When there is general agreement, the choices made by society are uncontroversial. When opinions differ, the problem is to find methods for bringing together different opinions in decisions which concern everyone. The theory of social choice is preoccupied precisely with this link between individual values and collective choice. Fundamental questions are whether - and, if so, in what way - preferences for society as a whole can be consistently derived from the preferences of its members. The answers are crucial for the feasibility of ranking, or otherwise evaluating, different social states and thereby constructing meaningful measures of social welfare.
Traditionally, the theory of social choice had only assumed that every individual can rank different alternatives, without assuming anything about interpersonal comparability. This assumption certainly avoided the difficult question of whether the utility individuals attach to different alternatives can really be compared. Unfortunately, it also precluded saying anything worthwhile about inequality. Nobel Prize winner Amartya Sen initiated an entirely new field in the theory of social choice, by showing how different assumptions regarding interpersonal comparability affect the possibility of finding a consistent, non-dictatorial rule for collective decisions. He also demonstrated the implicit assumptions made when applying principles proposed by moral philosophy to evaluate different alternatives for society. The utilitarian principle, for instance, appeals to the sum of all individuals' utility when evaluating a specific social state; this assumes that differences in the utility of alternative social states can be compared across individuals. The principle formulated by the American philosopher John Rawls - that the social state should be evaluated only with reference to the individual who is worst off - assumes that the utility level of each individual can be compared to the utility of every other individual. Later developments in social choice rely, to a large extent, on Sen's analysis of the information about, and interpersonal comparability of, individual utilities.