The Repugnant Conclusion
Is a world of 10 billion people with excellent lives worse than a world of 100 trillion people with lives barely worth living, if the total happiness in the second world is greater?
Philosopher Derek Parfit named this the 'Repugnant Conclusion' in 1984: that if we're trying to maximize total welfare, we seem forced to prefer a vast population of barely-satisfied people over a smaller population of thriving ones. Parfit spent much of his career finding this conclusion unacceptable and searching for an alternative. He never fully resolved it.
Parfit, D. (1984). Reasons and Persons. Oxford University Press.
The math that feels wrong
Take 10 billion people with flourishing lives, call it World A. Now imagine World Z: 100 trillion people, each with a life that's just barely worth living. Enough food, minimal shelter, the occasional moment of joy, but never really thriving.
If you add up the total welfare in World Z, it's greater than World A, because there are so many more people. Pure total utilitarianism says World Z is better.
Parfit called this conclusion repugnant. It seems obviously wrong that a world of barely-living trillions is better than a world of truly flourishing billions.
Why it's hard to escape
The problem isn't that total utilitarianism seems repugnant in isolated cases. It's that every alternative leads to problems too:
Average utilitarianism (maximize average welfare, not total) says you shouldn't add happy people to a world if they'd be below average, which also seems wrong. Critical level utilitarianism only counts welfare above some threshold, but where's the threshold, and why? Person-affecting views only count welfare of people who will actually exist, but struggle with obligations to future generations.
Every escape route from the Repugnant Conclusion leads into its own thicket.
What Parfit concluded
Parfit believed the problem was real and unsolved. He thought our intuitions about population ethics were genuinely inconsistent: we can't construct a theory that respects them all. He was honest about this rather than pretending he'd found a way out.
Some philosophers have since argued that the Repugnant Conclusion isn't actually repugnant if you imagine the lives in World Z vividly enough. They're still human lives, after all.