THE degree of social mobility in America has long been a point of dispute not only among social scientists but also among political advocates. The once popular acceptance of America as the golden land of opportunity, for example, rested on the belief that it offered unparalleled chances for those in the lower ranks of society to improve their position.  This theory of social mobility was also adopted, though for different reasons, by socialists attempting to explain the relatively low level of class consciousness among workers in America. They argued that because American workers could realistically hope to improve their circumstances and move out of their class, they were less likely than their more deprived European counterparts to support revolutionary movements. Socialists who made these assumptions, from Marx on, anticipated the emergence of radicalism among the working classes at a later period, when changes in the economic system would have sharply reduced upward mobility. And the notion is by now widespread that the chances for upward mobility would diminish-and have in fact diminished—as a result of the ongoing processes of urbanization, industrialization, and bureaucratization.