>> So the most ethically optimal population is one in which adding new members will cause life to no longer be worth living to at least the same number of other

>> So the most ethically optimal population is one in which adding new members will cause life to no longer be worth living to at least the same number of other members. I can't see how that follows. The paradox is comparing possible worlds which have differing average happiness and total populations. It doesn't follow that there exists some way to transform each world so that it matches the other. You aren't killing off the excess population in B to somehow reach A or vice-versa adding population. The paradox exists because you are comparing the happiness of a person which doesn't exist in one possible world with his (realized) happiness in another. There could be any actual relationship between population size and happiness but the paradox would still exist because you wouldn't be able to order the desirability of some possible worlds.