Is a huge world of barely-happy people better than a small, joyful one?

Parfit argued that many popular ideas about value seem to say yes, and he found that conclusion repugnant. The mere addition paradox, also called the repugnant conclusion, is a problem in population ethics identified by Derek Parfit in Reasons and Persons (1984). His formulation: for any 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, other things equal, would be better, even though its members have lives that are barely worth living.

The route there is a series of small steps, each plausible. A+ is A with extra people added, whose lives are good enough that they would rather exist, and it seems no worse than A, since the people in A are no worse off. B− seems better than A+, since it has greater total and average happiness. B merely merges the groups of B− into one, so it seems as good as B−. Together, these imply that B is better than A, and yet A, with its high average happiness, can seem better than B. Repeat the "mere addition" over and over, Parfit argues, and you end at a vastly larger population Z with only slightly positive welfare.

Where to resist is the debate. Switch from total to average utilitarianism and A+ becomes worse than A, but Parfit thought that absurd, since adding people with lives worth living would make the world worse simply because they lower the average. Larry Temkin and Stuart Rachels suggest "better than" may not be transitive. Others bite the bullet: Torbjörn Tännsjö says the intuition that B is worse than A is wrong, and Michael Huemer argues the conclusion is not repugnant. A more modest line, taken by philosophers including Hilary Greaves and Toby Ord, is that avoiding it is not a necessary criterion for a good theory of population ethics.