Prioritarianism and Uncertainty: On the Interpersonal Addition Theorem and the Priority View

This paper takes its departure from the Interpersonal Addition Theorem. The theorem, by John Broome (1991), is a re-formulation of the classical result by Harsanyi (1955). It implies that, given some seemingly mild assumptions, the overall utility of an uncertain prospect can be seen as the sum of its individual utilities. In sections 1 and 2, I discuss the theorem’s connection with utilitarianism and in particular the extent to which this theorem still leaves room for the Priority View. According to the latter, the utilitarian approach needs to be modified: Benefits to the worse off should count for more, overall, than the comparable benefits to the better off (cf. Parfit 1995 [1991]).
Broome (1991) and Jensen (1996) have argued that the Priority View cannot be seen as a plausible competitor to utilitarianism: Given the addition theorem, prioritarianism should be rejected for measurement-theoretical reasons. I suggest that this difficulty is spurious: The proponents of the Priority View would be well advised, on independent grounds, to reject one of the basic assumptions on which the addition theorem is based, the so-called Principle of Personal Good for uncertain prospects.
According to the Principle of Personal Good, one prospect is better than another if it is better for everyone or at least better for some and worse for none. That the Priority View, as I read it, rejects this welfarist intuition may be surprising. Isn’t welfarism a common ground for prioritarians and utilitarians? Still, as I argue, this welfarist common ground is better captured by a restricted Principle of Personal Good that is valid for *outcomes*, but not necessarily for uncertain prospects. We obtain this surprising result if we take the priority weights imposed by prioritarians to be relevant only to *moral*, but not to *prudential*, evaluations of prospects. This makes it possible for a prospect to be morally better even though it is worse (prudentially) for everyone concerned. The proposed interpretation of the Priority View thus drives a sharp wedge between prudence and morality.