4 percent per year; the highest known rate for a national population—arising from the conjunction of a very high birthrate and a quite low death rate—is that experienced in Kenya during the 1980s, in which the natural increase of the population approximated 4.1 percent per annum. Rates of natural increase in other developing countries generally are lower; these countries averaged about 2.5 percent per annum during the same period. Meanwhile the rates of natural increase in industrialized countries are very low: the highest is approximately 1 percent, most are in the neighbourhood of several tenths of 1 percent, and some are slightly negative (that is, their populations are slowly decreasing).

Population growth

The rate of population growth is the rate of natural increase combined with the effects of migration. Thus a high rate of natural increase can be offset by a large net out-migration, and a low rate of natural increase can be countered by a high level of net in-migration. Generally speaking, however, these migration effects on population growth rates are far smaller than the effects of changes in fertility and mortality.

Population “momentum”

An important and often misunderstood characteristic of human populations is the tendency of a highly fertile population that has been increasing rapidly in size to continue to do so for decades after the onset of even a substantial decline in fertility. This results from the youthful age structure of such a population, as discussed below. These populations contain large numbers of children who have still to grow into adulthood and the years of reproduction. Thus even a dramatic decline in fertility, which affects only the numbers at age zero, cannot prevent the continuing growth of the number of adults of childbearing age for at least two or three decades.

Eventually, of course, as these large groups pass through the childbearing years to middle and older age, the smaller numbers of children resulting from the fertility decline lead to a moderation in the rate of population growth. But the delays are lengthy, allowing very substantial additional population growth after fertility has declined. This phenomenon gives rise to the term population momentum, which is of great significance to developing countries with rapid population growth and limited natural resources. The nature of population growth means that the metaphor of a “population bomb” used by some lay analysts of population trends in the 1960s was really quite inaccurate. Bombs explode with tremendous force, but such force is rapidly spent. A more appropriate metaphor for rapid population growth is that of a glacier, since a glacier moves at a slow pace but with enormous effects wherever it goes and with a long-term momentum that is unstoppable.

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