316 |= ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
and thus to characterize the United States as the place of the amal- 
gamation of races and of mores. 
Owing to the almost total cessation of immigration about 1920, 
our foreign-born are now as a whole a rapidly aging population. 
“Whereas in 1920 about 15 percent of their number were under age 
25, the figure was only 4 percent in 1940. On the other hand, the 
proportion at ages 65 and older almost doubled within this period, 
from not quite 10 percent in 1920 to 18 percent in 1940.* 
The growth of the population of the United States has been af- 
fected not only by a century of unprecedented immigration, but also 
by a differential birth rate as between foreign, native, rural, and urban 
elements. It is well known that the foreign-born at first had larger 
families than the native-born. Nevertheless, there has been a steady 
decrease in family size, both in the population as a whole (fig. 1) and 
also among the foreign-born (fig. 2). The average size of family 
in 1790 was 5.7 persons for the area covered by the census of that 
date; in 1900 it had decreased by 1 person (5.7 to 4.6), both for the 
area covered in the first census and for continental United States as 
a whole; and in 1940 it had decreased still further to 3.15. 
That urban birth rates tend to fall below rural has been shown 
by the ratios of children under 5 years of age per 1,000 women aged 
20 to 44. This ratio is known as “effective fertility.” Between 1800 
and 1930 for the United States as a whole this ratio dropped from over 
900 to under 400. Yet in 1930 effective fertility for the farm popu- 
lation as a whole was over 500 and in one isolated county in Kentucky 
it was still over 900.4 The tendency of many foreign-born peoples to 
avoid rural areas and to congregate in cities may be a factor in the 
rapid lowering of their birth rate. 
The net effect of these and other more involved factors is a gradual 
slowing down of population growth in the United States. According 
to a conservative recent prediction, made on the basis of the censuses 
up to and including that of 1940, the population of the United States 
should level off at around 184 millions about the year 2100 (fig. 3).* 
This means that, if the present trend continues for another century 
and a half, natural growth will cause the population to increase only 
about 50 millions more. 
Although we have seen the virtual end of immigration into the 
United States and are aware that the natural increase in the popu- 
3 Statistical Bulletin, Metropolitan Life Insurance Co., June 1945. 
4See Lorimer and Osborn, 1934. 
5 Back in 1919 Raymond Pearl and his associates fitted a logistic curve to the census 
counts from 1790 to 1910. Their prediction of the 1920 population proved to be in excess 
of the actual figure by 16 parts in a thousand; it missed the count in 1930 by 2.5 parts in 
a thousand (in defect) ; and it went wide of the mark in 1940 (37.3 parts in a thousand 
in excess). This error may have been due in part to the change in immigration policy. 
The latest prediction takes these trends into account. 
