Nondictatorial social welfare functions with different discrimination structures

In societies where agents distinguish imperfectly among the alternatives, a necessary and sufficient condition for the existence of an Arrovian Social Welfare Function is the presence of an agent who only distinguishes two groups of alternatives or of two agents with complementary discrimination patterns. This result is based on the observation that differences in discrimination structures may lead to the absence of free triples, thus providing a way to escape Arrow's impossibility result.