The Repugnant Conclusion

1. Introduction

The Repugnant Conclusion is a central puzzle in population ethics, the area of moral philosophy that evaluates outcomes differing in how many people exist, who they are, and how well their lives go. The conclusion concerns a structural tension between:

• seemingly attractive, aggregative ways of assessing outcomes (especially total utilitarianism), and
• a widespread intuitive resistance to ranking vast populations of people whose lives are only barely worth living above smaller populations of people enjoying very high quality lives.

Derek Parfit introduced the label and the canonical formulation in Reasons and Persons (1984). He used a stylized comparison between two possible worlds. In world A, there are relatively few people, all living excellent lives. In world Z, there are vastly more people, each with a life “barely worth living.” Under plausible aggregative assumptions, the total sum of well-being in Z exceeds that in A, so Z is judged “better,” even though many readers feel that A is clearly more desirable. Parfit called this verdict “repugnant.”

The Repugnant Conclusion is not a single argument but a family of related results: whenever certain broad conditions about aggregation, impartiality, and population comparability are met, it appears to follow that some very large, low-quality population is at least as good as, or better than, a much smaller, high-quality one. Philosophers and welfare economists have since treated this as a test case for theories of population value.

The issue is methodologically significant: it pressures moral theorists to choose between revising deep-seated intuitions about quality of life, altering axioms such as Mere Addition and transitivity, or abandoning simple aggregative approaches. It has prompted the development of new population axiologies, fresh analyses of person-affecting ideas, and extensive formal work on social welfare functions.

2. Origin and Attribution

The expression “Repugnant Conclusion” and its standard formulation are commonly attributed to Derek Parfit. The conclusion appears in Part IV (“Future Generations”) of Reasons and Persons (1984), especially Chapter 19 and Appendix I, where Parfit systematically develops population ethics and the Mere Addition Paradox.

Parfit’s work did not emerge in isolation. Earlier discussions in utilitarianism and welfare economics had already explored related issues about total and average welfare, optimal population size, and the ethics of procreation. However, Parfit was the first to present the Repugnant Conclusion as a clearly named and central challenge to total utilitarianism and to population axiology more broadly.

Key attributions and context can be summarized as follows:

Although Parfit named and popularized the conclusion, subsequent authors have refined and rephrased it. Larry Temkin developed spectrum and intransitivity arguments closely related to Parfit’s; John Broome, Gustaf Arrhenius, and others formulated axiomatic impossibility results that generalize the phenomenon beyond strictly utilitarian frameworks. Some scholars therefore treat the Repugnant Conclusion as a manifestation of deeper structural features of any theory that aims to aggregate welfare across variable populations.

Nonetheless, standard usage in the literature continues to trace the conclusion, in its classic “world A vs. world Z” form, directly to Parfit’s 1984 text.

3. Historical Context in Ethics and Economics

The Repugnant Conclusion arose at the intersection of developments in 20th‑century ethics and welfare economics. In the decades before Parfit’s work, theorists were grappling with:

• the revival and formalization of utilitarian and consequentialist ethics,
• new tools in social choice theory, and
• mounting concern about obligations to future generations in light of nuclear risk, environmental degradation, and technological change.

Ethical background

Post‑war analytic philosophy saw renewed interest in precise argumentation about moral theory. Discussions of act vs. rule utilitarianism, the nature of well-being, and the structure of moral reasons set the stage for more rigorous inquiries into how to treat future persons. Debates over abortion, procreation, and intergenerational justice started to highlight questions about individuals who might or might not exist depending on present choices.

The Non-Identity Problem began to surface in work by Jan Narveson and others, foregrounding the difficulty of saying that acts harm or benefit specific people when those acts determine who comes into existence. This prepared the ground for Parfit’s more systematic treatment of population ethics.

Economic and formal background

In economics, Bergson–Samuelson social welfare functions, Arrow’s impossibility theorem, and growth theory had already focused attention on aggregating individual utilities and on criteria for social choice. Economists debated optimal population and overpopulation, considering whether social evaluation should track total utility or average utility.

By the 1970s, writers such as Yew-Kwang Ng and John Broome were analyzing population questions within welfare economics. They examined the trade‑offs between population size and mean welfare, foreshadowing the structural tension Parfit would spotlight.

Parfit’s formulation thus crystallized and connected strands from both ethics and economics, making explicit a conflict that had been implicit in prior work on aggregation and population size.

4. Formulation of the Repugnant Conclusion

Parfit’s standard formulation relies on a comparison between stylized possible worlds that differ in population size and quality of life.

Worlds A and Z

• World A: Contains a relatively small number of people. Each enjoys a life of very high well-being—rich relationships, meaningful projects, and little serious suffering. Parfit represents this graphically with tall bars of equal height.
• World Z: Contains a very large number of people. Each person’s life is just barely worth living: there is some net positive balance of pleasure or satisfaction over pain and frustration, but by a very small margin. Graphically, this is depicted as a very wide population base with very short bars.

Under total utilitarian evaluation, the relevant quantity is the sum of all individuals’ welfare. If Z has vastly more people than A, its total well-being can exceed that of A, despite each individual having much lower welfare.

Parfit then states the Repugnant Conclusion roughly as follows:

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For any possible population of at least ten billion people, all with a very high quality of life, there is a much larger possible population whose existence, if other things are equal, would be better, even though its members have lives that are barely worth living.

— Derek Parfit, Reasons and Persons (1984)

The formulation is deliberately general: “any” very high-quality population A is said to be outclassed, from the standpoint of total welfare, by some much larger, low-quality population Z. Many readers find it counterintuitive—or “repugnant”—to regard Z as better than A, or even no worse than A.

The Repugnant Conclusion is thus a comparative claim about population outcomes:

Subsequent work has adopted variations of this formulation, but they generally retain the core idea: given certain aggregative assumptions, sufficiently many barely good lives can outweigh fewer excellent lives.

5. Parfit’s Mere Addition Argument

Parfit does not present the Repugnant Conclusion simply as a bare consequence of total utilitarian arithmetic. Instead, he offers the Mere Addition Argument (or Mere Addition Paradox) to show how one can be led stepwise, by seemingly innocent comparisons, from an attractive population A to a “repugnant” population Z.

Structure of the population sequence

Parfit introduces intermediate populations, often labeled A, A+, B, C, …, Z:

1. A: A small population with very high well-being.
2. A+: Formed from A by adding an extra group of people whose lives are worth living but at a lower level than those in A, without worsening anyone in A. A and A+ are disjoint subpopulations; no one is made worse off in the move from A to A+.
3. B, C, …: Successive transformations involving small redistributions of well-being (e.g., making the better-off slightly worse off, and the worse-off slightly better off) while keeping or increasing total welfare.
4. Z: After many such small steps, one reaches a very large population in which everyone has roughly the same, low but still positive, level of welfare—Parfit’s world Z.

The key normative premise is the Mere Addition Principle:

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Adding extra people, whose lives are worth living, to a population cannot make the outcome worse, other things equal.

From A to A+, Mere Addition seems to imply that A+ is at least as good as A, since nobody in A is harmed and the new people have lives worth living. From A+ onwards, Parfit relies on further intuitive comparisons: population diagrams are adjusted in ways that many people judge to be no worse than or better than their predecessors (e.g., removing inequalities while preserving total welfare).

The paradoxical trajectory

By repeatedly applying Mere Addition and such apparently benign improvements, Parfit constructs a chain of pairwise comparisons:

Yet, when comparing A directly with Z, many people feel that Z is clearly worse. The Mere Addition Argument therefore appears to show that we cannot retain all of the following: (i) the Mere Addition Principle, (ii) the particular pairwise intuitions along the sequence, and (iii) the intuitive judgment that A is better than Z, together with transitivity of “better than.”

The argument’s significance lies in showing how the Repugnant Conclusion emerges not only from a single utilitarian formula but also from a web of seemingly plausible comparative judgments about population changes.

6. Logical Structure and Assumptions

Philosophers often treat the Repugnant Conclusion as the endpoint of a reductio-style argument: starting from widely accepted premises, one is led to a highly counterintuitive conclusion. The main logical components can be separated into axiological assumptions and structural assumptions about the “better than” relation.

Core axiological assumptions

A typical route to the Repugnant Conclusion relies on:

These assumptions, taken together, support the inference that sufficiently many low-level positive lives can, in aggregate, exceed the total well-being of fewer high-level lives.

Structural assumptions on betterness

Parfit’s Mere Addition Argument and related formulations usually rely on:

The Mere Addition Principle

A further assumption is the Mere Addition Principle:

• Adding extra people with lives worth living (and harming no one) does not make an outcome worse.

This is not logically entailed by totalism but fits naturally with it. It is crucial for the move from A to A+ and for justifying changes in population size as at least not worsening outcomes.

Logical pattern

The overall pattern can be schematized as:

1. Adopt totalism and related axioms.
2. Apply Mere Addition and small redistributions to construct a sequence A, A+, B, …, Z.
3. Use transitivity and continuity to infer that Z is at least as good as A, and often better.
4. Confront the intuitive judgment that A is better than Z.

Much of the subsequent literature examines which of these assumptions might be weakened, rejected, or replaced to block the derivation, while still preserving other desiderata for a plausible population axiology.

7. Total, Average, and Critical-Level Utilitarianism

The Repugnant Conclusion is most directly associated with total utilitarianism, but its discussion has prompted careful comparison among several utilitarian variants.

Total utilitarianism (totalism)

Total utilitarianism evaluates outcomes by the sum of individual well-being:

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An outcome X is better than Y iff the total well-being in X exceeds that in Y.

Under totalism:

• Adding a person with a positive level of welfare always increases total value.
• A very large population with low positive welfare per person can surpass the total welfare of a smaller, very happy population.

This structure straightforwardly generates the classic Repugnant Conclusion.

Average utilitarianism

Average utilitarianism instead evaluates outcomes by mean well-being:

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An outcome X is better than Y iff average well-being per person is higher in X than in Y.

Average utilitarianism tends to avoid the classic Repugnant Conclusion, because:

• Adding many barely worth-living lives typically lowers the average, making such outcomes worse than smaller, high-quality populations.
• A world like Z, where everyone has low but positive welfare, may have a much lower average than world A.

However, critics argue that average utilitarianism has other counterintuitive implications, such as preferring:

• very small high-utility populations over large, moderately good ones, and
• eliminating very unhappy individuals without offsetting gains, if this raises the average.

Critical-level utilitarianism

Critical-level utilitarianism modifies totalism by introducing a critical level c:

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The value of each life is measured as (welfare – c), so only well-being above c contributes positively to social value.

If the critical level is set above the “barely worth living” threshold, then:

• Adding lives at very low positive welfare can contribute little or even negative value, depending on c.
• This can prevent large low-welfare populations from outranking small, high-welfare ones, at least in many cases.

There are multiple variants:

Critics have argued that critical-level views face their own challenges, such as versions of the “sadistic” or “anti-Repugnant” conclusions, where adding good lives can sometimes lower value relative to alternatives with additional suffering.

These three utilitarian frameworks thus illustrate different ways of relating population size and quality of life, each with distinct connections to the Repugnant Conclusion and to other evaluative paradoxes.

8. Person-Affecting and Impersonal Views

The Repugnant Conclusion arises naturally within impersonal frameworks that evaluate outcomes by aggregate or structural features (total or average well-being) rather than solely by how particular individuals fare. In response, many theorists have explored person‑affecting views, which aim to restrict moral comparisons to effects on specific people.

Impersonal views

Impersonal utilitarian and aggregative views typically hold:

• Outcomes can be better or worse even when they involve different numbers and identities of people.
• The value of creating additional people with good lives can be assessed without reference to any specific existing person being helped or harmed.

On such views, the comparison between A and Z is straightforward: one asks which world has greater total (or otherwise aggregated) well-being.

Person-affecting views

Person‑affecting theorists contend that:

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One outcome is better than another only if it is better for someone—i.e., for some person who either exists in both outcomes or will exist regardless of which outcome is chosen.

This idea is sometimes captured by slogans such as “we are required to make people happy, not to make happy people.”

Applied to population ethics, person-affecting views typically:

• Focus on avoiding harms to existing or guaranteed future individuals.
• Deny that we have strong moral reasons simply to add extra people whose lives would be good, if they would not otherwise exist.

This can undermine key steps leading to the Repugnant Conclusion, because the move from A to A+ (adding new, worthwhile lives) may not be seen as morally better, or even comparable, if none of the additional people are independently owed existence.

Variants and challenges

There are several strands within the person‑affecting tradition:

These views aim to block or weaken the Repugnant Conclusion but face well-known difficulties, especially in dealing with cases where choices determine who exists (the Non-Identity Problem) and in providing complete and transitive comparisons across all population scenarios. Some critics also argue that purely person‑affecting approaches risk neglecting intuitions that it can be good, in some sense, for more happy people to exist.

9. Key Variations and Related Paradoxes

Beyond Parfit’s original formulation, philosophers have identified variants of the Repugnant Conclusion and related paradoxes that raise similar structural issues.

Weak, strong, and very weak repugnant conclusions

Some authors distinguish between:

These distinctions allow finer-grained analysis of which versions a given axiology entails.

Population spectrum and “egalitarian” paradoxes

Larry Temkin developed spectrum arguments and the idea of intransitivity in betterness. His examples move gradually from very good lives to very poor but still positive lives via many small steps, similar in spirit to Parfit’s sequence. Temkin argues that, under plausible judgments, one may conclude that:

• A is better than B, B better than C, …, yet A is not better than Z, challenging transitivity rather than accepting the Repugnant Conclusion.

Relatedly, various egalitarian and prioritarian approaches confront tensions between valuing equality, prioritizing the worse off, and avoiding repugnant-type conclusions.

Anti-Repugnant and Sadistic conclusions

Alternative axiologies designed to escape the Repugnant Conclusion often yield counterpoints:

• Anti‑Repugnant or “Reverse” conclusions: Some theories imply that a small population of very poorly off people might be preferable to a larger population of very well‑off people, or endorse extreme preferences for very small populations.
• Sadistic conclusion (a label used in the literature): Under certain non-total views (including some critical-level or person-affecting variants), it can be better to add a few lives with negative welfare rather than many lives with slightly positive welfare, given background conditions.

These paradoxes reveal trade‑offs among competing desiderata such as:

• respect for quality of life,
• aversion to large numbers of barely good lives, and
• avoidance of preferences that seem indifferent or hostile to additional happy people.

Together, these variations and related paradoxes show that the Repugnant Conclusion is part of a wider family of structural tensions in population axiology.

10. Standard Objections and Critiques

Philosophers and economists have advanced a range of objections focused on different components of the reasoning leading to the Repugnant Conclusion.

Intuitive repugnance and quality-over-quantity

Many critics start from the intuition of repugnance: they find it extremely implausible that a world of countless people living barely worthwhile lives could be better than a world of fewer but very happy people. This is sometimes sharpened into a quality‑over‑quantity objection:

• The objection holds that total utilitarianism overvalues numerical quantity and undervalues the depth, richness, and intensity of individual flourishing.
• On this view, the metric of “sum of welfare” is said to miss morally important aspects of human (or sentient) life.

Objections to Mere Addition

Some critics target the Mere Addition Principle. They contend that:

• Adding people whose lives are only barely worth living can make an outcome worse, even if no existing person is harmed.
• The presence of many near‑miserable lives may create a world that is, in some impersonal sense, worse overall, or at least not better.

Others suggest that the principle is ambiguous: what counts as “other things equal” may implicitly include broader evaluative considerations that are not captured by mere welfare sums.

Intransitivity and incompleteness objections

A further line of critique, associated especially with Larry Temkin, questions the transitivity and completeness assumptions:

• Some argue that in population ethics, A can be better than B, B better than C, yet A not better than C (or the relation may be indeterminate).
• If betterness is intransitive or incomplete, the chain from A to Z can break, preventing the inference that Z is better than A.

Person-affecting and non-aggregation objections

Person-affecting theorists object to the impersonal aggregation underlying the Repugnant Conclusion:

• They deny that creating additional people with positive welfare is straightforwardly good in the same sense as improving existing lives.
• On some versions, A and Z are not strictly comparable, because there is no shared set of individuals whose well-being can be contrasted.

Methodological objections

A final set of critiques concerns methodology:

• Some commentators question how much weight to give to intuitions about extremely large populations and barely worth-living lives.
• Others argue that the stylized nature of Parfit’s examples (e.g., highly idealized diagrams) may obscure relevant dimensions of value or moral status.

The literature therefore includes objections aimed at each of the central ingredients: the value metric (total vs. other), the structural principles (transitivity, completeness), the Mere Addition Principle, and the role of intuitions in theory choice.

11. Proposed Resolutions and Alternative Axiologies

In response to the Repugnant Conclusion and its variants, theorists have developed a wide array of proposed resolutions and alternative population axiologies.

Accepting the Repugnant Conclusion

One approach is to accept the Repugnant Conclusion:

• Proponents argue that our intuitions in extremely large-number cases are unreliable or distorted.
• They maintain that total utilitarianism’s clear, simple structure and other advantages outweigh its implication of the Repugnant Conclusion.
• On this view, the puzzle is primarily psychological, not theoretical.

Modifications within utilitarianism

Several proposals retain a broadly utilitarian framework but alter the aggregation rule:

These views typically avoid the classic Repugnant Conclusion but often incur other paradoxes, including “sadistic” or “anti‑Repugnant” conclusions, or extreme preferences for small, high‑average populations.

Person-affecting and asymmetric axiologies

Another broad strategy is to adopt person‑affecting or asymmetric views:

• Strict person‑affecting principles judge outcomes better only if they are better for particular individuals, blocking many cross-population comparisons.
• Asymmetric theories assign strong moral weight to avoiding the creation of lives not worth living, but little or no positive weight to creating new happy lives.
• These can avoid both the Repugnant and some anti‑Repugnant conclusions, but may lead to incompleteness or limited comparability among outcomes.

Lexical and sufficientarian approaches

Some theorists propose giving lexical priority or special importance to certain thresholds:

• Lexical priority to high-quality lives: Any loss in the number or quality of high‑flourishing lives may outweigh gains from adding many barely good lives.
• Sufficientarian views: Focus on securing lives above a sufficiency threshold of well‑being; once that is met, additional low-level lives may contribute relatively little or no value.

These approaches attempt to encode the intuition that a world like A is decisively better than any Z‑type world, while still allowing some aggregation.

Hybrid and pluralist axiologies

A further class of proposals are hybrid or pluralist views that combine multiple evaluative components:

• For example, a social welfare function might include terms for total, average, and inequality, or give separate weight to priority for the worse off.
• Some models employ tiered or piecewise rules: one principle governs comparisons below a threshold, another above.

Gustaf Arrhenius and others have shown that many attractive desiderata for population ethics are mutually incompatible, yielding impossibility theorems. Hybrid theories therefore often aim for a carefully chosen balance of trade‑offs, accepting some form of weakened or very weak Repugnant Conclusion while avoiding the strongest, most counterintuitive versions.

12. Implications for Policy, Climate, and Global Priorities

The Repugnant Conclusion has implications beyond abstract theory, especially where decisions can affect population size, quality of life, and the long-term future.

Demographic and social policy

In discussions of population policy—such as family planning, pronatalist or antinatalist measures, and migration—different stances toward the Repugnant Conclusion may suggest different priorities:

• Views close to total utilitarianism may see value in larger populations with positive welfare, other things equal.
• Theories that reject the Repugnant Conclusion often place stronger emphasis on ensuring high quality of life and securing sufficient standards for those who exist, potentially giving less weight to increasing numbers.

Policy analysts sometimes use social welfare functions inspired by theories in population ethics; whether these functions treat extra lives as strongly beneficial can influence evaluations of fertility policies or social support programs.

Climate change and environmental policy

Climate and environmental decisions affect both:

• the number of future people who may exist, and
• the conditions in which they would live.

If one accepts an axiology that favors very large populations with lives just above the “worth living” threshold, then policies that enable long‑run human survival and expansion might be judged overwhelmingly important, even if average quality is modest. Conversely, views that resist the Repugnant Conclusion may prioritize:

• preventing scenarios in which many future people scrape by near subsistence,
• protecting environmental quality and resilience over sheer population size.

Global priorities and effective altruism

In global priorities research and the effective altruism movement, the Repugnant Conclusion interacts with debates about longtermism (addressed more fully elsewhere). If future populations could be extremely large, totalist or near‑totalist axiologies may imply that actions affecting the probability or size of such populations dominate moral evaluation.

Organizations engaged in cause prioritization sometimes model the value of interventions that influence existential risks, technological trajectories, or global health and development. Their implicit or explicit stance on the Repugnant Conclusion can shape whether they:

• heavily prioritize safeguarding the existence of vast future populations, or
• give more weight to improving the well-being of currently existing and near‑future people.

Thus, while the Repugnant Conclusion is a theoretical construct, it has practical ramifications for how policymakers, economists, and ethicists evaluate trade‑offs involving population scale, sustainability, and quality of life.

13. Connections to the Non-Identity Problem and Longtermism

The Repugnant Conclusion is closely linked to other central issues in population ethics, particularly the Non-Identity Problem and debates about longtermism.

Connection to the Non-Identity Problem

The Non-Identity Problem arises when actions determine which particular individuals will exist. For many long-term decisions—such as climate policy, reproductive technologies, or large infrastructure projects—the set of future people differs depending on the choice made.

This interacts with the Repugnant Conclusion in several ways:

• Person-affecting vs. impersonal evaluations: Non‑identity cases challenge the idea that one outcome can be worse for particular individuals if those individuals would not have existed under the alternative. Impersonal aggregative views, which naturally give rise to the Repugnant Conclusion, can more easily compare such outcomes.
• Creation vs. quality of lives: In both problems, decisions are about whether to bring into existence people whose lives may be good, mediocre, or bad, rather than about changing fixed individuals’ welfare levels.
• Structural parallels: Some of the same principles (e.g., identity‑independence, anonymity, and aggregation across possible persons) feature in formal treatments of both phenomena.

Because of these connections, theorists often discuss the Repugnant Conclusion and the Non-Identity Problem together when assessing the viability of person‑affecting approaches and alternative axiologies.

Connection to longtermism

Longtermism is a family of views emphasizing that the very large number of potential future people can give overwhelming moral importance to how our actions affect the far future. The Repugnant Conclusion is relevant because:

• Under total or near‑total views, the potential vastness of future populations—if humanity or other sentient life persists for millions of years—can dwarf the value associated with present generations.
• If one accepts that adding very many barely good lives can make an outcome better, longtermist reasoning may support strong priorities for reducing existential risks and enabling large, flourishing future civilizations.

However, some critics of longtermism worry that an implicit acceptance of Repugnant‑Conclusion‑type reasoning may:

• Overemphasize population size relative to average or sufficient quality of life.
• Lead to policy preferences that favor ensuring mere survival and expansion over enhancing well-being or justice.

Conversely, longtermist thinkers sometimes argue that one can value the long‑term future strongly without endorsing the strongest forms of the Repugnant Conclusion, for example by appealing to scenarios in which future people would have very high quality lives.

In this way, the Repugnant Conclusion serves as a conceptual touchstone in debates about how to weigh the interests of vast, uncertain future populations against present-day concerns.

14. Current Debates and Open Questions

Contemporary work on the Repugnant Conclusion explores both formal and substantive questions, with no consensus resolution.

Trade-offs among axiomatic desiderata

A major line of research, especially in the work of Gustaf Arrhenius and others, involves impossibility theorems showing that no population axiology can satisfy all of a set of seemingly plausible conditions while avoiding every form of the Repugnant Conclusion. Examples of such conditions include:

• non‑elitism (everyone’s welfare matters),
• non‑anti‑egalitarianism,
• Pareto principles,
• and some weak version of the Mere Addition Principle.

These results raise questions about which desiderata to abandon or weaken, and whether some form (often a “very weak” form) of the Repugnant Conclusion is inescapable.

Status of intuitions in large-number cases

Another ongoing debate concerns methodology:

• How trustworthy are our intuitions about hypothetical worlds with astronomically large populations or lives barely worth living?
• Should theory give priority to simple, coherent principles even when they conflict with such intuitions, or should intuitions constrain acceptable principles?

Some philosophers explore debunking explanations for the repugnance intuition, while others emphasize its resilience and alignment with broader moral judgments about quality of life.

Completeness, comparability, and intransitivity

There is continued discussion over whether to:

• allow incomplete betterness relations (some pairs of populations are incomparable),
• accept intransitivity (chains A ≻ B ≻ C without A ≻ C), or
• retain classical complete, transitive orderings and accept some repugnant or anti‑repugnant verdicts.

Each option raises technical and philosophical questions about decision-making under moral uncertainty, policy guidance, and the nature of practical reason.

Expanding the domain of value

Some recent work asks whether incorporating non‑welfarist values can reshape the debate:

• considerations of justice, rights, capabilities, or respect for persons;
• environmental and non‑human values.

Open questions include whether such pluralistic frameworks inevitably face analogues of the Repugnant Conclusion, once they allow trade‑offs between population size and these additional goods.

Empirical and interdisciplinary angles

Finally, there is growing interdisciplinary interest:

• empirical studies of lay intuitions about population trade‑offs,
• connections to climate economics, risk analysis, and public policy,
• and implications for artificial intelligence safety and long-term technological development.

Whether these empirical insights will significantly reshape philosophical assessments of the Repugnant Conclusion remains an open topic of inquiry.

15. Legacy and Historical Significance

Since its introduction in 1984, the Repugnant Conclusion has become a standard reference point in population ethics and a widely cited challenge to simple aggregative moral theories.

Influence on ethical theory

The conclusion has:

• Helped to define population ethics as a distinct research area, alongside the Non-Identity Problem.
• Pressured utilitarian and consequentialist theories to refine or reconsider their treatment of population size and the value of additional lives.
• Motivated the development of numerous alternative axiologies—critical-level views, person‑affecting theories, sufficientarian and lexical approaches, and hybrid models—many of which were formulated explicitly in response to Parfit’s puzzle.

Impact on formal and applied disciplines

In welfare economics and social choice theory, the Repugnant Conclusion has informed:

• the design and critique of social welfare functions that handle variable populations,
• axiomatic work on the comparability and ranking of different demographic futures,
• and explorations of how economic growth, inequality, and population dynamics interact in normative evaluation.

In applied ethics and public policy, it has influenced debates about:

• optimal population and demographic policy,
• climate change and intergenerational justice,
• and global priorities concerning existential risk and the long‑term future.

Role in Parfit’s legacy and beyond

The Repugnant Conclusion is a central component of Derek Parfit’s philosophical legacy. Alongside his work on personal identity and reasons, it exemplifies his method of using carefully constructed thought experiments and diagrams to uncover hidden tensions in our normative commitments.

Subsequent generations of philosophers—including Larry Temkin, John Broome, Gustaf Arrhenius, Torbjörn Tännsjö, T.M. Scanlon, Jeff McMahan, and many others—have engaged extensively with Parfit’s challenge, either by defending some form of totalism, proposing alternatives, or exploring the limits of moral theory in large-population contexts.

Today, the Repugnant Conclusion continues to serve as a benchmark test for new theories in population ethics: any proposed axiology is routinely examined to determine which, if any, versions of the conclusion it entails, and at what theoretical cost. Its enduring role reflects both the depth of the underlying issues and the difficulty of reconciling all our normative intuitions about the value of future people.