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Population
Bulletin Vol. 53 No. 3 September 1998 Table of Contents |
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Population: A Lively Introduction Fertility
The study of population dynamics must begin with fertility. Fertility refers to the number of births that occur to an individual or in a population. In 1998, fertility rates of national populations ranged from an average of 1.2 children per woman in Italy, Latvia, Spain, and several other European countries to 7.4 children per woman in the West African country of Niger. The average for the United States was 2.0, and for the world, 2.9. On an aggregate basis, nearly 4 million babies were born in the United States in 1997, and about 21 million in China. Worldwide, 137 million human beings were born in 1997, nearly 261 per minute. Fertility must be distinguished from its sister term, fecundity, which refers to the physiological ability of individuals or couples to have children. Some individuals are infecund—unable to bear children because of disease or genetic dysfunction. Mothers who breastfeed their babies often are infecund temporarily because of natural hormones released by their bodies.2 There is documented evidence of women giving birth to 30 or more children (usually including twins, triplets, and other multiple births).3 Thus, for individuals, fecundity probably ranges from zero to about 30 children. The maximum fecundity of a population, which is composed of individuals with varying levels of fecundity, is thought to be about 15 children per woman.4 This is the theoretical maximum number of children a population of women could produce if they engaged in regular sexual intercourse from menarche, at around age 12, until they reached menopause, at around age 50, and never used any form of birth control. The theoretical maximum of 15 children is a far cry from real-life levels. Even in the world's highest fertility countries the average has rarely exceeded eight children per woman. What accounts for this large gap? In every society a variety of cultural, economic, and health factors interfere with the process of human reproduction. These factors include cultural values regarding childbearing (does the society value large or small families?); social roles (is the wife primarily a childbearer and childrearer?); economic realities (do parents rely on children to look after them in old age?); and the prevalence of diseases such as gonorrhea that impair fecundity. Cultural and economic factors do not affect fertility directly; they influence another set of variables that determine the rate and level of childbearing. In 1956, demographers Kingsley Davis and Judith Blake isolated the factors that control the probability that a woman of reproductive age (roughly ages 15 to 49) will produce a child. The classic list of factors identified by Davis and Blake were labeled the "intermediate variables" that determine a society's fertility level. A modified list of these variables is given in Box 1, arranged according to whether they affect (1) fecundity, (2) sexual unions (including marriage), or (3) birth control. These factors operate in every society, but the relative importance of each varies tremendously. In the 1980s, demographer John Bongaarts demonstrated that four of these variables explain nearly all the variation in fertility levels among populations. These four "proximate determinants of fertility," as he termed them, could be quantified and expressed as a simple formula, creating a versatile method for statistical analysis. The four proximate determinants are: (1) the proportion of women married or in a sexual union, (2) the percent of women using contraception, (3) the proportion of women who are infecund (because they are breastfeeding, for example), and (4) the level of induced abortion.5 These proximate determinants have a direct biological effect on fertility. The importance of each depends on social, economic, and health factors within a population. Contraceptive use and abortion are the key proximate determinants of fertility levels in the United States and most developed countries. In 1997, Spain achieved one of the lowest fertility rates on record for a nation—1.15 births per woman -- largely because of relatively high rates of contraceptive use. About 72 percent of Spanish women of reproductive age use contraceptives. Abortion plays a larger role in keeping Russia's fertility low because, until recently, Russian women have had easier access to abortion than to effective contraceptives.6 Where contraceptive use and abortion are rarely used, the postpartum infecundity and marriage determinants are more important. The Hutterites, a North American religious sect, averaged 12 children per woman in the 1930s—the highest fertility on record for any population—by promoting early and universal marriage and eschewing contraception and abortion. The importance of the intermediate variables differs around the world because of cultural practices and beliefs that affect people's behavior. In many African countries today, women marry young and rarely use contraceptives, yet fertility is kept to a six- or seven-child average through cultural factors. In accordance with ancient traditions and beliefs, women in many African societies breastfeed their babies until age 2 or 3, thus prolonging the infecund period following childbirth (postpartum amenorrhea). In some African societies, mothers are expected to abstain from sex for up to two years after childbirth, especially while they are breastfeeding. Abstinence also lowers a woman's chances of getting pregnant while she is still caring for a toddler. Polygamy, or having more than one wife at the same time, is another cultural practice that affects fertility. African husbands sometimes have more than one wife, and husbands often work away from home for months at a time, further reducing the time women are, in demographers terms, "exposed to the risk of pregnancy." In parts of West and Central Africa, a high prevalence of sexually transmitted disease has caused high levels of infecundity.7 The national fertility rates discussed above are total fertility rates (TFRs). The TFR is commonly used because it is easy to visualize what it stands for: the average number of children in a family. But the TFR is a synthetic rate; it does not measure the fertility of any real group of women. The TFR measures the fertility of an imaginary group of women who pass through their fictitious reproductive lives subject to the rates of childbearing experienced by real women in a given year. Although the TFR concept is not intuitively obvious, it is straightforward and easy to calculate from age-specific birth rates (see Box 2). Another attraction of the TFR is that it allows us to explore the concept of replacement-level fertility. This is the level of childbearing at which couples have an average of two children—just replacing themselves in the population. A population with replacement-level fertility eventually will stop growing (as discussed in the section on population growth). Actually, replacement-level fertility requires a TFR slightly above 2.0, primarily because some children will die before they grow up to have their own two children. In a country with low mortality, such as the United States, a TFR of 2.1 produces replacement-level fertility. In a high-mortality country such as Sierra Leone, replacement-level fertility would require a TFR greater than 3. The crude birth rate is the most easily obtained and most often reported fertility measure. It is calculated from the number of babies born in a given year (or any other time period) divided by the mid-year population, and it is expressed as the number of births per 1,000 population. In 1998, the estimated crude birth rate was 15 births per 1,000 in the United States and 23 births per 1,000 for the world. Crude birth rates in 1998 ranged from 8 in Latvia to 53 in Niger. As the name implies, this rate is an imprecise measure of a society's childbearing patterns. The crude birth rate is highly sensitive to the age structure of a population. The crucial factor is the percentage of the women who are in the reproductive ages, because these women produce the babies. And, within the group of women of reproductive age, younger women have higher birth rates than older women. Thus, a female population with a relatively high proportion of young women will produce more births than a population with a higher proportion of older women. More refined rates attempt to minimize the effects of age structure. In addition to the TFR, these include the general fertility rate, which measures the number of births per woman of childbearing age (ages 15 to 49), and the net reproduction rate, which measures the number of daughters born to a woman given current birth rates and her chances of living to the end of her childbearing years.8 These and other refined rates allow demographers to compare the fertility of different countries more accurately. Lifetime Fertility: Cohort Rates What if we want to measure the fertility of a certain group of women, for example, women born between 1940 and 1945? For women who are past their reproductive years, say above age 50, a completed fertility rate can be estimated from the average number of children they bore from the time they experienced menarche in their early teens until they reached menopause in their 40s or 50s.Completed fertility is a useful measure for comparing the fertility levels of different generations. In the United States, women born between 1906 and 1910 (the 1906-1910 birth cohort) produced what was then the smallest number of children per family in U.S. history, an average of 2.2 children per woman. Women from the 1931-1935 cohort, who became parents during the baby boom, produced the century's highest fertility—a completed fertility rate of 3.2 children per woman. Baby boomers—Americans born between 1946 and 1964—will probably average fewer children than the 1906-1910 cohort, but we will have to wait to find out until about 2010, when they have completed their childbearing years. Completed fertility is a cohort measure because it describes the fertility of a specific cohort of women. The TFR and crude birth rate are period rates because they measure fertility for a given period of time. Cohort rates tell us nothing about current fertility. Likewise, period rates, such as the TFR for 1996, cannot predict future fertility. We do not know what the completed fertility of women who were 20 to 24 in 1996 will be by the time they reach menopause around 2025. The difference between cohort and period rates explains how it is possible that, during the height of the U.S. baby boom (1957), the TFR reached 3.7 children per woman, yet no cohort of women born in the 20th century has recorded a completed fertility rate of more than 3.2 children. Fertility in the United States American women averaged more than seven children each until the early decades of the 19th century. Average fertility declined gradually thereafter, interrupted only by the baby boom following World War II. The TFR reached an all-time low of 1.74 children per woman in 1976, and it has remained relatively low ever since. After creeping up to 2.08 by 1990, the TFR slipped back to 2.03 by 1996. The baby bust of the 1970s came about in large part because of delayed marriage and the widespread use of contraception and abortion. Judging by the long-term fertility trend and the current social trends favoring low fertility—including postponement of marriage and childbearing to older ages, high divorce rates, and the large proportion of women in the labor force—U.S. birth rates are likely to remain low.9 Although most women say they expect to have at least two children, many women have delayed marriage and childbearing so long that they will have only one child or no children at all.10 Some 18 percent of women who were ages 40 to 44 in 1995 had never had children, and most of these women never will. The overall fertility rate in the United States has remained fairly stable since the late 1970s, but American women vary considerably in when and how many children they have. Among all women ages 15 to 44 in 1995, only 23 percent conformed to the two-child average. Forty-two percent had not yet had children by 1995, and 17 percent had three or more children. What accounts for these differences? The most predictable and obvious fertility differential is age, but income, race, religion, and many other social, economic, and cultural factors also influence childbearing. Age Biotechnology and medical advances are expanding the ages at which women can have children. But few women give birth before age 15 or after age 50. Over this roughly 35-year span, birth rates vary substantially by age (see Figure 1).
The postponement of childbearing is portrayed by the steep drop in the birth rate for women ages 20 to 24 during the 1960s and the 1970s. After 1975, the rate leveled off for women in their early 20s, and it edged upward for women ages 30 to 34. Many of these older mothers were having the children they had postponed earlier in life. Despite considerable media attention about increases in the number of women becoming mothers in their 40s, the birth rate for women ages 40 to 44 is lower in 1996 than it was in the 1960s. Finally, Figure 1 reveals that teen birth rates remained relatively low in the 1970s and 1980s, despite large increases in the proportion of teenagers who were sexually active. The rate edged up around 1990, but the share of teens who are sexually active has leveled off, and the teen birth rate has declined.11 Birth rates by the age of mother follow the same general pattern in most societies—rates are low in the teens, peak in the 20s, and decline thereafter. But comparisons of the age-specific rates in different countries reveal significant variations, as shown in Figure 2. In Japan, where the 1996 TFR was 1.4, there is a remarkable concentration of childbearing among women 25 to 29 years of age. These women produce more than 40 percent of all Japanese births. In the United States, birth rates are high for women throughout their 20s. Women ages 20 to 29 account for just over one-half of all U.S. births. In the southern African country of Zambia, where the TFR was an estimated 6.1 in 1996, birth rates, already high in the teen years, peak in the early 20s and then decline slowly into the 40s. Zambian women have higher birth rates than American and Japanese women at every age.
Race and Ethnicity In many countries, racial and ethnic minorities have higher fertility than the majority. Often these differences arise from religious beliefs and cultural traditions. Immigrants often maintain the childbearing patterns of their homelands when they arrive, but they and their children tend to incorporate the fertility patterns of their adopted country over time. Hispanics born in the United States have lower fertility than U.S. Hispanics who were born abroad, for example. Likewise, fertility differences among European ethnic groups in the United States (such as Irish, German, or Italian American) have greatly diminished over time.12A minority group's fertility differences also are linked to its socioeconomic status. The fertility of African Americans has not converged with the rates for non-Hispanic white Americans, although African Americans have lived in the United States since the nation's founding. This persistent difference likely reflects African Americans' lower socioeconomic status, relative to whites. In 1996, the TFR was 1.8 for non-Hispanic white women, 1.9 for Asian and Pacific Islander women, 2.0 for American Indian women, 2.2 for black women, and 3.0 for Hispanic women.13 Arabs in Israel and Asians in Russia are other examples of minority ethnic or religious groups whose fertility remains higher than the average for the country. But minorities do not always have above-average fertility. In Malaysia, for example, the ethnic Chinese minority has lower fertility than the indigenous Malay population. In the United States, the relatively small populations of Japanese Americans and Chinese Americans have the lowest fertility of any major ethnic group.14 Socioeconomic Status In nearly every contemporary society, the poor have more children than the rich. This also holds true for the United States, within all major racial and ethnic groups. Income is closely related to educational attainment, which is often easier to measure. Individuals who have completed more schooling tend to have higher-paying jobs. In general, fertility declines as the income and educational attainment of women increase. In 1995, for example, women ages 35 to 44 with five or more years of college averaged 1.4 children, compared with 2.0 children for women who completed high school only, and 2.7 children for non-high-school graduates.15 Numerous other social, religious, and cultural factors are associated with fertility differences. Most of these can be explained by age, income, or educational differences among these groups. In just about every culture, women who work outside the home have fewer children than those who do not, and rural women have more children than urbanites. People who actively practice a religion tend to have higher fertility than nonreligious people. The long-standing differences between major religious groups in many countries often are intertwined with ethnic and cultural differences. In the United States, Catholics traditionally had more children than Protestants, but this difference has largely disappeared.
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