There seems to be very little disagreement among market-oriented economists that the optimal number of people on the planet is much larger than the number of people currently alive (see here, here, and here for examples). Here are some reasons for skepticism about that claim.
- The main advantage of more people is a deepening of the market and the division of labor. More people means more ideas and more specialization. But the law of diminishing marginal productivity suggests that each additional unit of labor and of human capital is of less value. Furthermore, in a world of 7 billion people we are going to get roughly as many outlier geniuses as we do in a world of 9, 10, or 15 billion.
- Along with diminishing marginal benefits of people, there are rising marginal costs. The human footprint on the natural environment increases with population, and intrudes ever more into ever scarcer (and more socially valuable) undisturbed habitats. Some free-market types like Ron Bailey suggest that this is not the case by pointing to the possibility of peak farmland in the near future. But peak farmland is only achievable (if it is) through ever more intensive applications of synthetic fertilizer and pesticide. In one sense this capital-intensive agriculture may be “sustainable,” in the sense that human ingenuity will always find new fertilizers and new pesticides to keep agricultural productivity growing, but the negative externalities of these methods are considerable. The economic costs alone of invasive species are immense: think about the costs associated with the chestnut blight, Dutch elm disease, hemlock woolly adelgid, and emerald ash borer in North America alone. They run into the billions. A lot of people look around and say, “Well, I see a lot of green, so the environment must be doing OK.” But 91% of all land in the United States consists of human-disturbed habitats. Disturbed habitats are not necessarily bad for biodiversity, but undisturbed habitats are also important — and the fewer there are, the more valuable each remaining one is. More people means more disturbance, more invasions, more “dead zones,” and the like. And yes, the costs are not just economic, but aesthetic. I have no shame in admitting that I aesthetically value the environment, that other people do as well, and that those values should matter in any schedule of “social welfare.” Is a world without butterflies a world worth living in?
- People don’t like being crowded. Part of the reason why people move to suburbs and exurbs is not just high crime and costs in central cities, but distance from other people. Where do people go to “get away”? Generally rural and wilderness areas. The U.S. still has a lot of open space and could perhaps tolerate 50% more population without feeling intolerably dense, but even in this country, much or most of the wilderness is found in areas with little water or harsh climates.
- More people in a country mean more agency problems with the government. The people find it more difficult to constrain their rulers when their rulers don’t pay attention to individual voices, or even small clusters of people. As a country of over 300 million, the U.S. would face severe agency problems were it not for the federal system — and even so, agency problems are significant. In essence, the rulers are less constrained by the people. Higher populations around the world will mean more prevalent problems with mass democracy and mass dictatorship.
- More people will mean more infectious disease. It is a basic principle of ecology that a higher population of a species encourages greater parasitism on that species. As human populations have increased, so have human diseases. Epidemics of influenza have become more frequent. These viral infections are difficult to prevent and treat. Of course, as medical technology proceeds, humans will fight better against infectious diseases of all kinds. But organisms adapt, and medical technologies will of necessity focus on life-threatening diseases rather than chronic and periodic diseases that are not life-threatening. But even the common cold significantly decreases human well-being. In a future world much more densely populated, we could expect human beings to spend much of their lives ill with minor diseases.