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Fertility is a term used to describe something which is fertile and has the ability to grow or reproduce.

see also Birth Control


In Demography, fertility is the statistical analysis of having babies. Couples do it, and so too do entire societies, in a way. Therefore, demographers approach the topic at both the individual and the social level. At the individual level, the statistics are normally analyzed in terms of the mother, though admittedly the father plays a role as well. At the social level we measure fertility with different rates using birth statistics and census data. Research on fertility inside the United States has slowed in recent decades, because most of the important questions have been answered. nearly identical. There has been a shift from a focus on fertility as the basis of population growth to one of fertility as a component of health, marital and family-building patterns, and the connection to women's employment and careerism. There has been new attention to racial and ethnic patterns, and to the fertility of unmarried women. Attention has shifted to historical patterns in the US and Europe, going back to the year 1600. Even more important, attention has shifted to the rapid growth of population in developing countries, especially in Asia, Africa and Latin America. In terms of world population, the main issue is the overall number of births, and how it can be reduced.

Measuring Fertility

The most basic measurement of fertility is the number of births; combined with deaths and migration, it permits the calculation of population change. To compare countries or groups, we must relate the number of births to the total population ("crude" birth rate) or to the number of women at risk (various measures.)

Period Measures

"Period" measures refer to a cross-section of the population in one year. "Cohort" data on the other hand, follows the same people over a period of decades. Both period and cohort measures are widely used.

  • Crude Birth Rate. The CBR is simply the number of births in a year divided by the midyear population, times 1000. Since 1972, the CBR for the USA has been fairly steady, between 14.0 and 16.0. To calculate it you need to count the number of births and know how large the population is.
  • General Fertility Rate. GFR is the number of births in a year divided by the number of women aged 15–44, times 1000. It focuses on the potential mothers only, and takes the age distribution into account. To calculate it you need age data.
  • Child-Woman Ratio. CWR equals ratio of the number of children under 5 to the number of women 15-49, times 1000. It is especially useful in historical data. It does not require counting births. This measure is actually a hybrid, because it involves deaths as well as births. (That is, because of infant mortality some of the births are not included; and because of adult mortality, some of the women who gave birth are not counted either.)
  • Coale's Index of Fertility is a special device used in historical research.

Cohort Measures

Cohort measures relate to a "cohort" of women (that is, women in the same age group). The cohort of women born in 1920 were aged 15–19 in 1935-39; they were 45-49 in 1965-69. Since we know the age specific birth rates in each of those years we can measure the cohort fertility. But consider the cohort of women born in 1970; they were aged 15–19 in years 1985-89; they were age 20-24 in 1990-1994; they will be aged 45–49 in 2025-29. Obviously we do not know what the future rates will be, so we have to wait a few decades. What impatient demographers do is create a "synthetic cohort"—an imaginary group of women who have the same age-specific birth rates as we observe in a given year. The result is a snapshot of fertility in a single year (See Exercise 3 for how it works.)

  • age specific fertility rate. ASFR = number of births in a year to women in a 5-year age group, divided by the number of all women in that age group, times 1000. Thus if there are 10 million women age 25-29, and they had 1,179,000 babies in a given year, then ASFR = 117.9. The usual age groups are 10-14, 15-19, 20-24, etc.
  • Total Fertility Rate. TFR is the total number of births over a lifetime for a synthetic cohort. In the USA since 1972, TFR has flyctuated between 1.7 and 2.0.TFR equals the sum for all age groups of 5 times each ASFR rate.[1]
  • Gross Reproduction Rate. GRR is simply the number of girl babies a synthetic cohort will have. Since 49% of American babies are girls, GRR = .49 x TFR. It assumes that all of the baby girls will grow up and live to at least age 50.
  • Net Reproduction Rate. NRR starts with the GRR and adds the realistic assumption that some of the women will die before age 59; therefore they will not be alive to bear some of the potential babies that were counted in the GRR. NRR is always lower than GRR, but in countries (such as the USA) where mortality is very low, almost all the baby girls grow up to be potential mothers, and the NRR is practically the same as GRR. In countries with high mortality, NRR can be as low as 70% of GRR. NRR of course requires knowledge of the age-specific death rates, in addition to the age-specific birth rate, and the numbers of women in each age group. When NRR = 1.0, each generation of 1000 baby girls grows up and gives birth to exactly 1000 girls. When NRR is less than one, each generation is smaller than the previous one; in the 1990s, the NRR for the USA was slightly under 1. When NRR is greater than 1 each generation is larger than the one before. NRR is a measure of the long-term future potential for growth, but it usually is different from the current population growth rate.

Determinants of Fertility

Fertility declines sharply with age in a women, such that by age 30 her fertility has already fallen by nearly 40% and by age 40 her fertility has fallen by 2/3rds.[2] The reproduction ability of humans has factors extending to nutrition, sexual behavior, culture, instinct, endocrinology, timing, economics, way of life, and emotions.

Biological Determinants.

"Fecundity" is the biological ability to have children, It is a function of age, health and nutrition. Fecundity begins at menarche, is low for several years and then increases quickly to a maximum in the early 20s, then declines to menopause (about age 50). "Natural fertility" is the level of fertility reached without any form of conscious birth control. The Hutterites, a small religious group in the U.S. and Canada who resemble the better-known Amish, do not practice birth control. Therefore, their fertility pattern is used as a statistical model of "natural fertility" for a population that does not use any contraception. Hutterite women average 12 children each.

Social Components of Fertility.

The "Three-step Analysis" was introduced by Kingsley Davis and Judith Blake in 1956 that makes analysis of the proximate determinants of the fertility process clear.

  • Intercourse. The first step is intercourse, and an examination of the average age at first intercourse, the average frequency outside marriage, and the average frquency inside. Voluntary abstinence is a simple and highly effective technique to eliminate the risk of pregnancy.
  • Conception. Certain physical conditions may make it impossible for a woman to conceive. This is called "involuntary infecundity." If the woman has a condition making it possible, but unlikely to conceive, it is called "subfecundity." Venereal diseases, especially gonorrhea, syphilis and chlamydia are causes. Nutrition is a factor as well; women with less than 20 percent body fat may be subfecund, a factor of concern for athletes and people susceptible to anorexia. "It takes 50,000 calories to make a baby," argues demographer Ruth Frisch. There is subfecundity in the weeks following childbirth, and it can be prolonged for a year or more through breastfeeding. A furious political debate raged in the 1980s over the ethics of baby food companies marketing infant formula in developing countries. A large industry has developed to deal with subfecundity in women and men. An equally large industry has emerged to provide contraceptive devices designed to prevent conception. Their effectiveness in use varies. On average 85% married couples using no contraception will have a pregnancy in one year. The rate drops to the 20% range when using withdrawal, vaginal sponges, or spermicides. (This assumes the partners never forget to use the contraceptive.)The rate will be only 2 or 3% when using the pill or an IUD, and drops to near 0% for implants and 0% for tubal ligation (sterilization) of the woman, or a vasectomy for the man.
  • Gestation. After a fetus is conceived, it may or may not survive to birth. "Involunatry fetal mortality" involves miscarriages and stillbirth (a stillbirth is a fetus born dead). Voluntary fetal mortality is called "abortion," and is the single most-used form of birth control around the world. In the USA, the number of abortions has been steady at about 1.6 million a year.

Explanations for Fertility Transition

"The Fertility Transition" takes place when a society moves from high to low fertility. All sorts of social and economic changes take place at the same time during modernization, making it hard to sort out all the causes and effects. Policy makers who want to move a particular society from high to low fertility must have a workable theory of how to get there.

Supply-Demand Framework

Supply and demand is the economists' approach, looking at fertility from the viewpoint of a couple that has a potential supply of babies (the mother's potential fertility) and a demand. Part of the demand is caused by natural sexual behavior. (If unrestrained, it will lead to "natural fertility" or Hutterite levels of childbearing.) An important demand factor in agricultural societies is the usefulness of children and teenagers to help with the crops and animals. Children as young as 6 or 7 can pay their own way. A second factor (important in developing countries that lack a social security apparatus) is the need for children to provide support for the parents in old age. The more children they have the better off the parents will be.

Ghana is typical of the many societies where the women bear most of the cost and burden of child-bearing and child rearing, while men make the fertility decisions.

What are the motivations for wanting ("demanding") fewer children. First of all they are expensive, especially in urban settings. Second, as a society modernizes and develops a complex infrastructure, large families are at a disadvantage compared to small ones. When family resource are concentrated on a few children, they are likely to do better than when resources are spread over many children. Schooling is an example; two children who attend secondary school can be more effective in a modern society than six illiterate children.

A model of three preconditions for fertility decline was developed by demographer Ansley Coale. He asked what perceptions of the world were necessary if people were to consciously limit their fertility. a) Accept calculation, planning, and choice as valid (and thereby turn away from fatalism or the sense that God or the stars predetermines everything.) b) Perceive the advantages of having fewer children. However, families may perceive advantages and disadvantages in a very different way from government officials or outside experts. There are two factors here: whether or not fewer is better; And how people perceive advantages and disadvantages—who do they look to for the norms and standards of behavior? c) Know about and be able to master effective contraceptive techniques. This is the easiest part.

Innovation-diffusion and "cultural perspective".

Historically societies that have low fertility now formerly had high fertility. Using the Coale model we can see that people have to learn how to control their fertility. Typically, low fertility is a cultural value that originates in the more privileged classes and then diffuses throughout society. In each community and social group there are innovators who are the first to experiment with any new idea or technique; they tell their friends and family, who are the secondary adopters. The mass media may speed up the process. In the USA, the "Comstock Laws" made it illegal until 1970 to publish information about contraceptives or to send devices through the mail.[3] Family physicians quietly informed married couples about withdrawal and condoms. Until the 1970s abortions were illegal and for most women hard to obtain. American feminist Margaret Sanger (1883-1966) crusaded tirelessly to promote knowledge about birth control and family planning. She wanted sexual liberation for women, and to attain that they needed control of their reproductive functions. She recommended the diaphragm, which gives the woman more control.

High Fertility Countries: India, Ghana, Jordan

The need to replenish society is a major cause of high fertility. Before World War Two, mortality was so high in India that they needed the very high CBR of 37 (37 births a year per 1000 population) just to stay even (i.e. to keep a NRR of 1.0). In addition there was a strong commitment to the idea of the lineage. Everyone had a duty to ancestors and descendants to keep the line going, and indeed to maximize its strength or numbers. Thus there was a demand for high fertility.

Sometimes, a minority group will promote high fertility in order to gain relative power, as for example the Palestinians in Jordan today (vis-a-vis Israel), and a century ago, the French (vis-a-vis Germany).

Children are in demand in India, Ghana and Jordan because they provide agricultural labor in rural societies. In countries with a weak social welfare system, children provide security for the parents' old age. India is rapidly modernizing and presumably fertility will fall sharply.

In many societies (especially in Korea, China and northern India) there is a strong preference for sons. As a result, couples who had mostly daughters are socially embarrassed, and they kept having more babies in order to get some sons. (A paradoxical result: the more people insist on having sons, the more girls will be born!)

India before the 1950s, practiced natural fertility. There was very little birth control of any kind; fertility varied somewhat by education, but by no other social factor. The custom of very early arranged marriages (at age 12 or 13 for girls) increased fertility. Young widows were not allowed to remarry, which reduced the birth rate.

Low Fertility countries: USA, Japan, Britain

Low fertility countries have already undergone the demographic transition, and usually have also modernized their economy.

If rural societies favor large families, urban societies favor small ones. By concentrating family resources on fewer children, each child performs better. One of the reasons societies urbanize is that their population has abandoned fatalism and is engaged in much more complex organizational forms (like corporations). When the people give up fatalism and take control of their lives they have attained Coale's first prerequisite. In Japan the culture always favored small families; the widespread use of abortion as a population control device helped the country slash its birth rate in half in the dozen years after World War Two.

Education is strongly related to fertility in every country, and has been for more than a century: the more education the mother has, the fewer children. Literate mothers can help prevent infant mortality (so that fewer births are needed to achieve desired completed family size.) Literacy is highly correlated with the use of contraceptives. Education helps people give up fatalism and take more active control of their lives. They can set an ideal family size, reach it, and stop. They set smaller ideal family sizes and use contraceptives more often and more effectively. Educated women are more modernized, and are more likely to reject traditional norms and lifestyles that are conducive to high fertility. In developed societies, women seeking a college degree often postpone motherhood. Once they have a dregree they are more likely to pursue full-time careers in demanding jobs that are hard to combine with childrearing. On the other hand, more education usually means better nutrition for the mother and less use of practices like extrended breastfeeding. These factors tend to increase the fertility of better educated women, but they are usually overwhelmed by the factors that lower fertility. The difference in fertility between the best and least educated women varies from country to country. In most countries the difference is one or two. The greatest difference is in copuntries in the midst of the demographic transition. In Latin America, for example, women who never attended school average six or seven children, while high school graduates average only two or three.

Income matters rather little to fertility. High income people may be able to afford more children, but they do not have them. Partly this is because wives must reduce or postpone childbearing in order to help earn that high income, and in part it is because high income people usually have more education.

Other factors such as religion and ethnicity are sometimes important. Until the 1960s, Roman Catholics typically favored much larger families than did Protestants. Mormons still do. On the other hand, Catholics also promoted religious orders of celibate nuns, as well as a celibate all-male priesthood.

Baby boom: 1940-1964

Baby boom: fertility fell steadily in the USA, from 1800 to 1940. Then suddenly it started going up again, reaching a new peak in 1957. After 1960 fertility started declining rapidly. In the Baby Boom years (1940-1964) women married earlier and had their babies sooner; the number of children born to mothers after age 35 did not increase. After 1960 ideal family size fell sharply, from 3 to 2 children. Couples postponed marriage and first births, and they sharply reduced the number of third and fourth births.

In the United States in 1995, there were 110 live births per 1,000 women ages 20–24. Four decades earlier near the peak of the Baby Boom in 1958, there were 258 live births to women in the same age range.

Easterlin Model

American Economist Richard Easterlin developed an ingenious model or theory to explain the Baby Boom. He assumes first that young couples try to achieve a standard of living equal to or better than they had when they grew up. This is called "relative status;" in other words, young men in one cohort compare themselves now to where their own fathers in a previous cohort had been. Second Easterlin assumes that when jobs are plentiful, it will be easier to marry young and have more children and still match that standard of living. But when jobs are scarce, couples who try to keep that standard of living will wait to get married and have fewer children. For Easterlin, the size of the cohort is a critical determinant of how easy it is to get a good job. A small cohort means less competition, a large cohort means more competition to worry about. The assumptions blend economics and sociology, and they seem to make sense, but notice that Easterlin did not rely on surveys or interviews asking people what really motivated them.

Now to put the model to work. In the 20 years after World War Two, 1945-1965, the economy was strong and growing and demanded workers. But immigration was slight, and social custom prohibited most young mothers from working. Most important, the cohort (or number of people coming of age each year) depended on the number of births 15 or 20 years earlier, which was small. In 1940, there were 5.7 million men age 20-24, in 1950 there were 5.6 million, but in 1960, with a much larger economy, there were only 5.3 million. Employers scrambled to hire those young workers, and paid them well, enabling them to start families young. Furthermore, these men grew up in the Depression, and so there expectations of a standard of living were modest. Hence the Baby Boom. By the late 1960s times were changing. More immigrants were arriving (because of the new laws in 1965), mothers were now "allowed" to work (social customs changed drastically), and most important, the first of the Baby Boomers started flooding the job market (cohorts became larger). Thus by 1970 there were 7.8 million men aged 20–24. They had much more competition for jobs (from each other, the mothers and the immigrants), AND they grew up in the more affluent 1940s and so had higher standards to meet. (Easterlin also notes that they were crowded growing up, and had to compete for attention with siblings in large families and classmates in crowded schoolrooms.) Their solution was to delay getting married (perhaps by staying in school), and when married to have the mother work and to bear fewer children. Divorce was high because it was harder to make a marriage work. Indeed, the stress contributed to all sorts of other social problems, such as suicide and crime. Easterlin's model explains the Baby Boom very well. He and his students used it to explain the fertility changes in the early 19th century, when America was an agricultural society and to get married a young man had to have a farm. They tried to show that long-term cycles existed. When farm land was readily available couples married young and had lots of children, but when land became scarce they married later and had fewer children.

Unfortunately there are fatal problems with the Easterlin model. First of all, the sociologists have been unable to find the hypothesized motivations. Do people try to attain a standard of living similar to what they had when they were young by delaying marriage or manipulating family size? People deny it and the statistics for individuals show no effect. Do large cohorts in fact earn less than small cohort? The differences, over a lifetime, seem to be about 3% for men and 0% for women, so the effects are too small to be important. Easterlin theory's predicts an upturn in wages for young people entering the labor market in the 1990s (because they came from a smaller post-baby boom cohort). However, the wages seem to be lower, not higher. Do other countries show an Easterlin effect in their wage or fertility patterns over time? No. The theory seems to stop working circa 1980. With smaller cohorts trends toward lower fertility and lower wages were supposed to reverse themselves, but they did not. As Ronald Lee has concluded, Easterlin's grand theory now has few believers.

see United States Population


The French pronatalist movement from 1919-1945 failed to convince French couples of their patriotic duty to help increase their country's birthrate. Even the government was reluctant in its support to the movement. It was only between 1938 and 1939 that the French government became directly and permanently involved in the pronatalist effort. Although the birthrate started to surge in late 1941, the trend was not sustained. Falling birthrate one again became a major concern among demographers and government officials beginning in the 1970s.[4]

Fertility in religions

One good example of the role fertility played in religion is the Israelites' use of pagan gods such as golden calves to bring fertility to the lands and their women. In most of Mesopotamia fertile land was essential to one's well-being and survival, so most pagan gods were devoted to fertility and growth.

See also


  • Barrett, Richard E., Donald J. Bogue, and Douglas L. Anderton. The Population of the United States 3rd Edition (1997) compendium of data
  • Coale, Ansley J. and Susan C. Watkins, eds. The Decline of Fertility in Europe, (1986)
  • Eversley, D. E. C. Social Theories of Fertility and the Malthusian Debate (1959) online edition
  • Garrett, Eilidh ety al. Family Size in England and Wales: Place, Class, and Demography, 1891-1911(2001) online edition
  • Grabill, Wilson H.. Clyde V. Kiser, Pascal K. Whelpton. The Fertility of American Women (1958), influential study at the peak of the Baby Boom online edition
  • GuzmÁn, JosÉ Miguel et al. The Fertility Transition in Latin America (1996) online edition
  • Haines, Michael R. and Richard H. Steckel (eds.), A Population History of North America. (2000), 752 pp. advanced scholarship
  • Hawes, Joseph M. and Elizabeth I. Nybakken, eds. American Families: a Research Guide and Historical Handbook. (1991)
  • Klein, Herbert S. A Population History of the United States. (2004). 316 pp
  • Knox, P. L. et al. The United States: A Contemporary Human Geography. (1988). 287 pp.
  • Kohler, Hans-Peter Fertility and Social Interaction: An Economic Perspective (2001) online edition
  • Leete, Richard. Dynamics of Values in Fertility Change (1999) online edition
  • Lovett, Laura L. Conceiving the Future: Pronatalism, Reproduction, and the Family in the United States, 1890-1938, (2007) 236 pages;
  • Mintz Steven and Susan Kellogg. Domestic Revolutions: a Social History of American Family Life. (1988)
  • Pampel, Fred C. and H. Elizabeth Peters, "The Easterlin Effect," Annual Review of Sociology (1995) v21 pp 163–194
  • Population Reference Bureau, Population Handbook (5th ed. 2004) online (5th ed. 2004), A quick 72pp guide to population dynamics; explains how to calculate and use major rates, ratios, and other measures from the birth rate to the life table; includes tips on interpreting population data.
  • Reed, James. From Private Vice to Public Virtue: The Birth Control Movement and American Society Since 1830. 1978.
  • Tarver, James D. The Demography of Africa (1996) online edition
  • Weeks, John R. Population: An Introduction to Concepts and Issues (10th ed. 2007), standard textbook


  1. Another way of doing it: add up the ASFR for age 10-14, 15-19, 20-24, etc, and multiply by 5 (to cover the 5 year interval).
  2. See chart
  3. The Comstock laws forbidding pornography involving children are still in effect and rigorously enforced.
  4. Andres Horacio Reggiani, "Procreating France: the politics of demography, 1919-1945." French Historical Studies Spring 1996 v19 n3 pp 725-54