The researchers say that half of all new HIV infections in sub-Saharan Africa are among young people, and girls are two to three times more likely to be infected than boys. One of the major causes is 'sugar daddies': older boyfriends who give food, money or pay for school fees in return for sex. These men are more likely to be HIV-positive, and their young girlfriends less able to request that they use a condom.

The study finds that the risk of these sugar daddy relationships is significantly reduced in households that receive government child support grants. The longitudinal study, conducted in 2009-12, involved a team of researchers from UK and South African universities. They interviewed 3,515 teenagers, with 97% of them followed up a year later. Face-to-face interviews were conducted with children in four urban and rural areas in Mpumalanga and the Western Cape -- all in very poor areas with high rates of HIV infection. Households that received child-focused grants were compared with those that did not. The study found that teenage girls from households receiving grants were two-thirds less likely to take much older boyfriends, and half as likely to have sex in exchange for food, money or school fees.

The South African government currently gives a child support grant of around $35 a month per household to 11 million children under 18, and a foster child grant worth around $96 a month to another 600,000 nationally. These findings have major implications for HIV prevention in sub-Saharan Africa. Government grants in South Africa have expanded massively, and currently reach about 70% of eligible children according to studies by the Children's Institute at the University of Cape Town. If all those potentially eligible in South Africa were reached, 77,000 new relationships of teenage girls with sugar daddies could be prevented each year, says the study.

The study adds to emerging evidence from scientific trials from other African countries. In Malawi, cash transfers to teenage girls resulted in a lower prevalence of HIV because girls then chose to have younger sexual partners. In Tanzania, money was given conditionally on a negative sexually transmitted infection test result, which also resulted in lower HIV risks. However, this study in South Africa shows that such child-focused cash transfers can work not only in carefully controlled research trials, but also in the real world on a massive scale through government grant systems. Many sub-Saharan African countries are considering introducing social welfare systems for poor households with children, so these findings showing this is money well spent come at a particularly significant time.

Lead author Dr Lucie Cluver, from the Department of Social Policy and Intervention at the University of Oxford, said: 'This study shows that as long as they are given enough money to survive, girls will choose not to have a sugar daddy. It also shows how valuable it is to give not only to younger children but also to teenagers, who are most at risk of HIV-infection.'

Government grants do not provide the whole solution to the problem of HIV-infection among young people, adds the study. It also found that grants did not reduce risks for boys, and did not reduce other risks for girls, such as the likelihood of having unprotected sex or having sex when drunk.

Professor Mark Orkin, from the School of Public and Development Management, University of the Witwatersrand, Johannesburg, said: 'Child support grants do not make teenagers more sensible when it comes to safer sex. But what they can do is to provide enough financial security for girls that they do not have to choose their sexual partners through economic necessity'.