I have a preliminary theory of population ethics that seems to work reasonably well: total utilitarianism, with the constraint that the addition of a new person must improve the total utility of the world by more than the utility of the additional person.
For example, suppose you have a trillion rat brains on heroin that are experiencing an extremely positive level of utility. You could create a trillion-first brain, which would experience slightly less utility and also reduce the utility of the other brains, because there'd be less heroin to go around - but the new brain's utility would still be high enough to make up for the decrease, so total utility would be greater if you created it. But total utility would increase by less than the utility of the new brain, so by this theory, you shouldn't create it.
So this theory avoids the Repugnant Conclusion without committing to a view with other problems, like average utilitarianism.
