Concerns over population growth date back to the 18th Century (Malthus). Population growth reflects food constraints, disease and parental preferences. Columbus's discovery of America had a huge impact on world populations: Europeans brought tuberculosis and small-pox to America which literally decimated Native American populations, while the introduction of the American potato to Europe increased food productivity and permitted rapid population growth in Europe, ultimately providing the labor resources for the Industrial Revolution.
In general, higher-income nations now have lower population growth rates. These nations have higher infant survival rates but lower birth-rates (live births annually/1,000 population). This reflects better availability of contraception, and preferences for later marriages and fewer children. Since birth-rates don't account for the age structure of the population, the US Census calculates a total fertility rate, which is the number of live births the average woman has in her lifetime. The total fertility rate yielding a stationary population is the replacement rate; in the US, this is 2.11 children per woman. The US has been below this rate since 1972.
Population Growth and Economic Growth
Population growth may increase or decrease economic growth. If the marginal productivity of people exceeds the marginal cost, more people increases total output, but unless marginal productivity is also above average productivity, more people reduces per capita output. Theory suggests that fixed factors such as land imply declining marginal productivity of labor. However, technologies enhance the marginal productivity of labor over time. Economies of scale may also be supported by population growth.
Age structure effects determine proportions of children, working-age adults and elderly in the population. The "elderly effect" (characterizing many developed nations) inflates the proportion of elderly in the population via improved health care. The "youth effect" (characterizing many LDC's) reflects high birth rates and increases the proportion of children in the population and also reduces female work-force participation. Both effects reduce the proportion of the population in the work-force. We hypothesize that the youth effect slows economic growth more than the elderly effect: since older people save more than younger people, the elderly effect yields a larger savings pool supporting capital formation. Countries exhibiting youth effects will tend to have lower savings, less capital and lower productivity per worker. We don't have strong empirical support for this hypothesis, however.
Various factors explain why rapid population growth tends to be self-perpetuating and worsens income inequalities: (1) High-income families tend to have fewer children, and thus invest much more money in each child, than low-income families. (2) If we view children as investments, children in capital-rich nations yield a higher average "returns" to their parents than children in capital-poor LDC's, so parents in LDC's compensate by having more children. (3) Rapid population growth depresses wage rates vis-a-vis profits.
The three-phase demographic transition model indicates economic growth ultimately reduces population growth: Pre-industrial (Phase 1) societies exhibit fairly stable populations because high mortality rates compensating for high birth rates. Transitional (Phase 2) societies exhibit rapid population growth as mortality rates fall relative to birth rates. Industrial (Phase 3) societies recover population stability as birth rates decline to match low mortality rates. This model suggests that economic development may actually be our best hope for preventing Malthusian collapse.
Are there behavioral biases toward over-population? Are parents' child-bearing decisions economically efficient for society? Probably not. Tietenberg argues that "income equality is a public good" which tends to be reduced by population growth. However, public policies (subsidies for food and education, tax breaks) may distort family-planning incentives by reducing the costs of children to parents. In addition to promoting contraception, India and China have both tried very unpopular policies restricting families to one or two children each. It might be more efficient to reduce child-bearing incentives via tough child-labor restrictions and a tax on children. This would make children normal or even luxury goods instead of inferior goods.
Modeling the "market" for children, demand is shifted downward by industrialization; availability of alternative family savings vehicles; and decreases in infant mortality. Supply (marginal cost) is shifted upward by better job opportunities for women; and increasing food, housing and education costs (education costs include foregone children's wages). Ruttan indicates that improving the economic status of women is critical for controlling population growth: this stimulates economic growth and demands for improved child health, nutrition and education, particularly where contraception is available.