Economists have had problems interpreting the gravity model, particularly as applied to migration behaviour. Its physical analogy is unappealing and theories of economic behaviour do not generally give rise to gravity laws. Some years ago Niedercorn and Bechdolt (1969) published a paper in which they provide a derivation of the gravity model using utility theory. Their approach is, however, much more applicable to other flows of human interaction across space, such as travel or communication flows, than it is to migration flows. The major purpose of this paper is to indicate a set of assumptions which might underlie a gravity model of migration and to show that these assumptions are quite severe. One important implication is that we should fully expect any real world migration pattern to deviate from the predictions of a gravity model. Nevertheless, the gravity specification may be a useful starting point in the analysis of migration behaviour, as it imposes restrictions on the formulation of the estimating equations. Thus we may fruitfully study how and why the real world migration patterns diverge from gravity flows.
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