What does it mean to be “overpopulated”?

Posted by Dave on March 3, 2010 | 6 Comments

Population increases when the birthrate exceeds the death rate, and decreases when the reverse occurs. So what does the world look like when there are too many people?

One way to approach this question is to consider what the limiting factors in population are. When, exactly, does the death rate exceed the birthrate? A key consideration is mortality. If people live shorter lives, the death rate goes up, and population goes down (or at least grows more slowly). What causes mortality to increase? Obviously things like wars, pandemics, and famines have an impact, but economist Robert Fogel argues that these are actually a relatively small part of the global mortality picture.

Fogel, who won the Nobel prize for economics in 1993, runs the Center for Population Economics at the University of Chicago. In 1994 he wrote an influential working paper that arrived at a startling conclusion: By far the biggest historical contributor to mortality is the amount of energy consumed in food compared to nutritional requirements. Eat less, die sooner. In order to survive, a man in his 20s or 30s requires between 1,700 and 2,500 kcal of energy per day, depending on height. This provides exactly enough energy to eat and take care of basic hygiene. Any work — even just strolling for an hour or two — requires additional food.

In England in 1790, the total amount of food consumed was about 2,300 kcal per person per day. For men in their 20s and 30s about 2,700 kcal/day was consumed—on average—which means that many people were consuming barely enough to make it through the day. Indeed, by Fogel’s estimates, three percent of the labor force didn’t consume enough energy to do any work at all. In France, that figure was more like ten percent. The bottom 20 percent of the English work force had only enough energy for about 1 hour of heavy work or 6 hours of light work per day. No wonder English literature from that period is rife with descriptions of impoverished beggars. There simply wasn’t enough food in England to support them in any other line of work.

But total food consumed isn’t the only factor in determining whether a person will live. The key is to compare consumption to requirements. Smaller people need less food. In England in 1800, the average man weighed substantially less than today—about 140 pounds (56 kg). In 1700, the average was even lower: 118 pounds (46 kg). If men in 1700 had been as big as they were in 1800, they wouldn’t have had enough excess energy to do any work outside of agriculture.

Despite this, life expectancy was much lower in 1700 and 1800 compared to today. Fogel says this is because despite being smaller, plenty of people still weren’t getting enough to eat, and if you’re malnourished, you’re not going to live long. Take a look at this chart:

The solid black curves show weight/height combinations that are equal in mortality risk. So for example, a 70-kilo man who is 1.55 meters tall has roughly the same mortality risk as a 55-kilo man the same height. The red curve that divides the figure shows the ideal weight for a given height. So if you’re 1.75 meters tall the ideal weight for maximum life expectancy is around 77 kg. Either gaining or losing weight will reduce your life expectancy.

Plotted on top all this with straight dotted lines are actual average values for French men on four different dates. As you can see, in 1705, 1785, and 1870, the average weights are below the ideal for their heights. But by 1975, the value was nearly aligned with the ideal risk curve. To improve further, average heights would have to increase.

Fogel believes there’s a long way to go. Life expectancy can continue to increase and mortality decline, with resulting surges in population. What he doesn’t address in this paper is what might happen to birth rates under different economic scenarios. If birth rates decline, then the impact of decreased mortality will be diminished.

Fogel also points out that malnourishment doesn’t occur all at once. Suppose the population outstrips the food supply. One response is that people will simply become smaller—as parents have less food to give their kids, the kids won’t grow as large. Yes, they will die younger, but it won’t necessarily look like a famine.

But beyond Fogel’s argument, different cultures have different responses to population pressures. The most dramatic is China’s one child policy, but other countries are promoting birth control and other population control methods. Will these be enough to surmount the economic and technological forces that combine to increase population?

More importantly, even if the world could sustain a much larger population than it does now, do we want it to? If even the current population is dramatically affecting global climate, doesn’t it make sense to try to reverse population gains to stem global warming?

I’ll try to address some of these questions in future posts.

Fogel, R.W. (1994). The Relevance of Malthus for the Study of Mortality Today: Long-Run Influences on Health, Mortality, Labor Force Participation, and Population Growth. NBER Historical Working Paper No. 54.

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