Page 86 - Fall2020

Given a 2.5 ACRES PER PERSON feared that someday population would outstrip
During the first few decades of the twentieth food supply, but he also noted that the produc-
constant century, the question of how many people the tion of food and items like coal, iron and cotton
environment, earth could support was mainly an academic was actually increasing faster than the rate of
any consideration. One of the scholars who was population growth (Figure 4).
population most interested in the study of populations was In 1935, the Land Policy Section of the
the statistician Raymond Pearl, who earned his USDA’s Agricultural Adjustment Administra-
would increase PhD at the University of Michigan and became tion conducted the first real investigation to see
to the limit a professor at John Hopkins University in 1918. just how many people could be fed with Ameri-
of the Pearl’s mathematics were far more complicated can agriculture at that time. These researchers
and nuanced than the simple geometrical versus calculated that in 1930-1933, it took about 2.2
environment, arithmetical hypothesis of Malthus. acres to feed each American an “adequate diet
then remain in Along with his colleague Lowell J. Reed, at moderate cost.”

a steady state. Pearl developed what he considered to be a “law Included in this 2.2 acre figure was a 16-22
of population growth”: all populations, whether percent allowance for feeding the horses and
fruit flies or humans, tend to follow a logistic mules used for agricultural production; only
growth curve. The first half of a logistic curve about 1.8 acre per person was actually used
looks a lot like an exponential growth curve, but to grow livestock and crops for direct human
then it passes an inflection point, growth slows consumption. Tractors had already entered
down and eventually it levels out—looking American agriculture in 1935 but had not yet
somewhat like an elongated S (Figure 3). fully displaced horses; the researchers predicted
Pearl found that a logistic curve fit actual that the horse-tractor ratio would remain ap-
population data much better than an exponen- proximately the same in 1940.
tial curve. Given a constant environment, any While valuable as a description of American
population would increase to the limit of the agricultural productivity during the years the
environment, then remain in a steady state. data were collected, this USDA study was not
With advances in culture or technology, a new, especially useful for predicting the future. The
higher upper limit might be set, starting a new researchers noted that there was great poten-
growth cycle. tial for increasing agricultural yields by using
While admitting that accurate data on the already known methods such as fertilization;
total world population did not yet exist, Pearl controlling water supply, insects and diseases;
made some rough calculations and tentatively and farming more intensively. Moreover, the
predicted that world population would probably data applied specifically to the United States
level off around two billion people, unless a and were never intended to be extrapolated to
“new cycle, made possible and inaugurated by a global scale.
scientific discoveries” began. Like Malthus, he The USDA study languished in obscurity for

FIGURE 3. One of Raymond Pearl’s
logistic growth curves, charting and
projecting the population of the
United States.

Source: Raymond Pearl, “The Curve of
Population Growth,” Proceedings
of the American Philosophical Society.
1924;63(1):13.

84 Wise Traditions FALL 2020