Mere Addition Paradox

1. Introduction

The Mere Addition Paradox is a central puzzle in contemporary population ethics. It arises when we try to evaluate outcomes that differ both in how well people’s lives go and in how many people there are. The paradox suggests that a set of intuitive and widely endorsed moral principles, when combined, appears to lead to a highly counterintuitive verdict about very large populations.

At the heart of the puzzle is the Mere Addition Principle: the idea that, other things equal, simply adding extra people whose lives are worth living, and who do not make anyone else worse off, cannot make an outcome worse. When this principle is combined with familiar ideas from utilitarianism, egalitarianism, and social choice theory, we seem driven toward valuing a vast population whose members all have lives that are barely worth living more highly than a much smaller population of very happy people.

The paradox is formulated most famously by Derek Parfit in Reasons and Persons (1984). He uses a sequence of hypothetical populations—labelled A, A+, B, and, further along a chain, a “Z-world”—to illustrate how apparently innocuous comparative judgments can together generate this striking result. Each individual step in the sequence is designed to look, by standard ethical lights, like an improvement or at least not a worsening. Yet the end-state seems intuitively much worse than the starting point.

Philosophers treat the Mere Addition Paradox as a diagnostic tool rather than a standalone argument for any one theory. It is used to probe the limits of total utilitarianism, average utilitarianism, and person‑affecting views, and to motivate more complex or revisionary approaches to evaluating populations. There is no consensus about which of the underlying principles should be abandoned or modified, and debate continues over whether the paradox reveals a flaw in our theories, in our intuitions about large populations, or in the assumption that there must be a single, transitive ordering of all possible outcomes.

2. Origin and Attribution

The Mere Addition Paradox is widely attributed to Derek Parfit, who introduced and developed it in Part IV (“Future Generations”) of his book Reasons and Persons (Oxford: Clarendon Press, 1984), especially Chapter 19. Although earlier authors had discussed ethical issues about future generations and varying population sizes, Parfit’s formulation is generally regarded as the first clear, systematic presentation of the paradox in its contemporary form.

Parfit explicitly frames the Mere Addition Paradox as a step on the way to his more famous Repugnant Conclusion, but he treats it as a distinct, earlier stage of the overall argument. He introduces the populations A, A+, and B in diagrammatic form, with bar charts representing levels of wellbeing, to make the structure of the comparisons as transparent as possible:

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“We can first consider whether it could be worse if, in some future century, there are more people… whose lives are well worth living. I shall try to show that, though it may seem that this could not be worse, this belief leads to difficulties.”

— Derek Parfit, Reasons and Persons (1984)

The historical lineage of ideas feeding into Parfit’s work includes debates on utilitarianism and future generations, such as those by Jan Narveson in the 1960s and 1970s, and work in welfare economics on aggregation and social welfare functions. However, those predecessors typically did not isolate the specific pattern of reasoning that Parfit labels “mere addition.”

Subsequent literature almost uniformly credits Parfit’s Reasons and Persons as the canonical source. Later authors—including Larry Temkin, Gustaf Arrhenius, Torbjörn Tännsjö, John Broome, and others—explicitly take Parfit’s diagrams and terminology (A, A+, B, Z) as the starting point for further analysis.

3. Historical Context in Population Ethics

The Mere Addition Paradox emerged against the backdrop of a growing interest in future generations and population ethics in the late 20th century. Before that period, many moral theories focused primarily on fixed populations or on obligations among contemporaries, leaving questions about varying numbers of people relatively underexplored.

Pre-Parfit Discussions

Earlier work in utilitarianism and welfare economics had already raised puzzles about how to treat population size:

These contributions did not yet crystallize into a unified “population ethics” framework but provided tools and concerns that Parfit would later integrate.

Rise of Formal Population Ethics

In the 1970s and early 1980s, philosophers increasingly adopted formal methods from decision theory and social choice theory. They asked how to evaluate outcomes differing in:

• the number of people who exist,
• their identities,
• and their levels of wellbeing.

Parfit’s work was pivotal in giving this emerging field a recognisable shape. He coined or popularised several central problems—including the Non‑Identity Problem, the Repugnant Conclusion, and the Mere Addition Paradox—which together helped define population ethics as a distinct subdiscipline.

Position of the Mere Addition Paradox

Within this historical trajectory, the Mere Addition Paradox served as:

• a bridge between traditional utilitarian reasoning and novel questions about population size,
• a test case for applying aggregation principles (such as Pareto and anonymity) to variable-population settings,
• and a precursor to later impossibility theorems (e.g., by Gustaf Arrhenius) showing that no axiology can satisfy all seemingly reasonable conditions.

The paradox thus reflects a broader shift from intuitive, case-based moral theorising toward more formal, axiomatic approaches to future generations, while also highlighting the tensions that arise when such formalisms confront lay intuitions about what makes a world better or worse overall.

4. The Setup: Populations A, A+, and B

Parfit’s presentation of the Mere Addition Paradox begins with a carefully structured comparison of three hypothetical populations. These are depicted using bar charts, but the essential features can be described verbally.

Population A

Population A is the initial benchmark:

• It contains a relatively small number of people.
• Every person enjoys a very high level of wellbeing.
• Wellbeing is equal across all individuals.

A is meant to represent a highly desirable, flourishing society with no internal inequality in welfare.

Population A+

Population A+ is constructed from A by “mere addition”:

• All individuals in A remain, at exactly the same high level of wellbeing.
• A new, disjoint group of people—the “added group”—is brought into existence.
• These added individuals have lives worth living but at a lower positive welfare level than the original A‑people.
• There is no interaction between the original group and the added group that changes anyone’s wellbeing.

Formally, A+ is A plus extra lives with positive welfare, with the welfare of the original people held constant. This models the Mere Addition Principle directly.

Population B

Population B is introduced as an alternative way of distributing wellbeing:

• The total number of people in B is the same as in A+.
• Everyone in B has the same intermediate level of wellbeing.
• This level is:
  • Lower than the high level enjoyed by people in A,
  • Higher than the lower positive level of the added people in A+.

B can be thought of as resulting from a sequence of trades or redistributions starting from A+: some wellbeing is taken from the initially better-off and given to the initially worse-off until equality at an intermediate level is reached.

The evaluative relations among A, A+, and B—especially whether A+ is better than A, and whether B is better than A+—are the building blocks of the paradox.

5. Formal Statement of the Mere Addition Paradox

The Mere Addition Paradox can be stated in a more formal, axiom‑based way, abstracting from Parfit’s diagrams while preserving their structure. The focus is on how different populations are ranked according to a welfarist axiology, where only individuals’ wellbeing levels matter for overall value.

Basic Ingredients

Let:

• N(X) be the number of people in population X.
• wᵢ(X) be the wellbeing of individual i in X.
• V(X) be the overall value of population X according to the axiology in question.

We consider three specific populations:

• A: all existing individuals have high wellbeing h.
• A+: includes all individuals from A at level h, plus an added group at lower positive level l (0 < l < h).
• B: the same number of individuals as A+, all at an intermediate level m (l < m < h).

Principles Typically Assumed

The paradox usually invokes:

• Mere Addition Principle: If X is obtained from Y by adding people whose lives are worth living (w > 0) and leaving everyone else unchanged, then V(X) ≥ V(Y).
• Pareto or Weak Pareto: If everyone in X is at least as well off as in Y, and some are strictly better off, then V(X) > V(Y).
• Anonymity / Non‑elitism: The identity or label of individuals does not matter; only welfare distribution does.
• Transitivity of Betterness: If V(X) ≥ V(Y) and V(Y) ≥ V(Z), then V(X) ≥ V(Z).

Formal Paradoxical Pattern

In a simplified form, the paradox arises from the following schematic judgments:

1. V(A+) ≥ V(A), by Mere Addition (A+ is A plus extra lives worth living).
2. V(B) ≥ V(A+), by Pareto‑type and equality considerations (some individuals corresponding to the added group are better off in B; equality is higher; no one is much worse off).
3. By transitivity, V(B) ≥ V(A).
4. Yet structurally, B is a larger population at a lower average welfare than A.
5. Iterating similar steps can generate a sequence A → A+ → B → … leading to populations with more people at progressively lower welfare, while each step is still judged V(next) ≥ V(previous).

The paradox is then: under these formal assumptions and comparative judgments, one is led to rank a very large, low‑welfare population at least as good as, or better than, a small, high‑welfare population, despite strong contrary intuitions.

6. Logical Structure and Reductio Strategy

The Mere Addition Paradox is typically framed as a reductio ad absurdum: it starts from widely accepted principles and shows that, taken together, they lead to an apparently absurd or highly counterintuitive conclusion. The structure is logical rather than empirical.

Core Structure

The argument’s skeleton can be represented as:

1. Accept a set of axioms and judgments that seem independently plausible:

   • Mere Addition Principle,
   • certain Pareto and equality-based comparisons,
   • transitivity of “at least as good as,”
   • a negative assessment of very large low‑welfare populations.

2. Use these to derive a chain of comparisons:

   • A ≤ A+ (mere addition),
   • A+ ≤ B (redistribution and equality),
   • B ≤ C, C ≤ D, …, Y ≤ Z (further small adjustments).

3. By transitivity, infer A ≤ Z (and often Z > A).

4. Combine this with the intuitive judgment that A > Z.

5. Conclude that the set of commitments is inconsistent: at least one must be rejected or revised.

Role of Intermediate Populations

The intermediate populations (A+, B, and further stages) are central to the reductio strategy. Each step is crafted so that:

• only small changes are made,
• changes appear, by standard moral reasoning, to be improvements or, at worst, neutral,
• and no single step seems dramatic enough to trigger a strong intuitive reversal.

The “shock” is reserved for the endpoints: a very happy small population versus an enormous barely-happy one. The reductio works by showing that if each local step is accepted, then denial of the final comparative judgment seems irrational, yet accepting the final comparative judgment conflicts with deep-seated intuitive beliefs.

Logical Targets

The reductio does not assume in advance which principle is at fault. Instead, it shows that the following cannot all be true together:

Different philosophers respond by rejecting different components, but the logical role of the paradox is to force a choice.

7. Key Principles: Mere Addition, Pareto, and Transitivity

The Mere Addition Paradox relies on the interaction of several key evaluative principles. Each is independently attractive and has a substantial theoretical pedigree.

Mere Addition Principle

The Mere Addition Principle states that, other things equal, simply adding extra people whose lives are worth living, and who do not worsen the wellbeing of anyone else, cannot make an outcome worse.

Proponents regard this as a natural extension of the idea that additional happy lives are in some sense good. It also reflects an intuition that we should not regret the existence of extra people whose lives are, for them, positive overall, when their existence does not harm others.

Critics note that the principle is silent on how such additions affect average welfare, inequality, or other pattern-based values, and they question whether such additions might sometimes be worse overall even if no identifiable person is harmed.

Pareto-Type Principles

The paradox uses a version of the Pareto principle adapted to population ethics:

• Weak Pareto: If everyone in X is at least as well off as in Y, and some are better off, then X is better than Y.

In the step from A+ to B (and later steps), proponents invoke Pareto-style and egalitarian reasoning: some individuals corresponding to the added group are better off in B than in A+, while those corresponding to A‑people may be only slightly worse off; overall equality improves. Many find it plausible that such a move is at least not worse.

Debate focuses on whether these Pareto and equality considerations must always override the fact that some individuals are made worse off in comparisons like A+ vs. B.

Transitivity of Betterness

The transitivity of “better than” or “at least as good as” is a standard assumption in decision and value theory:

• If X is at least as good as Y, and Y is at least as good as Z, then X is at least as good as Z.

Transitivity ensures that outcomes can be placed in a coherent ranking. In the Mere Addition Paradox, transitivity is what allows the local judgments (A ≤ A+, A+ ≤ B, etc.) to generate a global judgment (A ≤ Z).

Some responses to the paradox challenge this assumption, suggesting that betterness in population ethics might be intransitive or incomplete, but the classical setup treats transitivity as part of the core framework whose tension with other principles needs to be examined.

8. Connection to the Repugnant Conclusion

The Mere Addition Paradox is closely linked to Parfit’s Repugnant Conclusion, but they are conceptually distinct. The paradox serves as an intermediate step that shows how certain plausible axioms seem to lead inexorably toward the Repugnant Conclusion.

From A, A+, B to Z

Starting from the comparison among A, A+, and B, Parfit imagines repeating similar steps:

1. Construct a new A′ analogous to B: a larger population with somewhat lower welfare than A but still high.
2. Form A′+ by merely adding more people with lower but positive welfare.
3. Transform A′+ into B′, equalising at an intermediate level.
4. Continue this process through further rounds.

This iterative sequence can, in principle, lead to a Z‑world:

• A population enormous in size.
• Each person’s life is barely worth living, i.e., just above the threshold where non‑existence would be preferable.

Because each local step is deemed not worse (or better) than the previous one, transitivity implies that Z is at least as good as A.

The Repugnant Conclusion

Parfit articulates the Repugnant Conclusion roughly as:

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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 imaginable population whose existence, if other things are equal, would be better, even though its members have lives that are barely worth living.

The Mere Addition Paradox provides the mechanism by which acceptance of Mere Addition and related principles seems to force acceptance of this conclusion.

Conceptual Relationship

• The Mere Addition Paradox highlights the tension among a cluster of principles via a finite, structured set of comparisons (A, A+, B, and a finite chain beyond).
• The Repugnant Conclusion is a general thesis about how any very good, small population can be “outweighed” by a sufficiently large, low‑welfare population.

Many discussions treat the Mere Addition Paradox as the “first step” or gateway argument leading toward the Repugnant Conclusion. Others analyse it separately, as it already raises issues about aggregation, equality, and the value of extra lives even before invoking the full extremity of the Z‑world.

9. Assessment of Intuitions about A, A+, B, and Z

A distinctive feature of the Mere Addition Paradox is its reliance on conflicting intuitions about how to rank populations. Philosophers often use the case to probe which of these intuitive responses should be taken as more reliable.

Intuitions about A vs. A+

Many find it natural to judge:

• A+ is at least as good as A, and perhaps better.

Reasons cited include:

• The added people in A+ have lives worth living; their existence seems good for them.
• No one in A is made worse off.
• From an impartial perspective, it may seem better that there are more people enjoying positive welfare.

However, some are uneasy about A+ because average wellbeing is lower and overall inequality is higher than in A.

Intuitions about A+ vs. B

Intuitive reactions to A+ vs. B are more divided:

• Many judge B at least as good as A+, on grounds that:

  • the previously worse-off group is significantly better off in B,
  • everyone now has the same level of wellbeing,
  • no one is drastically worse off than in A+.

• Others hesitate, emphasising:

  • people corresponding to A‑individuals are clearly worse off in B,
  • it is not obvious that gains to others plus equality justify making them worse off.

This step is crucial because it crystallises tensions between egalitarian and prioritarian concerns versus a reluctance to sacrifice the better-off.

Intuitions about A vs. Z

Reactions to the endpoint comparison, between A and a Z‑world, are far more uniform:

• A widespread intuition holds that A is much better than Z.
• Z is often described as cramped, monotonous, or barely tolerable; though each life is “worth living,” the overall quality seems poor.
• Many find it “repugnant” to claim that Z could be better than A.

Some theorists, however, argue that these intuitions may be distorted by:

• difficulty in grasping very large numbers,
• emotional responses to the phrase “barely worth living,”
• or a tendency to focus on average, rather than total, wellbeing.

Tension Among Intuitions

The paradox arises partly because:

• Intuitions supporting A+ ≥ A and B ≥ A+ seem strong locally.
• Intuitions supporting A > Z seem strong globally.

Assessing which of these to revise, and how much weight to assign each, is a central task in the subsequent debate.

10. Standard Objections and Critiques

Philosophers have developed several prominent lines of objection to the Mere Addition Paradox, targeting different steps or assumptions.

1. Rejecting the Mere Addition Principle

Some critics deny that adding extra people with lives worth living is always neutral or better. They argue that:

• Average welfare can be intrinsically important; lower average welfare may make A superior to A+.
• The pattern of distribution (e.g., avoiding large low‑welfare groups) may have value beyond individual wellbeing.

On such views, A can be better than A+, blocking the initial step of the paradox.

2. Questioning Intuitions about Z (Accepting the Repugnant Conclusion)

Others keep Mere Addition and related axioms but challenge the intuition that A must be better than Z. They suggest that:

• Our negative reaction to Z may stem from cognitive biases about large populations.
• Once we take seriously the immense number of lives with positive welfare in Z, it becomes plausible that Z could be better.

This response preserves the internal coherence of the principles at the cost of accepting the Repugnant Conclusion.

3. Attacking Intermediate Comparisons (A+ vs. B)

A third strategy focuses on the A+ → B step. Critics argue that:

• The move from A+ to B harms individuals corresponding to A‑people.
• Egalitarian gains and small benefits to many may not morally outweigh these harms.
• It is reasonable to judge B as worse, or at least not better, than A+.

If A+ is strictly better than B, the chain leading toward Z is broken.

4. Challenging Transitivity or Completeness

Some philosophers question the assumption that “better than” must be transitive and complete. They propose that:

• In population ethics, there may be intransitive cycles or incomparabilities.
• For example, A may be better than A+, A+ better than B, but B not better than A.

On such views, the chain of comparisons cannot be straightforwardly collapsed into a single ranking from A to Z.

5. Contextual or Multi-Dimensional Critiques

Other critiques emphasise that:

• Our values may be multi-dimensional, balancing total welfare, average welfare, and distribution.
• A single scalar ranking (V(X)) may be inadequate to capture these complexities.

These approaches often view the paradox as highlighting limitations of oversimplified axiologies, rather than as refuting any specific moral intuition outright.

11. Revisionary Axiologies and Proposed Resolutions

In response to the Mere Addition Paradox, philosophers have proposed various revisionary axiologies—alternative theories of value for populations—that aim to avoid or soften the paradoxical implications.

Accepting Mere Addition and the Repugnant Conclusion

One family of views retains the Mere Addition Principle and standard aggregation axioms:

• Total utilitarianism sums wellbeing across all individuals.
• Some “impure” or constrained total views make adjustments but still allow very large low‑welfare populations to be better than smaller high‑welfare ones.

These approaches “resolve” the paradox by accepting that the Repugnant Conclusion, though surprising, may be true.

Average and Critical-Level Views

Other proposals modify how additional lives contribute to overall value:

• Average utilitarianism evaluates populations by average wellbeing. Adding lower‑welfare people can make an outcome worse, so A may be better than A+.
• Critical-level utilitarianism introduces a welfare threshold (the critical level); adding a person benefits the world only if their wellbeing exceeds this level.
• Critical-range utilitarianism allows a range around the critical level within which additions are neutral or ambiguous.

These axiologies can block the move from A to Z but face other challenges, such as possible “sadistic” implications in certain trade‑off scenarios.

Person-Affecting and Narrow Person-Affecting Views

Person‑affecting theories hold that an outcome is better only if it is better for someone:

• On narrow versions, creating new people does not count as benefiting them, since “better for” comparisons presuppose existence.
• Thus, mere additions of new happy people are not automatically good, and there is no general pressure to regard A+ or B as better than A.

Such views can sidestep the paradox but must address how to evaluate choices that primarily affect who exists rather than how well existing individuals fare.

Intransitive, Pluralist, and Variable-Value Theories

Several more complex axiologies revise structural assumptions:

• Intransitive or incomplete orderings (e.g., Temkin’s work) allow A, A+, and B to stand in non-transitive or incomparable relations.
• Pluralist axiologies weigh multiple dimensions (total, average, distribution) without reducing them to a single metric.
• Variable value and rank-discounted utilitarianisms reduce the marginal value of additional lives as population size grows, limiting the appeal of extremely large low‑welfare populations.

These approaches aim to retain many intuitive verdicts while avoiding a straightforward path from A to Z, though often at the cost of increased theoretical complexity.

12. Implications for Utilitarianism and Person-Affecting Views

The Mere Addition Paradox has significant implications for both utilitarian theories and person‑affecting approaches in population ethics.

Utilitarianism

For total utilitarianism:

• The paradox highlights how summing wellbeing can favour large populations with low individual welfare over small, very happy populations.
• It brings into focus tensions between total and average welfare, and challenges the idea that maximizing the sum of welfare straightforwardly captures what is morally best.

For average utilitarianism:

• The paradox underscores the cost of rejecting Mere Addition: average utilitarianism allows that adding people with positive but below-average wellbeing can make an outcome worse, even though their lives are worth living.
• This leads to counterintuitive verdicts about procreation and immigration, among other things.

Utilitarians have responded by exploring hybrid, critical-level, or variable value views that attempt to reconcile core utilitarian commitments with more moderate population verdicts.

Person-Affecting Views

The paradox also pressures person‑affecting views, which often aspire to avoid counterintuitive claims about creating people:

• Narrow person‑affecting theories can deny that A+ is better than A, since the added people are not made “better off” relative to nonexistence (on some interpretations).
• This can defuse Mere Addition but may imply that there is no moral reason to create additional happy people, even when doing so would not harm anyone.

The paradox pushes these views to clarify:

• how to treat choices that affect who will exist rather than how well particular individuals fare,
• and whether moral evaluation can be limited to “benefits and harms to specific persons” while still capturing intuitions about the goodness of happy lives.

Overall, the Mere Addition Paradox exposes a deep fault line between impersonal views (like standard utilitarianism) that treat additional good lives as straightforwardly good, and person‑affecting views that resist such impersonal aggregation. It forces both camps to refine their positions or accept surprising implications about the value of creating new people.

13. Intransitivity, Pluralism, and Alternative Value Orderings

One influential line of response to the Mere Addition Paradox questions the assumption that population outcomes must be arranged in a single, transitive ordering of betterness. Instead, it explores intransitivity, pluralism, and more complex value structures.

Intransitivity and Incompleteness

Some theorists, notably Larry Temkin, argue that:

• Betterness relations between populations may be intransitive: it can be that A is better than B, B better than C, but C not worse than A (or even better than A).
• Alternatively, some pairs of outcomes may be incomparable: neither is better, worse, nor exactly equal in value.

Applied to the Mere Addition sequence:

• A may be better than A+,
• A+ better than B,
• yet B and A may be incomparable or B not better than A.

This blocks the transitive collapse from local to global comparisons that drives the paradox.

Pluralist Axiologies

Pluralist accounts hold that multiple, sometimes conflicting, values are relevant to assessing populations, including:

• total wellbeing,
• average wellbeing,
• equality,
• priority to the worse off, and possibly others.

On such views:

• A may be better than B in terms of average welfare,
• B may be better than A+ in terms of equality and benefits to the worse off,
• but there may be no single all‑things‑considered ranking that reconciles all dimensions without remainder.

Pluralism can explain why different intuitive judgments about A, A+, and B pull in different directions without insisting on a unique ordering.

Alternative Ordering Structures

Beyond intransitivity and pluralism, some proposals introduce more nuanced ordering concepts:

• Lexical or quasi‑lexical priority (e.g., giving decisive weight to avoiding very low welfare before counting additional benefits elsewhere).
• Threshold or satisficing orderings, where improvements beyond certain levels matter only weakly.
• Rank-discounted or variable value systems, where the contribution of an additional life depends on its position in an already ordered population.

These alternative structures aim to preserve many intuitive comparisons (e.g., that severe misery is especially bad; that early additions of happy lives are good) while loosening the constraints that generate the Mere Addition chain.

In all these cases, the Mere Addition Paradox functions as a test for whether a given alternative ordering can coherently accommodate our judgments about A, A+, B, and the prospect of very large low‑welfare populations.

14. Applications to Climate Ethics and Longtermism

Although the Mere Addition Paradox is a highly abstract puzzle, it has informed real-world debates in climate ethics and longtermism, where policy choices affect not only how well people live but also how many people will exist.

Climate Ethics

In climate policy, decisions influence:

• the size of future populations (through impacts on habitability, migration, and fertility),
• and their levels of wellbeing (health, economic security, environmental quality).

The paradox highlights tensions between:

• policies that maximize total wellbeing over very long time horizons (potentially favouring large future populations with modest welfare),
• and policies that prioritise high average welfare or stringent sustainability standards, possibly at the cost of lower total population.

Climate ethicists use ideas from the Mere Addition Paradox to analyse:

• whether it could be better to sustain a smaller, very well‑off population rather than a much larger, moderately well‑off one,
• and how to weigh the value of additional future lives when assessing mitigation and adaptation strategies.

Longtermism

Longtermism is the view that improving the very long‑run future is a key moral priority. Since the long-run future could contain vast numbers of people, the paradox directly bears on longtermist reasoning.

Questions include:

• If Mere Addition and totalist reasoning are accepted, does this strongly favour actions that increase the expected number of future people with lives worth living, even at some cost to current average welfare?
• Alternatively, if one rejects Mere Addition or adopts average or critical-level axiologies, does this weaken the case for prioritising population-increasing interventions?

Longtermist discussions of existential risk reduction (preventing events that would drastically curtail future potential) often implicitly assume that having many future lives worth living is extremely valuable. The Mere Addition Paradox encourages scrutiny of this assumption and invites exploration of how different population axiologies would reshape longtermist priorities.

In both climate ethics and longtermism, the paradox serves less as a direct decision rule and more as a conceptual framework highlighting that evaluations of large-scale policy choices depend critically on how we resolve deep questions about the value of additional people.

15. Legacy and Historical Significance

The Mere Addition Paradox has become a standard reference point in contemporary moral philosophy and population ethics, with a legacy extending across several domains.

Consolidation of Population Ethics

Historically, the paradox played a key role in:

• crystallising population ethics as a distinct area of research,
• motivating systematic study of axiological principles governing varying populations,
• and inspiring a wave of formal results, including impossibility theorems (e.g., by Gustaf Arrhenius) showing that no simple axiology can satisfy all plausible constraints.

Parfit’s A, A+, B, and Z diagrams are widely reprinted in textbooks and syllabi, making the paradox one of the field’s canonical teaching tools.

Influence on Subsequent Theorists

The paradox has shaped the work of numerous philosophers:

These and other contributions have turned Parfit’s original thought experiment into a broad research programme.

Methodological Significance

The Mere Addition Paradox has methodological importance as well:

• It exemplifies the use of thought experiments and formal diagrams to expose hidden tensions in moral theory.
• It encourages axiomatic analysis, where intuitive principles are explicitly articulated and then tested for mutual consistency.
• It illustrates how small, local judgments can accumulate into surprising global conclusions, a pattern mirrored in other areas of ethics and decision theory.

Ongoing Debates

Decades after its introduction, the paradox remains a live topic:

• There is no agreement on which principle(s) to surrender—Mere Addition, certain egalitarian or Pareto judgments, transitivity, or the anti‑Repugnant intuition.
• New proposals in risk ethics, global priorities research, and intergenerational justice continue to be evaluated partly by how they handle Mere Addition‑type cases.

As a result, the Mere Addition Paradox occupies a central place in the “history of ethics” in Parfit’s sense: it continues to shape how philosophers think about the moral significance of creating new people, the structure of value, and the ethical evaluation of humanity’s long‑term future.