In 1984, the philosopher Derek Parfit made a disturbing observation. Given a choice between a moderate population of happy people, and an extremely large population of rather miserable people, classical utilitarianism recommends the second option. The same paradox arises for many other methods of social welfare assessment. Parfit called this the Repugnant Conclusion. Various attempts to resolve this paradox have led to the development of a branch of welfare economics and moral philosophy called population ethics. The problem is find the right way to make trade-offs between the size of a population, and the welfare of the people in that population.
The class of rank-additive axiologies is a flexible framework for population ethics. It includes the rank-weighted utilitarian, generalized utilitarian, and rank-discounted generalized utilitarian rules. In this presentation, I will axiomatically characterize rank-additive axiologies and studies their properties in two frameworks: the actualist framework (which only tracks the utilities of people who actually exist), and the possibilist framework (which also assigns zero utilities to people who don't exist). The axiomatizations and properties are quite different in the two frameworks. For example, actualist rank-additive axiologies can simultaneously evade the Repugnant Conclusion and promote equality, whereas in the possibilist framework, there is a tradeoff between these two desiderata. On the other hand, possibilist rank-additive axiologies satisfy the Positive Expansion and Negative Expansion axioms, whereas the actualist ones do not.