A variant of urban ecology associated with the work of Eshref Shevky and Wendell Bell and their associates (see especially E. Shevky and M. Williams, The Social Areas of Los Angeles, 1949, and E. Shevky and W. Bell, Social Area Analysis, 1955). The original formulation offered an almost wholly descriptive account of residential differentiation in urban Los Angeles, distinguishing its social areas in terms of three (by no means clearly constructed) indexes of social rank, degree of urbanization, and segregation. The subsequent theoretical rationale, which emerged during the later study of San Francisco, hinges on the key concept of societal ‘scale’; that is, ‘the number of people in relation and the intensity of these relations’. The results of increasing scale are then identified with Louis Wirth's propositions about urbanism as a way of life. In the Shevky-Bell model, increasing societal scale is synonymous with the emergence of urban industrialized society, the prime mover for which is changes in the economy (caused, in turn, by technological innovation). In a later revision of the model, Dennis C. McElrath (‘Societal Scale and Social Differentiation’, in S. Greer (ed.), The New Urbanization, 1968) departs from this economic determinism, attributing changes in scale to changes in industrial organization and the distribution of skills, on the one hand, and the aggregation of population and redistribution of resources in favour of cities, on the other. A large number of studies employing the social area model—most of which take the validity of the basic scheme for granted—were conducted (mainly in the United States) during the 1950s and 1960s. Readers will have noticed—as did critics at the time—that, despite the aura of sophisticated quantitative analysis pervading this literature, many of the central concepts and causal relationships are ill-defined. The discussion of social trends accompanying urbanization fails to explain why, or how, social differentiation actually occurs. Ultimately, the model fails to relate the effects of modernization to the axes of residential differentiation, and the latter have to be hitched to some other theory if they are to advance beyond the level of mere description. The best general account of the theory, which is now largely of historical interest only, is Duncan Timms 's The Urban Mosaic (1971).