On May 3, the United Nations issued its 2010 Revision of World Population Projections, which, according to the media, predicts that the world’s population, expected to reach 7 billion by the end of this year, will be 10.1 billion by the end of the century. But the media reports have tended to be imprecise. The UN report offers three predictions—a high, medium, and low—depending on different assumptions. The high is almost 16 billion and the low 6.2 billion (which is actually lower than the current world population), and a cautious appraisal of the report is that it provides a plausible basis for thinking that the world population will probably be between 6 and 16 billion 89 years from now. Of course much could happen between now and then to push the world population far outside the range, such as an asteroid collision that wiped out the entire human population.
World population depends on fertility (the birth rate) and mortality (the death rate). In predicting the former, the UN divides countries into those with birth rates below the replacement level of 2.1 births per woman (42 percent of the world’s population lives in such countries), countries with birth rates slightly above replacement level (40 percent), and countries with very high birth rates (the remaining 18 percent). The death rates in reach group of countries is calculated, and then the expected population growth within each group is estimated using the current rate of population increase (birth rate minus death rate) as the baseline for each. (For the world as a whole, the UN report expects life expectancy at birth to increase from 68 today to 86 in 2100.)
The fall in population in countries with birth rates below replacement levels is expected to level off, which seems plausible, but what mainly drives the 10.1 billion 16 billion predictions of total population at the end of the century is the assumption that birth rates will continue to be very high in the countries (mostly in Africa, Asia, and South America) that currently have high birth rates. How realistic is such an expectation? Much depends on changes in the status of women. If employment and wage rates of women rise, thus increasing the opportunity costs of children, birth rates will decline. These changes are likely to be correlated with increased wealth, which in turn will accelerate the fall of death rates in these countries. But the decline in death rates is likely to be less than the decline in birth rates (notice how the UN report predicts a 26 percent increase in longevity [(86 – 68)/68], versus a 44 percent [(10.1 – 7)/7] increase in population (the midrange forecast), and if so the rate of population increase will slow in the high-fertility countries.
Really no one knows what the world population will be in 2100. The history of population projections is not promising. An example is how the Bureau of the Census, failing to anticipate the post-World War II “baby boom,” underpredicted U.S. fertility rates, then overpredicted them by failing to predict the end of the baby boom. And techniques of predicting fertility and mortality don’t seem to have improved much, judging by results. And the longer-range the forecast, the larger the forecasting error.
But suppose world population will reach 10.1 billion by the end of this century. Would that be a good or a bad thing? Arguably a good thing, on several grounds. One is that it would enable greater specialization, which reduces costs. Second is that it would increase the returns to innovation by increasing the size of markets, though an offset is that innovation can produce immensely destructive as well as constructive technology. Third, the more people there will be, the more high-IQ people there will be, and hence the faster the growth of knowledge will be; though a possible offset is that the more evil geniuses and other monsters there also will be; persons of great potential for evil, such as Hitler, Stalin, and Mao, presumably are rare. Fourth, if the total subjective welfare of the 10.1 billion exceeds that of a smaller population, or (depending on one’s version of utilitarianism) the average welfare of the greater population is greater than that of the smaller one, the world will be a happier place in a utilitarian sense (the excess of pleasure over pain will be greater).
The downside of population growth is the pressure it places on the environment and natural resources, especially the former, since the price system provides efficient rationing of resource use. Rapid population growth will increase the problem of global warming and the rate of extinctions and other biodiversity decline, as well as create congestion externalities, though these concerns have diminishing significance the longer the time span of concern. Continued population growth could combine with an acceleration of global warming to precipitate a global catastrophe (perhaps a catastrophic water shortage) within the next few decades, but 89 years from now the march of technology may enable such problems to be solved. Think of the technological advances of the last 89 years (that is, since 1922), and imagine a comparable rate of technological advance applied to the current level of technology, which is so much higher than that of 1922.
But the beneficent effects of population growth, like the estimates of that growth, are highly uncertain. The risk averse among us might prefer a lower rate of population growth in order to reduce the downside risks of that growth, even though the upside potential would be reduced as well.