Our measures of urban bias (ratio of domestic to world price of cereals; black market premium) or of shocks to agriculture are never significant determinants of either the level or the pace of urbanization. At any rate, they do not help explain why Africa is different.

One can question whether the particularly high rural-urban wage differentials in Africa in the post-colonial period is a symptom of urban bias. It is possible, but by no means certain. They may have reflected differences in productivity - the fact that they gradually decreased over time at about the same rhythm as in East and South Asia supports this hypothesis. Anyway, they are surely best understood as a symptom of elite - rather than urban-- bias.

So does Africa's urbanization process remain a puzzle? Africa, at the end of the colonial period, was underurbanized given its income and income structure due to the policies of the colonial powers. The 1960-80 period was characterized by very rapid urbanization, even more rapid than can be explained by a catch-up hypothesis, traditional urban-bias measures, agricultural shocks, or civil disturbances. It is largely explained, however, by rural-urban wage differentials. The slowdown in the pace of urbanization after 1980 is significantly greater than can be explained by our explanatory variables. However, given that Africa had urbanized "exceedingly rapidly" in the 1960-80 period, the slowdown that followed is not unexpected. In fact, if we let initial urbanization level take 1975 values for the 1980-90 observations (instead of 1960), the post-80 Africa dummy is no longer significant.

Alternative explanations could, of course, be poor data. One argument is that urbanization data in Africa are simple projections based on old census data and therefore the results presented here have limited meaning and interest. But if this were the case, we should find that Africa in the post-80 period continued to urbanize exceedingly rapidly. The bias would be in the other direction. Another point frequently made is that we may severely underestimate income and income growth in Africa, as much of the economy has gone underground to escape predatory governments. This point, while surely accurate,<sup>18</sup> is moot because income growth explains so little of the pace of urbanization.

A more interesting criticism is that the distinctions between urban/rural and formal/informal may be misplaced in developing countries, especially in Africa. Many workers straddle these divisions, whether by seasonal or circular migration between town and country, or moonlighting in the informal sector while holding a formal sector job during the day (Jamal and Weeks, 1998). Even the economic activities we use to distinguish between rural and urban sectors may not be appropriate. The growth of urban agriculture in response to rising food prices and shortages and general urban poverty is a good example of how strict rural vs. urban dichotomies may not be applicable to the modern developing world (Tacoli, 1998). This, however, is far from being a uniquely African phenomenon. In Nicaragua, 40% of the urban poor are employed in

> 18 The inclusion in Zaire's national accounts of its "non-registered economy" raises "effective" GDP to three times the current official measures of GDP (Cour, 1991.)

26