The productivity of labor depends upon the manner in which it is combined with the other factors' in production, and upon its quantity and quality. The quantity of labor is in general modified by the same influences which modify the quantity of population as a whole. Moreover, any influence which changes the proportion of laborers in the total population tends to affect the labor supply. For example, legislation which raises the age limit for compulsory education tends to reduce temporarily the labor supply.

The population is also influenced by migration and natural growth. Migration into a country is called immigration and migration from a country emigration. The excess of immigration over emigration represents a tendency to increase the population. The United States throughout its history has constantly had an excess of immigration over emigration. An excess of emigration over immigration represents a tendency to decrease of population, which may, of course, be counteracted by a natural increase of the population at home. All European countries during the past century have had an excess of emigration over immigration but in spite of this loss by migration, and because of the excess of births over deaths, they have with few exceptions been able to maintain or increase their numbers.

The change of population through natural growth depends upon the difference between the number of births and the number of deaths. The number of births annually per thousand of population is called the crude birth rate or simply the birth rate and the number of deaths per thousand population annually is called the death rate. Normally in all civilized countries there is a tendency for both birth rate and death rate to diminish. Where the death rate diminishes more rapidly than the birth rate population tends to increase.