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 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. Both effects reduce the proportion of the population in the work-force. Countries exhibiting youth effects will tend to have lower savings, less capital and lower productivity per worker.