How many people can planet Earth support? That is the thorny question that economist Partha Dasgupta tackles in Time and the Generations. Or, as he asks: “How should we evaluate the ethics of procreation, especially the environmental consequences of reproductive decisions on future generations, in a resource-constrained world?” Given that I have previously called overpopulation the elephant in the room that few wish to address, my interest was immediately piqued.
Intuitively, the answer to this question to me has always seemed: it depends. The IPAT equation formulated in the 1970s by Barry Commoner, Paul R. Ehrlich, and John Holdren is pretty clear about it, Impact = Population × Affluence × Technology. There is not going to be a single number, it depends on what we consider a good life. The more affluent we want our lifestyle to be, the fewer of us there can be before we require more natural resources than our planet can supply. This is the essence behind concepts such as Earth Overshoot Day and humanity’s global ecological footprint. These consider renewable natural resources such as timber, clean water, and fertile soil (but, it should be pointed out, ignore resources such as fossil fuels and ores that are essentially non-renewable).
So, superficially, this question does not seem like rocket science. But Dasgupta here turns it into something very much akin to rocket science. If your idea of stimulating dinnertime conversation includes the ideas of philosophers and economists such as Henry Sidgwick, John Rawls, Derek Parfit, or Frank Ramsey, with a healthy dollop of normative ethics, utilitarianism, and axiology, then this book will go down well. If the above means nothing to you, then, like me, you might struggle with this book. Dasgupta writes for an audience of fellow philosophers and economists here and takes as a given a certain level of background knowledge and familiarity with the vocabulary. Below I will try to demystify some of this and hopefully not butcher it too much in the process.
“Given that our planet has resource constraints, what level of economic activity can it support long term? And can we deduce an optimum population size from this?”
This book is part of the Kenneth J. Arrow Lecture Series, which honours the intellectual legacy of this economist. Dasgupta has revised and expanded on material from this and other lectures and turned it into a large essay, the Arrow Lecture, that sits at the heart of this book. It is driven by two questions. Given that our planet has resource constraints, what level of economic activity can it support long term? And can we deduce an optimum population size from this? To answer these questions, Dasgupta engages in a formal mathematical modelling exercise in both normative and population ethics. Normative ethics studies ethical action: how should one act, morally speaking? Population ethics studies the ethical problems that arise when our actions affect who and how many of us are born in the future.
As his starting point, Dasgupta takes Henry Sidgwick’s version of utilitarian thought. As an ethical theory, utilitarianism says that we should act to maximize happiness and well-being for individuals involved. When applied in the context of population ethics, I think this means asking how we can maximize well-being for humanity as a whole. Dasgupta starts with a model of a timeless world in which goods are produced and consumed and asks what level of population maximizes well-being (so-called total utilitarianism). He then modifies this theory to construct what he calls generation-relative utilitarianism. He adds a component of time in the form of existing people choosing how many children to have and asks how we can maximize well-being in that scenario. The aim of this whole exercise? A model that offers potential parents a normative theory of, and ethical guidance on, reproductive decisions.
The Arrow Lecture is accompanied by other material that, fortunately, is quite enlightening. I would even go so far as to recommend you read it first to get a better handle on the book’s outline. There is a foreword by economist Robert M. Solow, some emails from Kenneth J. Arrow to Dasgupta on the Lecture, three commentaries from economists Scott Barrett, Eric Maskin, and Joseph Stiglitz – with a response by Dasgupta, an epilogue, and a reprint of a closely-allied paper co-authored with his daughter Aisha Dasgupta. To make it self-contained, the last part of the Lecture actually borrows heavily from this paper, so there is a fair bit of overlap here.
“[Dasgupta’s] model [gives] some very rough numbers for optimal population size, with most estimates falling somewhere between 1.5 to 3.5 billion people.”
Where the modelling exercise was rather abstruse to me, the second half is where things get really interesting. He first offers the briefest of reviews of humanity’s impact on the planet, stressing repeatedly just how insanely fast, just mere decades, we have increased our standard of living and, with it, extraction of natural resources. Then he applies his model to get some very rough numbers for optimal population size, with most estimates falling somewhere between 1.5 to 3.5 billion people. This agrees with earlier work by Paul Ehrlich and co-authors, as well as the recent limit of 3 billion championed by Christopher Tucker.
Dasgupta is frank about the model simplifications made up to this point and the crudeness of some the estimates of our impact on the biosphere. But, as he points out, “What matters […] is not the exact figure but whether the footprint exceeds 1. On that there should be little question” (p. 105). Indeed, and for a fuller picture I refer readers to Vaclav Smil’s Harvesting the Biosphere, the recent Humans versus Nature, and my review of Cataclysms. The same hockey stick patternhockey stick shape that characterises the graph of atmospheric carbon dioxide concentration against time shows up in a broad class of other geochemical signatures. This is why stratigraphers are trying to get the term Anthropocene formally recognized. This pressure on the biosphere is also feeding into what is now widely called the sixth mass extinction (notably, Dasgupta was co-editor on Biological Extinction, published in the same year as this book).
There are three other things that Dasgupta writes worth mentioning here. First is his invocation of the economic concept of externalities. These are positive or negative consequences that our actions have for others that are not accounted for. An example of a negative externality is global warming caused by the burning of fossil fuels. We, its consumers, are not paying the cost of global warming which is instead inflicted on both existing and future people. Dasgupta argues this applies just as much for reproduction: adding another human being to an already overburdened planet has costs for all future people. For most, life is so sacrosanct that nobody wants to discuss this, and applying this kind of cold calculus is still taboo. High time somebody broached that topic.
“[…] the Sustainable Development Goals laid out by the United Nations […] are silent on population and the burden increased affluence will have on the planet.”
Second is that the Sustainable Development Goals laid out by the United Nations, which aim for example to reduce global wealth inequality, are silent on population and the burden increased affluence will have on the planet. One more reason why this conversation is so terribly overdue. Third – there is good news too – preferences for the number of children people wish to have is, what Dasgupta calls, socially embedded. It is subject to both people’s desire to compete with one another and to conform to a majority. Changing such preferences might be less costly than it first appears, though it will be none the easier for it.
One thing Dasgupta, by his own admission, stops short of is formulating policies as to how we could achieve optimum global population. Luckily, others are starting to be willing to discuss this. Equally important will be the tough discussion on how we can change our behaviour so as to live lighter on our environment, something I cover in my review of Planetary Accounting.
Time and the Generations takes no prisoners where its level is concerned, being primarily written for an audience versed in economics and philosophy. Though I struggled to get to grips with parts of the book, I found it refreshing and uplifting to see an economist champion something else than endless growth.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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