HUMANS can digest lactose, the main carbohydrate in milk, only with the help of an enzyme called lactase. But two-thirds of people stop producing it after they have been weaned. The lucky third—those with “lactase persistence”—continue to produce it into adulthood. A recent paper* argues that this genetic quirk helps explain why some countries are rich and others poor.

Justin Cook of the University of California, Merced, uses data on historical migration flows to estimate the ethnic composition of 108 countries in Africa, Asia and Europe in 1500. He then estimates what proportion of the population would have been able to digest milk, using data on the lactose tolerance of different ethnic groups (which he assumes has not changed much over the centuries). Pre-colonial countries in western Europe tended to have the highest rates of lactase persistence, Mr Cook estimates. Some 96% of Swedes had it, for instance. The lowest levels were in Sub-Saharan Africa and South-East Asia.

A one-standard-deviation increase in the incidence of lactase persistence, in turn, was associated with a 40% rise in population density. People who could digest milk, the theory goes, used resources more efficiently than those who couldn’t. They could extract liquid energy from livestock, in addition to the wool, fertiliser, ploughing power and meat for which others raised them. The white stuff may have helped in other ways too: its fats, proteins, vitamins and minerals added balance to the pre-colonial diet, reducing the incidence of disease. If used as a substitute for breast-feeding, animal milk could have reduced weaning time and, thus, the time between mothers’ pregnancies. All this suggests that milk-guzzling societies could support higher population densities (although it remains puzzling that lactase persistence evolved in parts of Africa, but did not spread).

When people are tightly bunched together, the theory goes, growth takes off. Rulers find it easier to build infrastructure and administer the law, including property rights. Cities can develop, which allows workers to specialise. Technological innovation explodes; bigger armies can defend what is produced. Small wonder, then, that places with high population density in pre-colonial times tend to be relatively rich today. No single factor can explain long-run economic outcomes, of course, but Mr Cook’s idea may be worth milking.

* “The role of lactase persistence in pre-colonial development”, by C. Justin Cook, Journal of Economic Growth, December 2014.