Optimum human population size

ABSTRACT: Although the tremendous size and rate of growth of the human population now influence virtually every aspect of society, rarely does the public debate, or even consider, the question of what would be an optimum number of human beings to live on Earth at any given time? While there are many possible optima depending on both the criteria defining “optimum” and on prevailing biophysical and social conditions, there is a solid scientific basis for determining the bounds of possibilities. All optima must lie between the minimum viable population size, MVP (Gilpin & Soulé, 1986; Soulé, 1987) and the biophysical carrying capacity of the planet (Daily & Ehrlich, 1992). At the lower end, 50-100 people in each of several groups for a total of about 500, might constitute an MVP.

At the upper end, the present [1994] population of 5.5 billion, with its resource consumption patterns and technologies, has clearly exceeded the capacity of Earth to sustain it. This is evident in the continuous depletion and dispersion of a one-time inheritance of essential, nonsubstitutable resources than now maintains the human enterprise (e.g., Ehrlich & Ehrlich, 1992; Daily & Ehrlich, 1992). Numerous claims have been made that Earth’s carrying capacity is much higher than today’s population size. a few years ago, for example, a group of Catholic bishops, misinterpreting a thought exercise by Roger Revelle (1976), asserted that Earth could feed 40 billion people (Anonymous, 1988); various social scientists have made estimates running as high as 150 billion (Livi-Bacci, 1989). These assumptions are based on preposterous assumptions, and we do not deal further with them here.

Nonetheless, we are left with the problem of determining an optimum within wide bounds. Above the minimum viable level and within biophysical constraints, the problem becomes a matter of social preference. Community-level, national, and international discussions of such social preferences are critical because achieving any target size requires establishing social policies that influence fertility rates. Human population sizes have never, and will never, automatically equilibrate at some level. There is no feedback mechanism that will lead to perfectly maintained, identical crude fertility and mortality rates to a substantial degree, through various cultural practices (Harris & Ross, 1987). In the future, societies will need to continue manipulation vital rates to reach desired demographic targets. Most important, societies must reach a rough consensus on what those targets should be as soon as possible because the momentum behind the growth of the present population ensures at least a doubling before any decline is possible (UNFPA, 1992).

This commentary, given at the First World Optimum Population Congress (convened in London, U.K., 1993) is a contribution to that necessary dialogue. What follows is a brief statement of our joint personal views of the criteria by which an optimum should be determined…

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