Increasing Returns to Marketing in Zambian Maize Markets

The article investigates the existence of market externalities due to increasing returns to marketing. Panel data on 198 pairs of maize markets in Zambia and correlated random effects linear and tobit estimators were used to model the relationship between market externalities and producer-to-wholesale marketing margins. This article is one of the first to explicitly account for unobserved local market heterogeneity in a developing country context. The results suggest that Zambian smallholder maize markets are substantially specialized with significant margin-reducing own externality effects and insignificant cross-externality effects. The results also indicate that the unobserved market effects exert a systematic influence on this relationship, a phenomenon that is impossible to measure with cross-sectional data.