AbstractRational Solidarity The notion of rational solidarity may ap¬pear as a contradiction of terms at first glance to a sociologist, in that actors are either rational or solidary. This dichoto¬my, however, leads to problems, for in¬stance in explaining public allocations in the welfare state. This is because much sociological theory is based on the di¬stinction between homo economicus and homo sociologicus and therefore cannot have a concept of collective rationality. This is a self-imposed limitation created at the level of general theory in the so-called problem of order or collective action. This limitation is illustrated in an analysis of the works of Talcott Parsons, Jon Elster and Jürgen Habermas. In order to develop a concept of collec¬tive rationa¬lity, sociology must change some of its basic assumptions. Following the thoughts of Niklas Luhmann it must re¬place action and individuals with com¬munication as the basic unit of society. By doing so, it will be possible to appreci¬ate the fact that solidarity is not some¬thing typical of archaic societies or small groups, but rather a medium for the allocation of collective goods by the welfare state. The idea of rational solidarity sym¬bolizes the difference between individual and collective rationality as a unity: one should sacrifice individual opportuni¬ties to achieve collective goods and solve social problems. Taken as a medium for the allocation of collective goods, there is more to solidarity than Luhmann will admit: solidarity is the nexus of functio¬nal differentiation.