Monday, February 21, 2011

Malthus, Demographics, Scrooge and Rwanda

Kevin Spacey in the T.V. show Wiseguy says,

"The population grows geometrically and the food supply grows arithmetically. Three things keep the balance: Famine. Disease. And war."

This is an accurate synopsis of Thomas Malthus' An Essay On The Principle of Population, published in 1798. Although Malthus never words this so darkly, he does say, "The power of population is indefinitely greater than the power in the earth to produce subsistence for man". This is because populations grow at a constant rate (e.g. 5% per year) while farm output tends to grow at a fixed amount per year (e.g. 5 additional pounds of corn per acre). Eventually, the compounding of the population outstrips the food supply, or as Malthus put it "the increase of population is necessarily limited by the means of subsistence."

Empirical Proof

As evidence that the population grows faster than food production, Malthus cites the aftermath of pandemics and famines around the world, "The effects of the dreadful plague in London in 1666 were not perceptible fifteen or twenty years afterwards. The traces of the most destructive famines in China and Indostan are by all accounts very soon obliterated. It may even be doubted whether Turkey and Egypt are upon an average much less populous for the plagues that periodically lay them waste. "
Much more troubling though, is the idea that Malthusian economics continues to exist today... 

"If they would rather die they had better do it, and decrease the surplus population." - Ebenezer Scrooge

The book Collapse by Jared Diamond (of Guns, Germs, Steel fame) analyzes several accounts from Rwanda... many pointing to Malthus' prescience. More specifically, Collapse emphasizes that while the genocide of Tutsis was ethnically motivated, murders still occurred in villages that were entirely Hutu. The book also does the straightforward arithmetic of:

Acres of Arable Rwandan Land / Population = 1/2 Acre per Person

It is impossible to feed someone with the agricultural output from a half acre of farmland, but the book further highlights the underlying cause of the war by quoting Rwandans saying, "We need another war. We still have too many people."


I think anyone will agree that the mathematical argument is valid, but the assumptions underlying the math are suspect.
  • Population grows faster than food production.
  • Food production grows by a constant amount, but can't grow at a constant rate.
  • Population grows at a constant rate.

There is ample evidence of populations growing at different rates (for example Europe isn't growing while North Africa is), but remember that contraceptives didn't exist in Malthus' day... so populations couldn't slow their growth rates down. Even when a population stabilizes around a level of sufficiency, it can overshoot due to unforeseen flooding, frost or drought, resulting in famine. Furthermore, agricultural productivity may be increasing, but will food output per acre always increase? Western countries may have the luxury of outsourcing food production to other countries, but developing countries can't... especially those in the grip of Malthusian economics. 
I consider it especially profound though, that on the basis of a quantitative argument, a demographer was able to predict wars at a time when society believed that the Industrial Revolution and modern science would eliminate famine and war forever. As Malthus demonstrated, eliminating famine is difficult with an increasing number of mouths to feed, and impossible when they're increasing faster than the food supply.
 "An Essay on the Principle of Population",  1798 by Thomas Malthus.