resources flow that possess the greatest positive productivity differential between the trading partners, thus ensuring larger amounts of surpluses usable for trade. In the latter case, in turn, they will tend to cluster at geographical passage points between contrasting regions involved in exchange, at the end points of natural transportation routes between such regions, or at critical nodes along such routes, such as bulk-breaking points and/or junctures where different types of transport come together (Bairoch 1990, 148, n. 26).5

Growth Institutionalized

The foregoing discussions explain the economic processes that create economic activity, concentrate it in particular locations, and propel population agglomeration at those locations beyond an urban threshold in the first place. But what are the mechanisms that allow urban systems to continue to expand once created, and, in so doing, to structure regional development patterns that naturally tend to become increasingly asymmetrical with time? The key concept here is “circular and cumulative causation.” Situated at the very core of the new economic geography, this idea was first articulated as a coherent concept by the economist and social theorist Gunnar Myrdal and was later elaborated and formalized for economics by Allen Pred (1966).6 The concept came to the attention of researchers interested in the rise of early civilizations through the early work of Jane Jacobs (1969). In its initial formulation during the 1930s and 1940s, and at its most complex, cumulative causation theory involved the recognition that economic, social, and cultural factors often reinforce each other, as Myrdal (1944) argued so eloquently in his pathbreaking analysis of modern race relations in the United States. During the 1950s, Myrdal narrowed the focus of the concept to more specifically address the dual questions of specific concern here, urban process and regional development. In this context, cumulative causation implies that population growth, production, innovation, and urbanization are invariably interlocked in a circular process whereby change in one realm does “not call forth countervailing changes [in other realms] but, instead, supporting changes, which move the system in the same direction as the first change, but much further” (Myrdal 1957, 13). From the point of view of urban growth and the development of regional disparities, Myrdal (1970, 279–80) argued that the most important “supportive changes” were the creation of economies of scale in production and increases in the rate of

innovation as a result of the expansion of knowledge. Both of these factors will figure prominently in later discussions of the Sumerian takeoff (chap. 8).

For the moment, suffice it to say that if we focus our attention on the economic side of the cumulative causation equation, the most important mechanism promoting differential regional growth is what economists often term “import substitution” (Jacobs 1969, 2000; Krugman 1995, 49). After a regional economy grows beyond a critical point by means of the iterative processes described by Jacobs and Krugman, it becomes profitable to replace imports of some commodities subject to scale economies with local production. This substitution creates a second burst of economic expansion that allows already growing centers to expand further by generating even greater diversity and employment, and thereby to draw in even more workers from the immediately surrounding countryside and possibly even from neighboring regions. The reasons for this have to do with the inevitable multiplier effects of increases in productive capacity. One is the creation of linked industries adding further value to semifinished goods produced by the initial industries (forward linkages). Another is the creation of new work in sectors providing services (backward linkages) to those industries (Pred 1966, 25–26). A final multiplier is provided by collateral employment in the managerial classes required to organize the larger number of workers created by these linkages, store and distribute their enhanced production, and keep records of these various activities. In turn, as managerial classes expand, new demands for internal consumption are created, and old demands for already existing products are intensified, adding further boosts to the economy of growing cities and regions.

In due course, the operation over time of these interrelated multipliers creates the enlarged population/market size necessary to induce even further rounds of import substitution processes. As this process is repeated on an ever larger scale, a circular (or, more precisely, spiral) relationship is created between population growth, market size, innovation, the range of productive activities that a region possesses, and the efficiency level of those activities: innovation is most likely and production is highest and most efficient where population and markets are larger, but markets are bigger where production and innovation are greater, so that city-led regional growth (or decline) always takes the form of a selfreinforcing snowball or cascade effect (Krugman 1995, 49).

The historical implications of processes of circular and cumulative causation are clear. Two such implications appear particularly salient for our understanding of early urban processes in ancient Mesopotamia. First, the impact of a new technology or innovation on a given society will always depend on when exactly that innovation is introduced in the cycle of mutual determinations that always exists between trade, production, population growth, commodity demand, market size, and increasing returns resulting from new economies of scale. Accordingly, depending on timing, the adoption of a new technology may lead to dramatically different parameters of an economy, and it follows from this that similar adoptions will differentially affect societies with varying developmental trajectories or with trajectories that are similar but of varying time depth. The importance of timing in determining how a society reacts to innovation is described by Krugman (1991, 487), who notes that “the details of the [human] geography that emerges—which regions end up with the population—depend sensitively on initial conditions. If one region has slightly more population than another when, say, transportation costs fall below some critical level, that region ends up gaining population at the other’s expense; had the distribution of population at that critical moment been only slightly different, the roles of the regions might have been reversed.”

Second, while an initial pattern of specialization in production may well arise from a variety of causes, once a pattern is established, that pattern is likely to become “locked in” by the cumulative gains from trade. Regional population growth rates and urban development are therefore highly historical (i.e., “path-dependent”) processes, so that if a region gains an initial advantage, those processes will concentrate new growth and its multiplier effects in the already expanding region rather than elsewhere (Malecki 1997, 49–50). Processes of cumulative causation thus ultimately account for the division of the world into core and peripheral areas. We will now turn to the issue of how this self-amplifying chain of events may have gotten its start in early Mesopotamia.