ECONOMISTS see reducing sexual inequality in education as a vital part of promoting development. The failure to educate girls limits economic growth in the developing world by wasting human capital. As a result, the UN set itself the target of eliminating gender disparity in education at all levels by 2015, as one of its Millennium Development Goals.

Although places like China, Bangladesh and Indonesia look likely to achieve the target, Africa, in particular, will not. For every 100 boys in secondary school on the continent in 2010, there were only 82 girls. The most common response is to channel more money to girls’ education. UN schemes finance school places for girls in 15 sub-Saharan countries. NGOs have got involved too. Camfed, a charity, now pays for almost 100,000 girls to be educated in Ghana, Malawi, Tanzania, Zambia and Zimbabwe.

But a new paper suggests that such initiatives are not enough to end inequality in education. The author, Martina Björkman-Nyqvist, compares 24 years of attendance rolls from primary schools in rural Uganda to rainfall records. She finds that girls’ attendance decreases much more than boys’ during droughts. This pattern became more pronounced after Uganda abolished fees at state schools in 1997. A reduction in rainfall of just 15% causes a drop of five percentage points in attendance by girls in seventh grade (aged about 12-13), but has no significant impact on boys. This pattern feeds through to test scores. Girls who stayed away during droughts did much worse in exams at the end of primary school than boys.

Why do families stop girls, but not boys, going to school during droughts? Some 80% of the workforce in Uganda is employed in rain-fed agriculture, so dry spells reduce income abruptly, by diminishing the harvest. The paper suggests that girls are put to work at home to support the family’s income in the short term. Boys, however, are kept at school, since education produces greater rewards for them in the job market than it does for girls. Droughts are especially damaging to the education of older girls, who have a greater potential to earn, or to free parents to do so by taking on household chores.

To keep girls in school, Ms. Björkman-Nyqvist concludes, development agencies may need to focus on measures to boost household income when times are tough, such as weather insurance or savings schemes. Something along those lines seems to have occurred in Victorian Britain, according to recent research from Oxford University. It finds that working-class women and girls typically bore the brunt of sudden falls in household income, by eating less. This situation gradually improved through the growing use of insurance schemes provided by friendly societies to help smooth out consumption.

Reducing sexual inequality in modern Africa may reap benefits for men as well as women. According to Britain’s Department for International Development, an increase of 1% in the number of girls with secondary schooling boosts growth in annual income per head by 0.3%. Girls’ education improves health, too. About half the reduction in child mortality over the past 40 years in Ghana can be attributed to improvements in female education. “If you teach a boy, you educate an individual”, an African proverb says, “but if you teach a girl, you educate a whole nation.” There are no proverbs, alas, about how to get girls into school and keep them there.