Introduction

What is population ethics? The straightforward answer is that it is the investigation of various ethical issues that arise when we consider how our actions affect both who is born and how many people are born. But, of course, it is more complicated than that.

One complication is that population ethics divides into two forms: the axiological and the normative. The axiological form considers how we ought to value different populations, whereas the normative form considers how we ought to act in relation to populations, for instance which populations we ought to bring about. There are at least three elements that influence how we value a particular population: (1) the number of people who exist; (2) the well-being of the people who exist; and (3) the identities of the people who exist. Possible populations vary along these lines and the study of population axiology attempts to determine how to rank them, taking account of these differences. For instance, would it be better if one excellent life existed than if two different decent lives existed? Do we make the world better by creating more happy people? Is a world in which many people live with a very low quality of life better or worse than a world in which very few people live with a very high quality of life? (We use the terms “quality of life” and “well-being” synonymously.) The normative form of population ethics, in contrast, asks similar questions but in normative terms. Do we do something wrong when we create a person with a decent life when we could instead have created a different person with an excellent life? Do we have an obligation to create more happy people? Do we have a duty to create many people with a very low quality of life rather than create very few people with a very high quality of life? In order to determine how we ought to govern our actions with respect to different populations, the normative form of population ethics often applies traditional ethical theories—for example, utilitarianism, Kantianism, and contractualism—and then evaluates the results. Exactly how the axiological and normative forms are related is in itself an important issue. Only simple forms of consequentialism would claim that we ought to bring about the best population but there are other more complex theories about the bridge from the axiological to the normative. (As, for example, Arrhenius shows in his contribution to this volume.)

The axiological and normative questions in population ethics are theoretically challenging but they are also directly relevant to many urgent practical decisions and policies. The answers to these questions matter because they affect the moral standing of future people in the decisions we currently make. Future people pose an especially hard problem for our decision-making, since their number and identities are not fixed; rather, they depend on the choices we make. Climate change is a clear example as the increase in temperature makes areas uninhabitable and will kill some people. However, because the increase in temperature will affect people’s decisions about where to live, when to have children, and how many children to have, climate change will also affect which particular people come to exist and prevent many possible people from ever existing. Many other policies also influence the size and make-up of the population, both directly and indirectly. For example, many health policies both prevent deaths and affect procreation decisions. Population-control policies, such as China’s one-child policy, obviously affect population size. Even taxation policies, such as tax relief for parents, can indirectly affect population size and make up. If we are to adequately assess these policies, we must be able to determine the value of different populations and how we ought to act in relation to them.

Despite these seemingly straightforward questions, what makes the task of formulating an adequate population ethics so difficult is that it is plagued by several very difficult problems and even paradoxes.

Perhaps the most notorious of these problems is the Non-Identity Problem (Parfit 1984). Often when we think about what we owe to future people, we assume that these people will exist at some point in the future, no matter what we do now. We take for granted that it would be wrong to perform actions that would harm them, and that the outcome would be worse if we did (assuming other things are equal). However, the Non-Identity Problem complicates this simple picture, for the existence of some future people depends on what we do now. One small-scale example is procreation (Parfit 1984: 358–359). Suppose that you can either (a) create a child now that, due to your poor financial situation, will have a not especially happy life, although its life will still be worth living or, (b) wait and create a different child later, who will have a much happier, and hence, better life, since by then your financial situation will have improved considerably. Suppose that all other things are equal. It then seems wrong to create the child now, and, furthermore, the outcome of this action seems to be worse than the outcome of postponing the creation. But can we really say that? If you create the child now, it seems that you do not harm her in any way, for she has a life worth living and she does not have it worse than she would have had if she had not been created. If she had not been created, she would not have had any welfare at all, since she cannot have any welfare if she does not exist. How can it then be wrong to create the child now and how can the outcome of the action be worse? No one will be harmed (or so it seems), and it also seems plausible that an action is wrong only if it harms someone, and that an outcome is worse than another only if someone is worse off in the former than in the latter.

There are large-scale versions of the Non-Identity Problem as well (Parfit 1984, 351–355). Suppose you can choose between two policies: conservation of natural resources and depletion of these resources. If we deplete these resources, the quality of life for people over the next few centuries will be slightly higher than it would have been if we had instead conserved. But after the next few centuries, quality of life will fall dramatically and will remain much lower, for many centuries, than it would have been if we had instead conserved. Suppose we decide to deplete the resources. As a result, many millions of people have a quality of life that is much lower than the quality of life that people would have enjoyed if we had instead conserved. However, these millions of people have lives that are worth living. Moreover, if we had chosen the alternative policy, these particular people would never have existed. This is because our choice of policy has certain wide-reaching effects. For example, because of our choice, many people end up living in different places, meeting and procreating with people they would otherwise never have met, and giving their children a different upbringing. This means that while our choice of policy is the reason why many millions of people have a relatively low quality of life, it is also a cause of their existence. They would not have existed if we had chosen differently. Different people would instead have existed. Hence, the millions of people with a low quality of life cannot plausibly claim that our choice of policy harmed them when their existence depended on our choice and in fact they have lives that are worth living. But, surely, we nevertheless would like to say that it is wrong to choose depletion and that the outcome of this action is worse than that of conservation.

The Non-Identity Problem is a challenge to both axiological and normative accounts of population ethics. It seems difficult to avoid the surprising and disconcerting conclusion that our seemingly negligent actions are not wrong, and their outcomes not worse, because they do not make future people worse off than they would otherwise have been. Of course, this is only a problem if we share the intuition that there is something wrong with such actions and something bad about their outcomes. There are some who are willing to bite the bullet and accept that these seemingly wrong actions are in fact permissible (e.g., Boonin 2014), but most disagree and therefore look for alternative ways to “solve” the Non-Identity Problem. One alternative would be to abandon the ideas that an action is wrong only if it harms someone, and that an outcome is worse only if someone in it would be worse off. A more radical solution would be to argue that bringing a person into existence can harm that person even if it is not true that she is thereby made worse off than she would otherwise have been, and even if her life is worth living (Shiffrin 1999).

Another central paradox is the Mere Addition Paradox (Parfit 1984, 419–438). To illustrate, consider the example given in Figure I.1: Population A consists of a small number of individuals, all leading lives with a very high level of well-being. Population A+ consists of all the individuals in A, plus some extra individuals leading lives with a level of well-being that is slightly lower than in A, but still very high. Population B has the same number of individuals as in A+, all leading lives with the exact same level of well-being, that is lower than the level in A and higher than that of the extra individuals in A+. Population B+ modifies B in a parallel way to how A+ modifies A: B+ has all the individuals in B, plus some extra individuals leading lives with a level of well-being that is slightly lower than in B, but still very high. C modifies B+ in a parallel way to how B modifies A+, and so on all the way to population Ω.

Comparing the value of the populations in the sequence leads to the following observations. A+ seems at least as good as A since A+ is a mere addition of positive well-being; it contains some extra people with positive well-being and the well-being of the A population is not affected. More generally, it seems that a mere addition of positive well-being is always at least as good as no mere addition (Parfit 1984). B seems better than A+, since B has higher total and average of well-being and has perfectly equal distribution of well-being. More generally, it seems, an increase of total and average well-being that also creates perfect equality is always an improvement (Ng 1989). Call this Weak Inequality Aversion. Mere Addition implies that B+ is at least as good as B, and Weak Inequality Aversion implies that C is better than B+. Indeed, we can reapply these principles until we reach the conclusion that Ω is better than the immediately preceding population. By the transitivity of the relation “is better than” we must conclude that Ω is better than A. But this result may seem unacceptable. A is a population whose members have very high well-being. Ω is a population whose members have lives barely worth living.

It may seem that, generally, any acceptable theory should avoid the Repugnant Conclusion which (in Parfit’s famous version) holds that

The Repugnant Conclusion: for any possible population of at least ten billion people, all with a very high quality of life, there must be some much larger imaginable population whose existence, if other things are equal, would be better even though its members have lives that are barely worth living

(Parfit 1984, 388).

But now we have a problem. If we accept Mere Addition, Weak Inequality Aversion, and want to avoid the Repugnant Conclusion, it seems we get a contradiction: A is both better and worse (and hence not better) than Ω. We have to give up one of these compelling principles, if we want to avoid a contradiction. However, as Arrhenius (2000a; 2000b; 2011) has shown, even if we give up one of these principles there are other paradoxes lurking.

One way to avoid the contradiction is to bite the bullet and accept the Repugnant Conclusion. This is what some advocates of total utilitarianism do. According to this theory, provided the people who exist have lives that are (even just barely) worth living, the existence of a sufficiently large number of such people would have a greater total of well-being, and hence would be better, than the existence of a smaller number of people with very high quality of life.

However, total utilitarianism is not the only theory that has been shown to lead to the Repugnant Conclusion. As shown above, this holds for any theory that accepts Mere Addition and Weak Inequality Aversion (and some other background assumptions, such as that betterness is transitive, that is, if a is better than b and b is better than c, then a is better than c). Many philosophers see avoiding the Repugnant Conclusion as a necessary feature of any satisfactory population ethics and try to find a solution to the problem. Others believe that the Repugnant Conclusion may not in fact be repugnant at all and ought to be embraced or at least grudgingly accepted. Finally, some believe that it is simply impossible to find a population ethics that avoids the Non-Identity Problem (above) and the Repugnant Conclusion and that does not lead to other similarly unpalatable conclusions. All three approaches are represented in this volume.

Another salient problem arises from two claims that many people seem to accept. The conjunction of these two claims is known as the Procreation Asymmetry:

To many, each of (i) and (ii) seems, on its own, prima facie plausible. Yet it is surprisingly difficult to provide a satisfying explanation of the conjunction of (i) and (ii). Why is it that the fact that a life would be worth not living has reason-giving force against creation while the fact a life would be worth living has no reason-giving force in favor of creation?

Not all theorists think it is worthwhile to search for an answer to this query, and instead they deny the Procreation Asymmetry. For example, total utilitarians claim that we should maximize total well-being and this can be done not only by refraining from creating lives with negative well-being (lives worth not living) but also by creating lives with positive well-being (lives worth living). Since total utilitarians think we have a duty to increase well-being wherever possible, they would say that if we can increase well-being by creating a worth-living life, we ought to do so.

The question of how to respond to the paradoxes in population ethics has spawned a huge, though largely inconclusive, literature. Part I of this Handbook therefore focuses on different responses to (or “ways out of”) these paradoxes. The responses canvassed here are diverse.

Ruth Chang’s contribution addresses what are known as continua arguments, and in particular, continua arguments for the Repugnant Conclusion. Continua arguments create a continuum of outcomes varying by population size and well-being, such that each successive outcome is better than its predecessor, and because of transitivity, the final outcome is better than the initial outcome. She argues that neither incommensurability, incomparability, indeterminacy, nor Parfit’s own proposed “lexical imprecision” view can successfully “break the chain” and provide a solution.

Nils Holtug also confronts the Repugnant Conclusion. He defends a version of prioritarianism that implies this conclusion, as well as several other counterintuitive conclusions. However, he argues that prioritarianism entails more palatable versions of these conclusions than utilitarianism and other rival axiologies. He also argues that different ways of avoiding these conclusions are available to prioritarians, if they are willing to adjust the prioritarian function, but that such adjustments lead to further problems.

Geir Asheim and Stephane Zuber argue that there is a dilemma between assigning “dictatorship” to a single worst-off person, thereby succumbing to a tyranny of non-aggregation, and assigning “dictatorship” to an (unlimited) number of better off people, succumbing to a tyranny of aggregation. They claim that this corresponds to a dilemma between, on the one hand, a Reverse Repugnant Conclusion (a very small number of very well-off people is seen as better than a much bigger population with fairly good lives), and, on the other hand, the Repugnant Conclusion or Sadistic Conclusion. They argue that the dilemma can be resolved by allowing that the evaluation of populations might change with population size, even though the relative distributions of well-being remain unchanged.

Walter Bossert’s chapter discusses some important characteristics of critical-level utilitarianism and compares this axiological view to alternative views. He points out that critical-level utilitarianism with a critical level above neutrality can avoid the Repugnant Conclusion but implies what Arrhenius calls the Sadistic Conclusion. Bossert illustrates that it is impossible to obtain a general population axiology that avoids both of these conclusions, provided that some additional plausible requirements are imposed. The chapter illustrates one of the difficult trade-offs in population ethics: embracing the Sadistic Conclusion may be the price that one must pay to avoid the Repugnant Conclusion.

Wlodek Rabinowicz’s chapter aims to accommodate the axiological intuition of neutrality within a version of neutral-range utilitarianism. The intuition is that there is a range of welfare levels such that adding people with lives at these levels makes the world neither better nor worse. Rabinowicz proposes an interpretation of the neutral range according to which a life within this range is neither good nor bad for the person who has it and is incommensurable with non-existence. The resulting version of neutral-range utilitarianism entails not only that lives are incommensurable with non-existence but also that some lives are incommensurable with each other, and hence, that some lives are not comparable.

John Broome’s chapter is a response to Rabinowicz’s. Broome argues that if some lives are not comparable, this makes no significant difference to the utilitarian ethics of population. Neutral range utilitarianism can accommodate the same axiological intuitions and will be vulnerable to the same problems that motivated Broome to reject the theory in his book Weighing Lives (2004). Furthermore, Broome doubts that Rabinowicz’s personalized interpretation of the neutral range can advance our understanding of population axiology, since, Broome thinks, the axiological intuition of neutrality is directly about the general goodness of populations, not about personal goodness.

A number of philosophical and methodological assumptions underly the paradoxes discussed in Part I of this volume. Rejecting or revising some of these assumptions may hold the key to formulating an adequate population ethics. The contributions in Part II aim to critically assess some of the most important assumptions in this area.

One important methodological assumption is that our intuitions are reliable guides to axiological and normative truths. For example, if we believe that an adequate population axiology must avoid the Repugnant Conclusion, this is because we find this conclusion deeply unintuitive, and the fact that we find it deeply unintuitive is then taken as strong evidence that it is false.

Yet intuitions are not always reliable. They are subject to various distorting effects and cognitive biases. This might be true of our intuitions about axiological and normative matters. Johan Gustafsson argues in his chapter that our intuitions about the Repugnant Conclusion are unreliable as evidence against total utilitarianism. According to Gustafsson, there is an insensitivity in our intuitive understanding of the axiologically relevant factors involved in comparisons of different populations, and this insensitivity has its greatest impact when these factors are extremely proportioned in opposite ways, as in the cases used to illustrate the Repugnant Conclusion.

Larry Temkin’s chapter argues that the Mere Addition Paradox challenges some of our deepest assumptions about the nature and structure of the good. It distinguishes two opposing views of the goodness of outcomes, an Internal Aspects View and an Essentially Comparative View, and argues that the latter view underlies our judgments about the comparative goodness of many outcomes, including those featured in the Mere Addition Paradox. An Essentially Comparative View, Temkin argues, challenges the assumption that “all-things-considered better than” is a transitive relation, and has profound implications not only for axiology but also for rationality and practical reasoning.

Gustaf Arrhenius’s chapter considers whether the axiological paradoxes translate directly into corresponding paradoxes for normative theories. Perhaps one reason for thinking that they do not is that the axiological paradoxes depend on the transitivity of “better than,” and one might think that because there is no convincing analogue to transitivity on the normative level, the paradoxes will not reappear on that level. Arrhenius argues that this claim is false and that the axiological impossibility theorems can be proved without an appeal to the transitivity of “better than.”

Erik Carlson’s chapter responds to Arrhenius’s impossibility theorems and his contention that there is no satisfactory population axiology because plausible adequacy conditions are mutually inconsistent. Carlson argues that if non-Archimedean theories of welfare are not excluded, there are in fact population axiologies that satisfy all of Arrhenius’s adequacy conditions and Arrhenius’s results therefore fail to conclusively demonstrate that there is no acceptable population axiology.

Teruji Thomas’s chapter discusses the principle of separability, which says (roughly) that in comparing the value of two possible outcomes one can ignore any people who exist in both and whose welfare is the same in both. The chapter surveys the motivations for separability and its theoretical implications for population ethics. It also systematically explores how axiological theories that satisfy requirements of separability and impartiality can avoid the Repugnant Conclusion.

Dean Spears and Marc Budolfson’s chapter questions the assumption that avoiding the Repugnant Conclusion is a desideratum for a population axiology. They argue that our aversion to the Repugnant Conclusion derives from an aversion to a more general axiological claim that is implied by “all plausible axiologies,” including average, critical-level, and variable-value versions of utilitarianism, prioritarianism, and egalitarianism. This is an important claim, since avoidance of the Repugnant Conclusion is usually the primary motivation for adopting an average, critical-level, or variable-value axiology. If Spears and Budolfson’s argument is sound, then these axiologies fail to avoid the repugnance of the Repugnant Conclusion, even if they avoid the Repugnant Conclusion as it is stated by Parfit.

One assumption that is often implicit in debates in population ethics is that our theories should provide practical guidance for agents facing real-world problems, such as climate change. A natural thought is that we should first discover the correct theory, and then implement whatever policies it prescribes. However, the paradoxes canvassed in Part I of this book present a formidable obstacle for such an approach. They show that we cannot consistently accept all our favorite axiological and normative principles. It may seem that in order to apply theory to practice, we must first decide which of these conflicting principles to abandon. But we are deeply uncertain which principle(s) to abandon, and hence, deeply uncertain which theory is correct. Indeed, given the lack of consensus regarding how the paradoxes should be resolved, our uncertainty may be irresolvable. It may therefore seem that for many real-world issues for which our choice of theory would be relevant, decision-making is paralyzed.

However, some argue that deep evaluative and normative uncertainty need not paralyze decision-making. There are well-established approaches to decision-making under empirical uncertainty, and some have sought to develop corresponding approaches to decision-making under evaluative and normative uncertainty. Along these lines, Krister Bykvist’s chapter sketches the contours of an approach to rational decision-making under evaluative uncertainty. This approach extends some of the tools of standard decision theory, including certain dominance and expected value maximization principles, to the evaluative setting. Bykvist demonstrates the practical usefulness of the approach by applying these principles to a range of cases in which different axiologies yield conflicting claims regarding the betterness ordering of the agent’s alternatives. Given that the agent cares about the value of population changes but is uncertain which betterness ordering (if any) is correct, she can rationally choose an alternative by acting in accordance with one or more of the extended decision theoretic principles.

Several chapters in this Handbook defend person-affecting approaches to population ethics. These approaches attempt to explain the betterness relation between outcomes in terms of a personal betterness relation.

Ralf Bader’s chapter rejects impersonal versions of utilitarianism that he claims treat persons as “mere containers” of impersonal good, and considers the theoretical prospects for different person-affecting forms of utilitarianism. It argues that person-affecting total utilitarianism requires metaphysically problematic commitments because it presupposes that existence and non-existence are comparable in terms of personal good. It concludes that same-number person-affecting utilitarianism is the only version of utilitarianism that avoids both objectionable axiological implications and problematic metaphysical commitments.

Melinda Roberts’s chapter aims to develop the beginnings of a plausible person-based maximizing consequentialism. According to Roberts, such a theory must not only accommodate the Procreation Asymmetry but also explain how we can have a moral reason to give someone a greater chance of existence with a good life. She proposes a person-based view that satisfies these desiderata, and defends her view against a charge of inconsistency as well as an objection based on the Non-Identity Problem.

Matthew Adler’s chapter sets out a conceptual framework that he calls “claims across outcomes.” According to this framework, outcome x is morally better than outcome y only if some person has a claim in favor of x over y, and the person has such a claim only if x is better than y with respect to her well-being. In previous work, Adler has defended his framework for fixed population cases. His chapter extends this framework to variable population cases and addresses several objections that arise in this context, including the objection that a person’s existence and her non-existence are incomparable with respect to her well-being.

One question in applied population ethics is that of ideal population size and the legitimate role of government in regulating it. Many people believe the world is currently over-populated and the human population ought to be reduced or at least prevented from increasing. There are two related issues here. The first is whether the world really is over-populated, and relatedly, what the optimal population size actually is. Hilary Greaves’s chapter argues that to answer this question we need a population axiology and an account of which states of affairs are achievable in practice. Assuming a total utilitarian axiology, Greaves surveys economic models of optimum population size at a time and of the optimum population path through time, finding that even the most sophisticated of these models cannot, by themselves, provide quantitative answers to questions of optimal population size. Greaves also considers two important arguments in favor of deliberately reducing the birth rate to bring us closer to the optimal population path. She finds both arguments problematic and inconclusive.

Aisha Dasgupta and Partha Dasgupta’s chapter studies some important factors underlying “population overshoot.” One concerns the desire for children. The authors criticize demographers’ methods of estimating women’s desire for children, arguing that these methods fail to account for the social-embeddedness of preferences related to family size. The authors also make use of the estimate of humanity’s current ecological footprint to generate their own estimate of how many people Earth can support at a reasonable standard of living without further diminishment of the biosphere.

The second question is, if it is indeed the case that the world is overpopulated, what, if anything, should be done about it? Are individuals morally obliged not to reproduce, or to have only one child even if the incremental harm to others created by their choice is minimal? Serena Olsaretti argues that egalitarian justice demands that even if having children creates burdens for others, everyone should share equally in the costs of providing for them. This is because new generations bring with them societal benefits as well as burdens. Moreover, Olsaretti argues, the fact that third parties have a complaint against parents for producing too many or too few children presupposes that everyone is entitled to share the benefits of children; but this would seem to imply that everyone must share the costs as well. Sarah Conly’s chapter argues that since population increase is one cause of the climate emergency, and, given that voluntary efforts to reduce the fertility rate are ineffective, appropriate government coercion to reduce birth rates is permissible. She argues that individuals have a moral duty to refrain from actions that contribute an increment to a great harm (such as the harm of climate change), and that even if no individual were morally responsible for incremental harms, we could still justify government action to coordinate efforts to avoid the collective harm that overpopulation brings.

Martin Kolk provides an interesting account of how demographic theory can help illuminate certain issues in population ethics. He argues that many population ethicists implicitly assume a Malthusian perspective according to which there is a negative relationship between population size and average welfare. But, as Kolk points out, demographic theory includes a number of alternative perspectives that are often ignored in population ethics. He suggests that population ethicists consider population models that combine these different perspectives, to form a more complete picture of the relationship between population size, population growth, and average welfare. He also describes some implications of demographic theory for intergenerational inequality, reproductive rights, and the risk of human extinction.

Another applied population ethics question is how we ought to consider the value of a possible person’s existence in our decisions. Jeff McMahan’s chapter argues that causing someone to exist with a good life benefits that person, and that such benefits have moral significance. This result is supported by McMahan’s proposed response to the Non-Identity Problem and his view about how to accommodate the first claim of the Procreation Asymmetry (i.e., claim (i) discussed in Section 1.4). McMahan considers how the benefit of bringing someone into existence with a good life weighs against a similarly-sized benefit to an existing person. John Broome’s chapter in Part III (his second contribution to the Handbook) notes that one consequence of climate change that is rarely discussed is that it will change the future population. He then considers the “intuition of neutrality” discussed earlier to argue that the impact of climate change must take into account the fact that it will affect population change.

Some issues in applied population ethics intersect with other important issues, both at the level of theory and at the level of practice. For example, Elizabeth Harman’s chapter provides an intriguing analysis of gamete donation as a “morally permissible moral mistake”—an act that one ought morally not to do (since it involves isolating oneself from one’s biological child) but that is nevertheless morally permissible and morally good (since it gives certain people the opportunity to have and raise children). Based on her discussion of gamete donation, Harman draws out some general lessons about the relationship between what one is morally obligated to do and what one should do, all things considered, and about the nature of moral reasons.

Julia Mosquera’s chapter discusses the relationship between population ethics and the ethics of disability. She argues that while reducing the incidence of disability is desirable in certain important respects, it may also exacerbate inequality. This is because, she argues, as the minority of disabled individuals within a population shrinks, these individuals are subjected to a greater number of pairwise relations of inequality. Egalitarian duties of justice toward disabled individuals may therefore grow stronger as their numbers dwindle.

Finally, most literature in applied population ethics focuses on human populations—but should we not also be concerned about non-human animal populations? Tim Meijers and Axel Gosseries explore how the different theoretical questions and answers in population ethics bear on ethical questions related to our interactions with non-human animal populations. They consider four different hypotheses corresponding to four different ways in which non-human animal population ethics may be thought to be distinctive or special as a subfield. Overall, they suggest, animal and human population ethics may be difficult to distinguish clearly once we abandon our implicitly speciesist commitments.