Birth rates are measures used by social scientists and journalists to provide some indication of the role that new births contribute to a country's total population growth each year, as well as potential future increases when the new cohort reaches childbearing age. The most common form is the crude birth rate (CBR), which is crude in the sense that it compares the number of births to the number of men, women, and children in a given society even though only women of certain ages can reasonably be expected to give birth. The CBR is usually expressed as the number of births in a given period for every 1,000 live persons counted in the midpoint of that period and must not be confused with the total fertility rate (TFR), which includes only women of childbearing ages in the denominator.

In 2006, CBRs ranged from a high of 50.7 births per 1,000 in Niger to a low of 7.3 births per 1,000 in Hong Kong. Birth rates reflect two factors: the proportion of the population composed of fertile women (ages 15 to 44) and the prevalence of childbearing among them. (The TFR is based only on the latter of these). When women of childbearing age constitute a large proportion of the population and exhibit a high prevalence of childbearing, the outcome is predictable: significant population growth due to high levels of childbearing. Small proportions and low prevalence of childbirth among such women will naturally lead to population stagnation or decline due to low levels of childbearing. However, in some instances low prevalence of childbearing among fertile women is offset by their over-representation in the population; sometimes high birth rates are driven not by the high prevalence of childbearing among fertile women but simply by the high number of fertile women in a given society.

In 2005, birth rates were slightly higher in Ireland and Chile (14.4 per 1,000 and 15.2 per 1,000, respectively) than in the United States (14.1 per 1,000) even though childbirth was more common among American women of childbearing age than among women in the other two countries. This paradoxical finding is attributable to the fact that larger proportions of Irish and Chilean women are of childbearing age (45 percent and 46 percent, respectively) compared with women in the United States (41 percent).

Through the 20th century, birth rates fell precipitously throughout the industrialized world, and less developed countries have begun to follow suit. Sudden drops in birth rate have a cumulative effect: The fewer babies born now, the fewer potential mothers there will be later. This has led to stagnant and even declining populations in some countries. This situation is aggravated by the simultaneous decrease in death rates, which has left relatively small birth cohorts charged with providing for larger birth cohorts who are surviving to retirement age, and well beyond, in unprecedented numbers. Immigrants have kept population growth robust in many such countries. However, by 2050 Mexico and other developing countries will experience similar population shortfalls; only time will tell if they can count on immigrants to span the difference between the number of native-born workers and the number needed to support burgeoning senior populations.

Bibliography:

1) Central Intelligence Agency. 2007. The World Factbook. Dulles, VA: Potomac Books.

2) Preston, Samuel H., Patrick Heuveline, and Michel Guillot. 2000. Demography: Measuring and Modeling Population Processes. Malden, MA: Blackwell.

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