Migration and Natural Increase in the Growth of Cities

Migration and Natural Increase in the Growth of CitiesProblems too difficult to solve analytically may often be dealt with by simulation.But simulation will not always provide a solution; a problem too difficult for a closed form solution may also suffer from some inherent ambiguity.In such a case, simulation serves to reveal the ambiguity and to show the numerical consequences of different interpretations; one may come to understand that the difficulty of a problem resides less in the mathematics than in the verbal statement.Our apparently straightforward question is whether migration or natural increase contributes more to the growth of cities.That any observer can distinguish the entry of a migrant from the birth of a baby is not enough to provide the unambiguous causal statement that we seek.To analyze one year's growth, we need only compare the count of net in-migrants with the count of births less deaths.But if the migrants are of an age to reproduce immediately, while those just born will not reproduce for at least 15 years, then that additional fact works for migration.And moreover the relative importance of the two may be different at different time intervals.The rapid expansion of cities in the lessdeveloped countries has been a focus of attention of policymakers and of several academic disciplines over recent decades.With Mexico City having passed 12 million, and headed towards 30 million by the end of the century, and Cairo at 9 million, administrators seek policies that will adapt the pace of growth to urban resources and jobs.The extent to which the growth is due primarily to the natural increase of the cities as against migration from the countryside arises naturally when policy regarding urbanization comes under discussion.Europe's cities of the Renaissance and long after had death rates so high that the only way they could increase was by migration.But they provide no model for present day cities, which urgently pose the question of how far control of the size of cities can be through migration and how far through natural increase. THE IMMEDIATE ANSWEROne way of answering the question is by simply noting the present rates.Andrei Rogers starts with the fundamental identity r = b -d + i -0, where increase T is equal to births b minus deaths d plus in-migrants i minus out-migrants o.This is true for either the absolute numbers or for rates based on the current urban population; it applies to a single city or to any defined group of cities.The contribution of natural increase to the population after one year is bd , and the *We are grateful to colleagues at IIASA and elsewhere for su estions on this article.In particular we found those of Andrei Rogers and his associates especially h e l p 8 The extensive computing was funded by IIASA.