What is the optimal human population? An eminent economist weighs in

Efforts to specify the optimal human population on Earth are as complex as they are controversial. A recent book from Cambridge University economist Sir Partha Dasgupta develops a theoretically rigorous approach to this perennial question, finding that an optimal human population might range from 500 million to 5 billion.

by Phil Cafaro

It is gratifying to see the great increase in recent years in philosophical attention to population ethics. While Sarah Conly’s book One Child: Do We Have a Right to More? has set the agenda for debate among rights-based approaches to population ethics, Partha Dasgupta’s recent Time and the Generations: Population Ethics for a Diminishing Planet, seems likely to do so for many utilitarians. It joins a growing list of efforts to specify an optimum human population on Earth.

Dasgupta, Frank Ramsey Professor of Economics at Cambridge University, takes humanity’s environmental predicament seriously, writing that “the enormous economic success we have enjoyed in recent decades may be a down payment for future failure” as we spend down natural capital and threaten essential ecosystem services (p. 227). Interestingly, he believes it is a mistake to reduce these environmental threats to global climate change, as is often done, writing: “Global climate change attracts attention among intellectuals and the reading public not only because it is a grave problem, but also because it is possible to imagine meeting it by using the familiar economics of commodity taxation, regulation, and resource pricing without having to forego growth in living standards in rich countries” (p. 102). A focus on climate change also leads analysts to concentrate on technological solutions, rather than consider reducing consumption or limiting human numbers. But while developing and deploying new technologies can play a role in reducing environmental impacts, it is unlikely to lead to real sustainability, Dasgupta believes, in a world where natural capital is unpriced or underpriced, and the goal is always more growth. Besides, new technologies often have unanticipated negative environmental side effects.

All this suggests a need to return to the old equation I = PAT and attend to P (population) and A (affluence, per capita consumption) as well as T (technological change). Dasgupta does so by working out estimates for a sustainable global population at various average income levels, with income standing in as a proxy for consumption. In a paper titled “Socially Embedded Preferences, Environmental Externalities, and Reproductive Rights,” written with Aisha Dasgupta and included in Time and the Generations, they note that current global GDP is about $110 trillion and that according to the Global Footprint Network, humanity’s ecological footprint is 60% above what is sustainable. This suggests, very roughly, that a world GDP of $70 trillion could be sustainable. Looking at international surveys, they note that in wealthy countries additional income above $20,000 per year is not statistically related to greater reported happiness, and that this same amount was the average income in high-income countries in the early 1980s, when people were about as happy as they are today.

“If we now regard 20,000 international dollars as the desired standard of living for the average person,” the Dasguptas write, “the maximum sustainable population comes to 3.5 billion,” less than half our current one (pp. 257-258). If $10,000 is set as an acceptable average income, the maximum sustainable global population doubles to 7 billion. Wherever we set desired average income, there is a trade-off between average income and the number of people who can be sustained globally. We can ignore this trade-off only by decreasing Earth’s long-term carrying capacity, and only temporarily. But according to the Dasguptas, this represents a failure of stewardship and disregards the rights of our descendants.

Reproductive rights are important, the Dasguptas believe. But “to insist that the rights of individuals and couples to decide freely the number of children they produce trump all competing interests is to minimize the rights of all those (most especially, perhaps, future people) who suffer from the environmental externalities that accompany additions to the population.” (p. 234). Our children and grandchildren have a right to sufficient food, shelter, and physical security, and to opportunities to develop and flourish. All these can be compromised on an overpopulated planet.

The Dasguptas believe that “the concept of reproductive rights, as currently framed, undervalues family planning” (p. 249), for two reasons. First, it ignores the potential effects of excessive numbers of children on others: a matter of justice. Second, it ignores the fact that people might want lots of children even if it won’t be good for them, their existing children, or future members of their societies: a matter of basic welfare. “A poor woman, suffering from iron deficiency and living in a setting where she is compelled to have sex and bear children, has a need for contraception for her own benefit that could remain undetected in her responses to questions regarding her desire for children,” they write. “To infer needs solely from wants is therefore to undervalue the significance of family planning” (p. 247).

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“The clash of rights we have identified in this essay arises from the fact that much of the biosphere is treated as a free good,” the Dasguptas write (p. 261). We need to reduce such environmental externalities through sensible public policies, while realizing that having children is an inherently social act involving claims on limited resources. That means that procreation may need to be managed for the common good, justifying a certain amount of “population engineering,” in Colin Hickey’s phrase. Best to face this fact and make explicit policies with a clear-eyed understanding of the limits we face and the ethical goals we hold.

The body of Time and the Generations seeks to do just that, in a long technical essay titled “Birth and Death,” written at the intersection of philosophy and economics. This essay lies in a long line of utilitarian attempts to specify an optimal human population, going back through Derek Parfit’s seminal book Reasons and Persons (1984) and beyond. In this essay, Dasgupta intriguingly attempts to arrive at an ethically plausible compromise between the philosophical approaches of Average Utilitarianism (in which average human well-being is held all important) and Total Utilitarianism (which takes aggregate human well-being as its central value), both of which generate ethically counterintuitive implications regarding population policies. In their place, he argues for Generation-Relative Utilitarianism. The basic idea is to discount the well-being of future generations (in order to facilitate practical planning and focus on creating flourishing human societies, rather than generating the maximum possible tonnage of human flesh), while not discounting our descendants’ well being too much (in order to preserve a reasonably just commitment to it). (Generation-Relative Utilitarianism thus appears to be a “variable value” theory; for a good introductory discussion of philosophical theories of optimal population from an anthropocentric perspective, see Hilary Greaves’ 2017 article “Population Axiology”).

Relying on Generation-Relative Utilitarianism, Dasgupta develops a formal theory that relates population, per capita consumption and biospheric capacity (K), suggesting, as in his earlier paper, that we have to work out optima for the first two variables while respecting the third. Failing to do so makes morally unjustified demands on future generations, who will have to bear the costs of diminished biospheric capacity. Calculating hard numbers under such a framework necessarily involves assumptions and considerable uncertainty; this includes uncertainty regarding total biospheric capacity, uncertainty regarding a proper discount rate for thinking about future people’s well-being, and uncertainty regarding how best to balance average consumption against the number of consumers (as one goes up the other must go down).

But not complete uncertainty! Within a range of plausible answers to these questions, Dasgupta delivers a range of optimal global populations between 0.5 and 5 billion. Like his earlier paper, this more rigorous effort suggests that humanity is already grossly overpopulated relative to global ecological carrying capacity and relative to the per capita consumption reductions that people are likely willing to undertake to remain within it. So does a revision of the earlier “Socially Embedded Preferences” paper, “Population Overshoot,” in the forthcoming Oxford Handbook of Population Ethics. This new effort defines per capita environmental impact in terms of average production, rather than average consumption, as in the earlier version. It sets an optimal sustainable global population at 1.8 billion people.

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Dasgupta’s central argument is strictly anthropocentric, in line with most ongoing philosophical debates about optimal population. The biosphere is treated essentially as a resource (or source of all resources) for human use; carrying capacity and optimal population are functions of what is possible or best for humans. Any limits to drawdown on biocapacity are understood as boundaries beyond which human happiness is no longer maximized, or unnecessary human suffering is caused. This anthropocentrism might be obscured because Dasgupta is very worried about biodiversity loss. Yet within his theoretical model, biodiversity loss can only register as important via declining biocapacity available for human use.

From one perspective, this anthropocentrism strengthens the overall argument for attending to population. As Professor Dasgupta wrote in kind response to a query from me, he deliberately made “minimalist assumptions” in developing his moral arguments, to show that even if all people care about is ourselves, we need to limit our population.

Yet Dasgupta, like most ethicists, realizes the moral insufficiency of anthropocentrism. As he writes in the final paragraph of “Birth and Death,” a truer ethics will recognize the intrinsic value of Nature and direct us accordingly:

It’s a mistake to seek justification for the preservation of ecological diversity, or more narrowly the protection of species solely on instrumental grounds; that is, on grounds that we know they are useful to us or may prove useful to our descendants. Such arguments have a role, but they are not sufficient. … A full justification bases itself as well on how we see ourselves, on what our informed desires are. In examining our values and thus our lives, we are led to ask whether the destruction of an entire species-habitat for some immediate gratification is something we can live with comfortably. (p. 120)

For Dasgupta, clearly, such destructive behavior is wrong. Considering our duties toward future generations and the future of Earth’s biosphere should lead to less selfish behavior, since “the idea of intergenerational exchange is embedded in the perspective of eternity” (p. 120). In considering an optimal human population, it cannot be unimportant whether we share the landscape justly with other species. We should grant some portion of biospheric capacity to other species and thus limit human use to some fraction of K smaller than 1. Whether that fraction should be ½ K (as under E.O. Wilson’s “Half Earth” proposal, which would preserve most existing species from near-term anthropogenic extinction) or a different fraction is a matter for further ethical debate and biological research.

Justice toward other species can be plugged into Dasgupta’s model. For example, assuming we reserve half the Earth primarily for the use of other species, the range of optimal global human populations shrinks from 0.5 – 5 billion to 0.25 – 2.5 billion people. Beyond any such calculations, adding a direct moral responsibility to avoid extinguishing other species strengthens our duty to effectively address human overpopulation.

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19 thoughts on “What is the optimal human population? An eminent economist weighs in

  1. Whatever is the maximum, the present number is blindingly, obviously too many!
    One does not need to be a professor to make that determination.
    Personally I’d suggest five billion as a maximum.

    1. “The present number is too many.” Yes indeed, you won’t get any argument from us about that! But it’s important that when scholars like Dasgupta or Chris Tucker crunch the numbers, they regularly find that to be the case. “Time and the Generations” is a rigorous study. For your friends who might not be on board yet about population issues, his book (or this summary) might be a good gift.

  2. Even if the proposition that Malthusian ‘population growth’ is responsible for environmental degradation on earth is valid, without clear causative evidence, two major and pressing factors are ignored here.

    With fertility rates already dropping and population growth stalling, how can population growth be stopped prematurely without major disruption in the short term, or why bother if clearly starting a decline?

    Fossil fuels? Governments could implement robust environmental and carbon emissions regulation quickly but are impeded by delaying tactics supported by the fossil fuels sector e.g. citing population growth as the issue via the junk science Limits to Growth, hence, creating regulatory inertia and protecting future income streams of fossil fuel producers and related.

    1. You are right that humanity needs to phase out fossil fuel use and phase in renewables, as quickly as possible. But I’ve never seen population matters used as an excuse not to do that.

      You write that “population growth is stalling.” At least globally, that’s false. In recent years, the global population has increased by 80 to 85 million annually. That’s a huge increase; in absolute terms, close to the peak annual increases seen in the 1960s and 70s.

  3. Dinosaurs ruled the world for 160 million years. I can guarantee with absolute certainty, not merely 99.99% certainty, that within the next 100,000 years all, and I do mean all, resources necessary for civilization, as we know it, to exist and function will be exhausted and no longer available for use by humanity. In simple words, no later than 100,000 years from now humanity will be back in the Stone Age and the supportable population will probably be under 25 million. And 100.000 years is a very, very short period of time to the 160 million years that the dinosaurs ruled the earth. Jason G Brent jbrent6179@aol.com

    1. Jason: I offer that you need to look at, and talk about, today and the next 100 years. Don’t waste your mind, living in a melodramatic far future.

  4. Well how many is largely influenced by whether we all want to live in tents and ride bicycles or have some level of comfort and dignity, but for now, I’ll stick with David Pimentel’s heavily researched number for decades ago, which determined that over all and AT BEST Mother Earth can put up with only about 2 billion of us long term, with it key that if the U.S. would get it’s ghastly, high-consumption population of an astounding 333 million, highest per-capita carbon-emission citizens anywhere from increasing (Wall Street has other plans.) and then, slowly, reduce it’s population to the 200 million we were in 1972, it’d give Mother Earth a badly needed breather!

    1. If you’re assuming that without a personal car you cannot have a life of comfort and dignity, this would mean that the vast majority of human beings that have ever existed (everyone until the 1950s, and billions still today) has had and is still having an undignified life. This seems a bit extreme to me.
      There are many options between abject poverty and a car an oversized house for each one of us.

      1. Greetings Gaiabaracetti.
        It would be interesting to know what YOUR opinion of what optimum exists between poverty and car, big house etc., is?
        I am not an ’eminent economist’ – but I KNOW that there are not enough resources to sustain a greater population, hence thousands of children are suffering malnutrition and death.
        Even if we shared more equally there would still be insufficient!
        Also I note that you failed to mention that in the halcyon pre 1950 days that the life expectancy for most people was considerably lower than now – do you wish to ‘turn the clock back’ to those days?
        The simple fact is, whatever fact and figures you quote, that there are too many human beings on the planet!

  5. I agree about climate change. I’ve long begun thinking that now almost everyone (including politicians, mainstream media, finance and industry even) appears to be on board with doing something about climate change because they think they’ve figured out a way to make money from it and keep the economy growing (“green” jobs etc). Never mind that transitioning to renewable energies, while necessary, is proving environmentally devastating and we’re just getting started… also, it’s easy to put a number on: co2 emitted, degrees… other things are much harder to measure – you can write a heartbreaking poem about land lost to intensive agriculture (John Clare did), but in our numbers- and growth-obsessed society, it’s nothing.

    One thing I partly disagree with is that it’s just future generations we’re hurting with overpopulation. If the meadow next to someone’s house is paved over and apartments are built on it; if natural places are flooded with people, if prices go up and conflicts arise etc… you’re harming people now, not just later.

  6. Seems to me gaiabaracetti finds the right balance. Mathematicians talking to economists talking to statisticians talking to other mathematicians talking to . . . isn’t going to get the job done. Populationists have some wonderful writers and wonderful speakers . . . but they could use better publicists!

  7. John, I am not disputing that there are too many people on the planet for all of us to have a decent life and share this Earth equitably with all the species that inhabit it. I am not the most qualified person to give you an exact number of how many of us there “should” be, and I do agree that it depends on what level of quality of life and consumption – and green spaces – we want or need to keep. History never goes back, so I am not proposing we go “back” to anything. I certainly would like to keep good healthcare available as long as possible, although part of the health sector is very wasteful and sometimes unnecessary or counterproductive, and life expectancy is not the only criterion to measure good health and a good life (there’s a very good article on Low Tech Magazine online on this).
    ALL I wanted to say is that some of the things we think we cannot live without are not so indispensable. My proposal would be to figure out some kind of slightly-variable budget for each human being, in terms of resources and / or energy, and let each use it as they please. For example, if you really want to have a car you won’t get to travel as much abroad or keep many pets, or if you really like owning clothes or books or electronic appliances you would need to sacrifice having a big house, or whatever combination roughly fits into your budget. Of course, easier said than done, and some countries might prefer a larger population and lower per capita consumption, or vice versa. Still, my point is that we don’t all need to live exactly the same way, and the richest in the world certainly should not be our example in terms of lifestyle. They’re not even any happier than regular folks.

  8. With current technology an earth of two to three billion of us could be an Eden, at seven approaching eight things are getting bad, fast, and an Earth of nine to ten billion of us will just be one huge living Hell.

  9. Read the book “Blip” by Chris Clugston. He proves in that book, in my opinion, that the collapse of civilization will commence around the year 2050 due to the inability of the earth to provide the resources our civilization needs to continue. In simple terms, he proves that we will run out of natural resources by the year 2050, Professor Jared diamond has stated that there is a 49% chance that by the year 2050 there will be a major civilization of people. In any event, it can be stated with absolute certainty, not merely 99.99% certainty, that 1000 years from now our civilization will no longer exist due to the inability of the earth provide the necessary resources. 1000 years from now the human population will be below 100 million and will exist at the Stone Age level. Jason G Brent jbrent6179@aol.com

  10. As a start, I have been advocating for a world population of 2.5 billion, plus a 50% decrease in consumption rates by rich countries ASAP, If we don’t heed the warnings by scientists, including myself, climate change, plus other forms of environmental pollution, will do us all in.

  11. Industrial and domestic waste generation needs to become far costlier for companies and the individual. Material ownership and any commercial action must build in the future cost of disposing and recycling all materials owned and used in far greater accounting detail. Let’s face it; it’s not done now and cheating is the rule. Recycling can be far more enhanced, specifically and fully applied, and policed. For example, with the use of integrated RFIDs. It’s not just about plastic bottles, it’s about plastic house cladding, plastic flooring, machine and automobile parts and so on, the majority of which may be “considered” recyclable – but isn’t actually recycled. Land dumps must be certified against leakage and the ability to decompose. Heavy fines need to apply. Our lifestyle needs to be reigned in. GDP must fall. Let’s have more leisure time, forego production increases, and find the way back to revive nature, and live with and in it. Along the way populations will fall.

  12. The human population will collapse; either voluntarily (unlikely) or nature will take its course. Either way, by then we will have taken out millions(?) of other species. Is our species worth that much?

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