We provide foundations for robust normative evaluation of distributions of two attributes, one of which is cardinally measurable and transferable between individuals and the other is ordinal and non-transferable. The result that we establish takes the form of an analogue to the standard Hardy-Littlewood-Pólya theorem for distributions of one cardinal attribute. More specifically, we identify the transformations of the distributions which guarantee that social welfare increases according to utilitarian unanimity provided that the utility function is concave in the cardinal attribute and that its marginal utility with respect to the same attribute is non-increasing in the ordinal attribute. We establish that this unanimity ranking of the distributions is equivalent to the ordered poverty gap quasi-ordering introduced by Bourguignon [12]. Finally, we show that, if one distribution dominates another according to the ordered poverty gap criterion, then the former can be derived from the latter by means of an appropriate and finite sequence of such transformations.
Introductory remarksThe normative foundations of the comparison of distributions of a single attribute between a given number of individuals are by now well-established. They originate in the equivalence between three statements that are considered relevant answers to the question of when a distribution x can be considered normatively better than a distribution y. Given two distributions x and y with equal means, these statements, the equivalence of which was first established by Hardy et al. [30] and popularised later on among economists by Kolm [32], Atkinson [3], Dasgupta et al. [15], Sen [47], Fields and Fei [21] among others, are the following:(a) Distribution x can be obtained from distribution y by means of a finite sequence of progressive -or equivalently Pigou-Dalton -transfers. (b) All utilitarian ethical observers who assume that individuals convert the attribute into wellbeing by means of the same non-decreasing and concave utility function rank distribution x above distribution y. (c) The Lorenz curve of distribution x lies nowhere above and somewhere below that of y, or equivalently, for all poverty lines, the poverty gap is no greater in distribution x than in distribution y and it is smaller for at least one poverty line.This remarkable result, which can be generalised in a number of ways, points to three different aspects of the inequality measurement process. 1 The first statement aims at capturing the very notion of inequality reduction by associating it with elementary transformations of the distributions. The second statement is fundamentally normative and it assumes that society has an aversion to inequality which, in the utilitarian framework, is reflected by the concavity of the utility function. To some extent the first statement helps in clarifying the meaning of the restriction imposed on the utility function in the second statement. While these two conditions shed light on two different facets of the inequality concept, they do ...