Pigou-Dalton and Separability: Is Equity-Regarding Moral Betterness Separable Across Persons?

The Separability axiom (shorthand for person-separability) arises frequently in the literature on social choice and formal ethics. It is a fixed-population axiom (an axiom that constrains the ranking of a pair of worlds in which the very same individuals exist), which says: If some individuals are equally well off in the two worlds, the moral ranking of the two is independent of those individuals’ well-being levels. Variable-Population Separability is a logically stronger version of Separability that applies to any pair of worlds. It says: if some individuals are equally well off in the two worlds, the moral ranking of the two is independent of those individuals’ well-being levels and their existence. The question of Separability has often been tied up with utilitarianism. A utilitarian world-ranking satisfies Separability; thus, an argument for utilitarianism is, automatically, an argument for Separability. This paper engages Separability from a different angle. It starts from the Pigou-Dalton axiom: the standard axiom expressing a concern for the <i>equitable</i> distribution of well-being. The Pigou-Dalton axiom states: a pure, gap-diminishing transfer of well-being from someone better off, to someone worse off, leaving everyone else unaffected, is a moral improvement. Utilitarianism violates Pigou-Dalton.<br><br>Notably, Pigou-Dalton itself is neutral on the issue of Separability. Some prominent Pigou-Dalton-respecting world-rankings satisfy Separability (in particular, prioritarianism, leximin, and Pigou-Dalton-respecting sufficientarianism), while others do not (in particular, generalized Gini-world/rank-weighted world-rankings). The paper reviews and critically evaluates the main arguments for and against Separability, given the non-utilitarian concern for equity embodied in the Pigou-Dalton axiom. This is an important topic for social choice theorists and ethicists who endorse that axiom—and a topic that has not, to date, been systematically addressed. The paper reaches a tentative conclusion in favor of Separability, but defending that conclusion is not its main ambition. Rather, it chiefly aims to foster robust debate about whether Separability aligns with non-utilitarian, Pigou-Dalton-respecting, moral value.