Recent literature in development economics associated ethnic diversity and poor growth performance. The 'ethnic conjecture' should be firmly rejected as a meaningful hypothesis to explain slow growth in African States, since it is grounded on a slippery and ill-conceived concept. Alleged ethnic groups are neither 'objective categories' nor their classification is exogenous with respect to economic and political issues. Overwhelming historical evidence points out to these negative conclusions. Extensive literature in the social sciences has argued against both “objective” partitions and exogeneity. The rigid ethnic classifications in ethnology are now rejected. When such perceived, partitions of ethnic identities are often not commonly shared by the people involved, as regards both their definitions and relevance. The paper criticises the literature on ethnicity and growth performance in development economics along four mail lines: the flimsy semantics of ethnicity; the alleged exogeneity of the ethnic fraction alisation variables; the fallacious idea of causality in growth processes that the ethnic conjecture illustrates; the political con-clusions that result from the ethnic conjecture.