Longer secondary schooling substantially reduces the risk of contracting HIV, particularly for girls, according to new research from Botswana. The researchers estimate that pupils who stayed in school for an extra year of secondary school had an eight percentage point lower risk of HIV infection about a decade later, from about 25 percent to about 17 percent infected.

The study, which also shows expanding secondary schooling to be a very cost effective HIV prevention measure, used a recent school policy reform as a 'natural experiment' to determine the impact of increased years of secondary schooling on risk of HIV infection.

Implemented in 1996, the reform led to an average increase of 0.8 years of schooling among teenagers, by providing free grade 10 education as part of junior -- rather than senior -- secondary school. This policy presented a unique opportunity to estimate the causal effect of length of schooling on risk of HIV infection by comparing birth cohorts exposed to the reform versus those unexposed, using data from national HIV surveys collected in 2004 and 2008.Although education is known to be closely associated with health, whether or not formal education actually protects against HIV infection has been hotly debated for over two decades.

Botswana has one of the highest rates of HIV in the world, with around 22% of adults aged 15-49 years infected in 2013. The authors investigated the causal effect of an additional year of schooling on HIV status in 7018 men and women at least 18 years old at the time of the surveys. Individuals born in or after 1981 (who would have started junior secondary school in 1996 or later) were classified as exposed to the reform.The researchers estimate that individuals who gained an extra year of secondary schooling due to the policy were 8 percentage points less likely to test positive for HIV about a decade later, when most of those exposed to the policy were in their mid 20s. The effects were particularly strong among women, with each additional year of secondary schooling reducing infection risk by 12 percentage points. Additionally, education may expand economic opportunities and reduce women's participation in higher risk transactional sexual relationships. Secondary schooling may be particularly effective in reducing HIV risk by targeting a critical period of growth in adolescence."