Projections of population numbers to future dates have been made by many people in many ways. Very often the projection is on a simple basis such as, "If the present rate of population increase should continue, the population in the year ________ will be ________." Sometimes different rates of increase are assumed and computations made for each, as was done by DeBow in the Compendium of the 1850 Census. Mathematical methods have been used in large variety, such as linear or parabolic extrapolation, geometric or exponential progression, and projections of fitted curves, such as the logistic. Methods have also been used that take explicit account of the components of population change (births, deaths, and net migration). Some methods go further and take explicit account of the factors that affect the components of population growth. A relatively recent development is the projections of births by cohorts of women, made by the Scripps Foundation for Research in Population Problems.
The components of population change are always involved, at least implicitly, whether or not the makers of the population projections consciously take them into account. The projections may seem reasonable on the basis of the facts used and unreasonable on the basis of direct consideration of the outlook for the components of population change. The methods may be descriptive of what happened before but there may be an indication in other data of some impending change that should be taken into account. In particular, it appears that in the future, fertility will be the main component of population change in the United States, and that many projections of population are in effect indirect and sometimes unreasoned assumptions as to the future course of fertility.
Mortality probably will be a less important source of variation than fertility in future population growth in the United States because mortality in recent years has been so low that not even an improbable complete elimination of deaths of females before the end of the reproductive period could greatly augment the supply of future parents. For example, according to United States life tables for 1953, about 95 percent of white female infants and 88 percent of nonwhite female infants would live to age 40. (Babies born in 1953 and subsequent years will of course be subject to different, probably lower, death rates as they pass through life.) Changes in mortality that do not increase the number of potential parents may re