Q and A: Why does it take so long to slow or stop population growth?

Growth through natural increase occurs when the birth rate exceeds the death rate. For example, the U.S. birth rate in 2005 was 14 births per 1,000 people and the death rate was 8, yielding a net increase of six persons for every 1,000 persons in the United States, or approximately 1.7 million additional persons for that year. This rate of natural increase occurred in spite of a very small average family size measured by the total fertility rate—an estimate of the number of births to women during their lifetimes.

The rate of natural increase of a population depends on birth and death rates, which are strongly influenced by the population age structure. Births occur primarily to people in the younger-adult age groups. If there are comparatively more young adults than older adults where mortality is highest, then even at replacement fertility levels (when each woman has about an average of two children) there will be more births than deaths.

Hence, a relatively large number of couples each having one or two children can still produce a large excess of births. This phenomenon is known as population momentum.

In the United States, birth rates are higher than death rates at present, partly due to the relatively young age structure of the U.S. population. Immigrants, who are younger on average than the U.S.-born population, play a significant role in keeping the United States younger than most other developed countries. For example, among U.S. Hispanics, 40 percent of whom are foreign-born, there are approximately 10 births for every death.

The momentum of population growth in less developed countries will only be slowed when the large number of young adults resulting from previous high fertility have passed out of the childbearing years and a succeeding smaller generation reproduces at replacement level fertility. This momentum is very pronounced in China, where women have about two children, but the number of women having children is now much larger than in the previous generation. Thus, even though it has reached replacement level fertility, China's population continues to grow.


Age-sex structure: The composition of a population as determined by the number or proportion of males and females in each age category. The age-sex structure of a population is the cumulative result of past trends in fertility, mortality, and migration. Information on age-sex composition is essential for the description and analysis of many other types of demographic data.

Baby boom: A dramatic increase in fertility rates and in the absolute number of births. In the United States this occurred during the period following World War II (1946 to 1964).

Birth rate (or crude birth rate): The number of live births per 1,000 population in a given year. Not to be confused with the growth rate.

Death rate (or crude death rate): The number of deaths per 1,000 population in a given year.

Less developed countries: Less developed countries include all countries in Africa, Asia (excluding Japan), and Latin America and the Caribbean, and the regions of Melanesia, Micronesia, and Polynesia.

More developed countries: More developed countries include all countries in Europe, North America, Australia, New Zealand, and Japan.

Mortality: Deaths as a component of population change.

Rate of natural increase: The rate at which a population is increasing (or decreasing) in a given year due to a surplus (or deficit) of births over deaths, expressed as a percentage of the base population.

Total fertility rate (TFR): The number of children women are having today. The average number of children that would be born alive to a women during her childbearing years if she conformed to the age-specific fertility rates of a given year.