In the ointment

ECONOMIC historians have long supposed that Africa’s historically low population density shaped its development. Rulers struggled to exercise control over scattered populations, the theory goes. Malfunctioning states inhibited growth because property rights were insecure and infrastructure was worse.

But why was it that land in precolonial Africa was so abundant, and people were so scarce? A new paper* by Marcella Alsan of Stanford University blames the tsetse fly. The pest, much like the mosquito, lives off the blood of people and animals and in the process transmits disease, in this case a parasite that causes sleeping sickness. To domesticated animals, on which it likes to feed, its bite is fatal. Its prevalence, the paper argues, made it considerably harder for Africans to develop agriculture.

Using historical data on climate, Ms Alsan constructs a “tsetse-suitability index” (TSI) to measure the extent to which the insect must have thrived in different places. She compares the TSI with anthropological records of precolonial African farming.

She finds that a one standard-deviation increase in the TSI is associated with a 23 percentage-point decline in the likelihood that the local people kept domesticated livestock. That, in turn, made it harder to farm: a one standard-deviation increase in the TSI is associated with a six percentage-point reduction in the use of ploughs.

The tsetse fly is found only in Africa, so Ms Alsan tested her results in areas outside Africa with similar climates; she did not find the same pattern. That suggests that it was the fly itself that was holding back agriculture. Without the help of animals, farming becomes much less productive and is viable in fewer places. Those Africans surviving by foraging, meanwhile, would have had to spread out over large areas in order to feed themselves. Doing so may also have lowered the risk of infection. No wonder, then, a higher TSI is correlated with a lower precolonial population density.

The tsetse’s bite still smarts today. Thanks to the weak governments it engendered, economies in tsetse-infested areas have struggled to grow (although colonialism hardly helped). The higher the TSI, Ms Alsan finds, the lower the current level of economic development.

^{* “The effect of the tsetse fly on African development”, by Marcella Alsan, American Economic Review, 2015}