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90. The Repugnant Conclusion

When ethics leads somewhere we don't want to go

Population

10 billion

Welfare

100 (flourishing)

A+

Population

20 billion

Welfare

100 / 80

Flourishing (100)

Mediocre (50)

Barely worth living (1)

Step 1: Mere Addition

Is A+ worse than A? We've added people with good lives (welfare 80). They don't affect the original A people. Adding happy people can't make things worse... right?

Conclusion: A+ is at least as good as A.

🔴 THE REPUGNANT CONCLUSION

Through seemingly reasonable steps, we've concluded that Z is better than A.

A world of 10 billion flourishing people is worse than a world of trillions living lives barely worth living.

Each step seemed reasonable. The conclusion is repugnant. Where did we go wrong?

The Paradox

"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."

— Derek Parfit, Reasons and Persons (1984)

This is called the Mere Addition Paradox because each step involves "merely adding" people with lives worth living. The paradox arises because:

Each individual step seems intuitively correct
The final conclusion seems obviously wrong
Yet logical consistency demands we accept it—or reject one of the steps

Proposed Solutions

Reject Transitivity

Maybe "better than" isn't transitive. A+ ≥ A and B ≥ A+ doesn't mean B ≥ A. Controversial but logically coherent.

Average Utilitarianism

Maximize average welfare, not total. A+ is worse than A because average drops. But this has its own problems...

Critical Level

Only count lives above some threshold. Lives "barely worth living" don't add positive value. But where's the threshold?

Accept the Conclusion

Some philosophers bite the bullet: maybe Z really is better. Our intuitions about "repugnance" might be wrong.

In 2000, Gustaf Arrhenius proved an impossibility theorem: no theory of population ethics can simultaneously satisfy a set of plausible axioms. Something has to give.

Why It Matters

This isn't just abstract philosophy. Population ethics affects real decisions about:

Climate policy: How do we weigh future generations?
Existential risk: Is preventing extinction about saving people or total welfare?
Family planning: Is it better to have more children with fewer resources each?
Longtermism: Should we prioritize creating more future lives?

Parfit spent decades searching for "Theory X"—a satisfactory population ethics. He concluded he hadn't found it. The puzzle remains unsolved.