The TV show “Farmer Wants a Wife” has been produced in over 30 countries around the world. Why is it so popular? The problem was noticed already in the 1960s by Pierre Bourdieu: many men in rural France remained single because many more women than men migrated out. How common is this phenomenon? Does it imply that there are also not enough partners for women in urban areas? While a growing body of literature is dedicated to the reversed education gender gap and an insufficient number of well-educated men, the problem may often be even more basic: there are not enough men or women around.

To answer these questions, I start with some of the oldest methods in demography. Already in 1885, Ernst Ravenstein looked at sex ratios to formulate his “laws of migration.” He concluded that “females are more migratory than males,” particularly on short distances. This led to skewed sex ratios at the local or regional level. So far, most studies were limited to single cases. However, this phenomenon appears to be very common.

In my new paper in Population and Development Review, I show that sex-selective migration leads to imbalanced subnational (rural or urban) sex ratios among young adults in most countries of the world. In almost all cases, more boys are born, but more young men die. So, the crucial factor shaping sex ratios in any place is sex-selective migration. In Europe and the Americas, many more women migrate from rural to urban areas. The pattern is opposite in Africa. However, with socio-economic development, migration becomes more feminized there. The most extreme case are the Gulf countries with up to four times as many young men as women in Qatar due to the immigration of men.

I also propose a simple and informative way to analyze spatial distribution of population by relating population composition to population density. Interestingly, Ravenstein’s observation is no longer true for his original case, the United Kingdom. However, the general pattern in Europe is clear: the lower is the population density of a region, the larger is the deficit of young women. Policies tackling rural depopulation should acknowledge that it is mostly women who leave.

The most striking case is Germany. After re-unification, many women migrated from the post-communist East to the West. In consequence, the poorer Eastern regions suffer from a shortage of women. In turn, there is a surplus of women (or shortage of men) in the West. In both parts of the country, the correlation between sex ratios among young adults and population density holds.

My study opens paths for many further research questions. Why is rural-urban migration much more sex-selective in some countries than others? Which cities and countries attract more women? Why is the United Kingdom an outlier in Europe? How fast and to what extent will the rural-urban migration in Africa feminize with economic and social development, and demographic transition? How do sex ratios relate to patterns of urbanization and sub-urbanization or economic development and peripheralization? Each country –with its own dynamics of sex-selective migration, economic structure, and level of development– may be an interesting case study.