There is a tradition in the social sciences which views the problem of social choice as how to construct a morally valid procedure for translating information about individual preferences among alternative distributive states into a social preference among those states. Arrow's impossibility theorem constructed a formidable barrier to the solution of this problem, as he showed that there could be no social choice procedure which satisfied a set of seemingly weak ethical postulates. One of these postulates, the so-called independence condition, ruled out from consideration any information that could be obtained about individual preferences through interpersonal comparisons of welfare. In later work, Arrow has suggested that this requirement might be too restrictive and that a weak form of interpersonal comparison should be allowed. This type of comparison, the 'extended sympathy' comparison, involves only ordinally significant operations, and in this paper I consider the consequences for social choice of allowing this kind of comparison. Following a natural modification and strengthening of Arrow's conditions, I prove that one and only one type of procedure can satisfy the revised set of conditions -a procedure which may be considered a generalized form of Rawls' difference principle.
I. INTRODUCTION
Since the publication of Arrow's Impossibility Theorem, 1 there has been much discussion of how Arrow's paradox for social decision-making may be avoided. Arrow showed that there could be no procedure for translating individual preferences into a social preference which obeyed a set of seemingly weak and reasonable ethical postulates. Among these is the condition of the independence of irrelevant alternatives, which required that the social choice among alternatives was to depend only on the orderings of individuals with respect to these alternatives. This condition ruled out the relevance of any information that could be acquired about individual preferences through interpersonal comparisons of welfare, and, in particular, information about something which has customarily been called 'preference intensity'. In a later work, Arrow has suggested that this independence condition might be too strong and that a weaker notion of interpersonal comparison should be Theory and Decision 10 (1979) 311-328.