The percentage of the land surface area of each country covered by sandy desert, dunes, rocky or lava flows, was calculated on the basis of the desert layer of the Collins Bartholomew World Premium digital map data (Collins Bartholomew, 2005) and the country boundaries described above. This was initially computed as a cruder measure of soil (in)fertility for an early draft of the paper and is no longer used in the final version. Nunn and Puga have left it in the dataset in case it is of use to other researchers.