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  The spread of a phenomenon over space and through time. There is a long and distinguished tradition of diffusion studies in cultural geography. According to Sauer (1941), it was Ratzel who \'founded the study of the diffusion of cultural traits, presented in the nearly forgotten second volume of his Anthropogeographie\' (see anthropogeography). In Sauer\'s view, diffusion — \'the filling of the spaces of the earth\' — was a \'general problem of social science\': \'A new crop, craft or technique is introduced to a culture area. Does it spread, or diffuse vigorously or does its acceptance meet resistance?\' The specific contribution of geography, so Sauer argued, was to reconstruct diffusion pathways and to evaluate the influence of (physical) barriers (see Sauer, 1952; more generally, Wagner and Mikesell, 1962). Both of these tasks were pursued energetically by various members of the Berkeley School, but they reappeared in a starkly different guise in Hägerstrand\'s much more formal study of innovation diffusion. In fact, Hägerstrand\'s original Swedish monograph was introduced to Anglo-American geography in a short note by Leighly (1954), himself an associate of Sauer. \'No one who essays in the future to interpret the distribution of culture elements in the process of diffusion can afford to ignore Hägerstrand\'s methods and conclusions\', Leighly declared, and he drew particular attention to Hägerstrand\'s emphasis on the importance of \'chance\' (see stochastic process). Even so, it was some 14 years before an English translation of Hägerstrand\'s Innovation diffusion as a spatial process appeared (1968), although in the interim his basic ideas had already become better known (see Duncan, 1974). The theoretical structure behind Hägerstrand\'s model is summarized in the figure. An interaction matrix suggests the contours of a generalized or mean information field, which structures the way in which information circulates through a regional system; these flows are modulated by both physical barriers and individual resistances which together check the transformation of information into innovation and so shape successive diffusion waves which break onto the adoption surface (Gregory, 1985; see also Haggett et al., 1977). Most discussion has been about the operationalization of the model — about Hägerstrand\'s use of Monte Carlo simulation methods, the comparison of \'observed\' and \'predicted\' patterns of adoption, and the detection of a neighbourhood effect — rather than about its theoretical basis (but see below). Within the modelling tradition initiated by Hägerstrand, the most important developments have included:

{img src=show_image.php?name=2022.gif } a formalization of the mathematical relations between the structure of the mean information field and the form and velocity of diffusion waves, indicating the connections between different distance decay curves and the classic neighborhood effect (although it is, of course, scarcely surprising that a distance-bound mean information field should generate a broadly contagious pattern of adoptions); {img src=show_image.php?name=2022.gif } a demonstration that the Hägerstrand model is only a special instance of the simple epidemic model, and the subsequent derivation of more complex epidemic models — particularly through the remarkable contributions of Haggett, Cliff and Ord (see Haggett et al., 1977, ch. 7), whose replication of a range of supposedly \'spatial processes\' (see process) has confirmed; {img src=show_image.php?name=2022.gif } the recognition of hierarchical diffusion, typically through central place systems (see central place theory) and frequently operating alongside the distance-bound, contagious processes of the classical model (see Hudson, 1969; Pedersen, 1970); {img src=show_image.php?name=2022.gif } the incorporation of rejection and removal processes and the modelling of competitive diffusions (see Webber, 1972).These changes have entailed a move away from simulation techniques towards the use of more analytical methods, which Agnew (1979) sees as a shift from empiricism and instrumentalism towards realism. But to some critics outside the abstract domains of location theory or medical geography (where these advances have been immensely important: see Cliff et al., 1981) the move towards a post-positivist formulation remains strategically incomplete. Blaikie (1978) even spoke of a \'crisis\' in diffusion research, which he attributed to a reluctance to reverse out from a \'spacious cul-de-sac\' — by which he meant a preoccupation with spatial form and space-time sequence — and Gregory (1985) attributed the \'stasis\' of diffusion theory to an unwillingness to engage with social theory and social history in ways which clarified both the conditions and the consequences of diffusion processes. In the most general terms, this critique notes that the spatial circulation of information is the strategic element in most applications of the Hägerstrand model and its derivatives to human geography. Although the flow of information through different \'propagation structures\' and contact networks has been exposed in more detail (e.g. Blaikie, 1973, 1975; Brown, 1975), most critics claim that the primacy accorded to the pattern of these pathways obscures two other, more fundamental, elements (cf. the critique of behavioural geography).

The Hägerstrand model \'is concerned with the locational attributes and communication habits of potential adopters\' and \'does not explicate the process by which potential adopters are identified\' (Yapa and Mayfield, 1978). The alternative is a model of biased innovation which takes as primary the ways in which \'social access to the means of production\' can be closed off through (for example) the class structure of the mode of production. In this perspective, \'non-diffusion is not to be equated with the passive state of lack of adoption due to low levels of awareness [or] apathy. … It is an active state arising out of the structural arrangements of the economic [base of] society\' (Yapa, 1977; see also Blaikie, 1978 and Gregory, 1985). Claims of this sort carry with them a demand for diffusion theory to be located within a more general political economy, although it is clearly not necessary to endorse the economic reductionism of classical Marxism implicit in Yapa\'s commentary. At the very least, the significance of ethnicity and gender needs to be admitted alongside that of class (see Blaikie, 1975).

The Hägerstrand model postulates \'a uniform cognitive region\' and does not explicate the selective social processes through which information flows are differentially constituted as socially meaningful. Hence Blaut (1977) has called for the forging of closer links with the older tradition of cultural diffusion, which is capable of treating diffusion in what Blaut takes to be \'its broadest, most adequate sense\' by locating diffusion theory within a more general cultural geography. Such a manoeuvre is not the return to Sauer which it seems to be, however, for it is clear from Blaut\'s other writings that here too the model is to be Marx (see Blaut, 1980). Whatever one thinks of this — and there is certainly a vibrant cultural perspective within historical materialism — no model of biased innovation can equate \'resistance\' with ignorance or insufficient information. Access to information, like any other means of production, is indeed socially structured, but resistance equally (and, one might say, usually) connotes a process of sustained struggle: considered and collective action on the part of people whose evaluation of the available information may be strikingly different to that of the \'potential adopters\' (see Gregory, 1985).

Hägerstrand has himself conceded the force of some of these objections; rather than explore the claims of contemporary Marxism, however, he has returned to some of those themes which exercised him before he constructed his diffusion model, and he has developed them beyond the confines of its spatial formalism into time-geography.

Many of the continuing tensions within diffusion studies are exemplified by the recent interest in the diffusion of AIDS. Mapping and modelling the spread of the disease has been the major focus of geographical inquiry (see, e.g., Shannon and Pyle, 1989; Casetti and Fan, 1991; Shannon, Pyle and Bashshur, 1991; Smallman-Raynor, Cliff and Haggett, 1992). Where this work has referred to the Hägerstrand model at all, it has usually been to its capacity to describe patterns of spread: their spatial analysis has involved epidemiological modeling. An interest in the cultural, social and political geography of AIDS diffusion has been much slower to develop — and even then it has often been subordinated to the objectivist logics of spatial science (see, e.g., Wallace and Fullilove, 1991) — but there have been many relevant studies outside the discipline (Shilts, 1987; Crimp, 1988; Patton, 1990), and, laterally, studies within the discipline that have been based on a vigorous critique of spatial science (Brown, 1995). (DG)

{img src=show_image.php?name=bkhumgeofig20.gif }

diffusion The structure of Hägerstrand\'s diffusion model

References Agnew, J.A. 1979: Instrumentalism, realism and research on diffusion of innovation. Professional Geographer 31: 364-70. Blaikie, P. 1973: The spatial structure of information networks and innovative behavior in the Ziz valley, S. Morocco. Geografisker Annaler B55: 83-105. Blaikie, P. 1975: Family planning in India: a sociogeographical approach. London: Edward Arnold; New York: Holmes and Meier. Blaikie, P. 1978: The theory of the spatial diffusion of innovations: a spacious cul-de-sac. Progress in Human Geography 2: 268-95. Blaut, J. 1977: Two views of diffusion. Annals of the Association of American Geographers 67: 343-9; Blaut, J. 1980: A radical critique of cultural geography. Antipode 12: 25-9. Brown, L.A. 1975: The market and infrastructure context of adoption: a spatial perspective on the diffusion of innovation. Economic Geography 51: 185-216. Brown, M. 1995: Ironies of distance: an ongoing critique of the geographies of AIDS. Environment and Planning D: Society and Space 13: 159-83. Casetti, E. and Fan, C. 1991: The spatial spread of the AIDS epidemic in Ohio: empirical analyses using the expansion method. Environment and Planning A 23: 1589-608. Cliff, A.D., Haggett, P., Ord, J.K. and Versey, G.R. 1981: Spatial diffusion: an historical geography of epidemics in an island community. Cambridge: Cambridge University Press. Crimp, D., ed., 1988: AIDS, cultural analysis, cultural criticism. Cambridge, MA: MIT Press. Duncan, S.S. 1974: The isolation of scientific discovery: indifference and resistance to a new idea. Science Studies 4: 109-34. Gregory, D. 1985: Suspended animation: the stasis of diffusion theory. In Gregory, D. and Urry, J., eds, Social relations and spatial structures. London: Macmillan, 296-336. Hägerstrand, T. 1968: Innovation diffusion as a spatial process, trans. A. Pred. Chicago: University of Chicago Press. Haggett, P., Cliff, A.D. and Frey, A.E. 1977: Locational analysis in human geography, 2nd edn. London: Edward Arnold; New York: John Wiley. Hudson, J.C. 1969: Diffusion in a central place system. Geographical Analysis 1: 45-58. Leighly, J. 1954: Innovation and area. Geographical Review 44: 439-41. Patton, C. 1990: Inventing AIDS. London: Routledge. Pedersen, P.O. 1970: Innovation diffusion within and between national urban systems. Geographical Analysis 2: 203-54. Sauer, C. 1941: Foreword to historical geography. Annals of the Association of American Geographers 31: 1-24. Sauer, C. 1952: Agricultural origins and dispersals. New York: American Geographical Society. Shannon, G.W. and Pyle, G.F. 1989: The origin and diffusion of AIDS: a view from medical geography. Annals of the Association of American Geographers 79: 1-24. Shannon, G.W., Pyle, G.F. and Bashshur, R.L. 1991: The geography of AIDS: origins and course of an epidemic. New York: Guilford Press. Shilts, R. 1987: And the band played on: politics, people and the AIDS epidemic. New York: Penguin. Smallman-Raynor, M., Cliff, A.D., and Haggett, P. 1992: London international atlas of AIDS. Oxford: Blackwell. Wagner, P.L. and Mikesell, M.W., eds, 1962: Readings in cultural geography. Chicago: University of Chicago Press. Wallace, R. and Fullilove, M. 1991: AIDS deaths in the Bronx 1983-1988: spatiotemporal analysis from a sociogeographic perspective. Environment and Planning A 23: 1701-23. Webber, M.J. 1972: The impact of uncertainty on location. Cambridge, MA: MIT Press. Yapa, L.S. 1977: The green revolution: a diffusion model. Annals of the Association of American Geographers 67: 350-9. Yapa, L.S. and Mayfield, R.C. 1978: Non-adoption of innovations. Economic Geography 54: 145-56.

Suggested Reading Blaikie (1978); Cliff et al. (1981). Gregory (1985); Wagner and Mikesell (1962), part 3.



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