Introduction

Urbanization is closely linked with the process of ST and industrialization. ST refers to the reallocation of economic activities across the three broad sectors (agriculture, manufacturing and services) that are associated with productivity increase and economic growth. As countries urbanize, their economies modernize toward highly productive sectors such as industries and manufacturing. The analysis of ST and productivity changes are key to understanding the mechanisms behind urbanization. Reversely, analyzing urbanization is essential to understand the causes of SC.

In traditional growth patterns, ST and urbanization are deeply interrelated. Both phenomena are characterized by the same movement of labor from the rural and agricultural sectors to the manufacturing and urban sectors. This process normally translates into higher productivity levels and overall economic growth. However, the observed lessons from developed countries does not seem to apply to all developing countries, where urbanization is often not accompanied by industrialization. The example of African countries is particularly striking. Africa's transformation trajectory is very dissimilar to the conventional pattern of growth whereby sustainable urbanization proceeds with transformation from agriculture to productive manufacturing. Contrarily, African cities are crowded with migrants from rural areas with little skill sets and mostly engaged in low- value services rather than high- value manufacturing.

Recent empirical evidence indicates that ST could occur without change in labor productivity, especially in the case of African economies. Most importantly, this situation is common in natural resource exporters (Jedwab et al., 2013). Many African countries tend to specialize in the export of natural resource, which constrain conventional transitions experienced in industrialized settings. Natural resource– exporting countries tend to urbanize without significantly increasing their GDP share of manufacturing output. It is worth noting that the most urbanized countries export natural resources: oil (Angola, Gabon and Nigeria), diamonds and/ or gold (Botswana, Liberia and South Africa). In these countries, urbanization takes place without significant change in the economic structure. A recent analysis looked at ST and productivity growth in eleven sub- Saharan countries during the past 50 years (McMillan et al., 2014). It suggests that the early postindependence period had brought an augmentation of manufacturing activities due to the reallocation of resources, resulting in economic growth. However, in the following period from the mid- 1970s to the 1980s, the process of SC attenuated.