When Sandra Tembo walks to the Mbare vegetable market in Harare, Zimbabwe, she passes a billboard: "Your future is brighter without a sugar daddy," it says. Tembo, a 20-year-old dressmaking student, says she's sure her friends "realize the risk" that comes with sugar-daddy relationships, which are common in sub-Saharan Africa and involve girls who have sex with older men in exchange for gifts and cash. "But they say being broke all the time also has its dangers, as you could starve."

Along with prostitution and promiscuity, sugar-daddy arrangements are fueling the AIDS epidemic in sub-Saharan Africa. Older men have a far higher prevalence of HIV than their younger cohorts. In Zimbabwe, for example, less than 5 percent of 15- to-19-year-old men test positive for HIV, while that number soars to about 30 percent for 30-to-34-year-olds, according to the U.S. Agency for International Development.

Now the World Bank is proposing to pay girls as a way to keep them in school and prevent AIDS. In a study that began in 2008 in Malawi, where about one in eight adults has the illness, the Bank paid a group of girls an average of $10 a month, plus money for school fees if they regularly attended class. A year and a half after the program began, the girls' infection rate was 60 percent lower than the rate for a control group that received no payments: 1.2 percent, vs. 3 percent. The beneficiaries also had fewer and younger sexual partners, according to the World Bank report.

Experts say that even though the girls weren't explicitly required to abstain from sugar-daddy relationships, the cash payments helped them avoid the temptation of taking gifts from older partners, which averaged $6.50 a month for girls who participated in the Malawi study. "Since they got money, they didn't need to interact with older men," says Mayra Buvinic, the World Bank's director of gender and development. Buvinic says the payouts are the first nonbiomedical approach to AIDS prevention that she knows of that had a "significant effect" on lowering HIV prevalence in Africa.

Distributing money to the less well-off in exchange for certain commitments—such as keeping kids in school or getting regular medical checkups—is a tactic that has been successfully employed in countries from Mexico to Brazil. Cash may be the "ethical policy instrument" of the 21st century, says Mead Over, a health economist at the Center for Global Development, a research organization in Washington. Another World Bank study in Tanzania paid cash to participants when they tested negative for a group of sexually transmitted diseases

Some aid organizations and health experts say the approach could backfire. Peter R. Lamptey, a Ghanaian physician at the nonprofit group Family Health International, argues that most African countries can't afford cash payouts. "Medical incentives aren't for us," he says. "You can keep them in Washington. We could end up creating an environment of dependency that cannot be sustained."

There's also a non-monetary element to sugar-daddy relationships, so cash payments alone may not end them. Rosemary Zulu, who teaches sexual and reproductive health for the London-based nonprofit Restless Development, recalls how married men in her native Zambia would show up at school and lavish gifts of clothes, mobile phones, and cash on their teenage paramours. "It takes a very strong teenager" to withstand the pressure to accept the gifts, says Zulu, 26. Sophie Harman, a senior lecturer in international politics at City University London, adds: "Paying people to influence their sexual behavior won't solve the wider problems of abuse, esteem, neglect, and inequality that cause them to get HIV."

The bottom line: Cash payouts may be the first successful non-biomedical approach to reducing HIV prevalence in sub-Saharan Africa.

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