At the International AIDS Conference in Vienna this past weekend, the World Bank announced the results of an unusual study on HIV/AIDS prevention — it gave cash to girls in Malawi just for staying in school. Girls between ages 13 and 22 in the southern district of Zomba were paid $15 a month for a year; their behaviors were compared to a control group that was not paid at all. Results showed that the girls who were paid to stay in school seemed to make wiser choices about when to have sex with and with whom:

18 months after the program began in January 2008, biomarker data show that HIV infection rates among girls who received cash was 1.2% versus the control group's 3%. This translates to 60% lower prevalence. Girls in the cash group also had a lower infection rate of herpes simplex virus type 2, the common cause of genital herpes (0.7% vs. 3%). Those findings hold even for a third group of girls who got cash without any schooling or other strings attached.

How did it happen? The key seems to be an "income effect" on the sexual behaviors of young women receiving cash payments. A year after the program started, girls who received payments not only had less sex, but when they did, they tended to choose safer partners, says Berk Ã–zler, a senior economist at the Development Research Group who conducted the study with Sarah Baird of George Washington University and Craig McIntosh at the University of California, San Diego. In fact, the infection rate among those partners is estimated to be half of that of partners of the control group.

The cash transfers may have led to a drop in the so-called "transactional sex." At the beginning of the study, a quarter of sexually-active participants said they started relationships because they "needed his assistance" or "wanted gifts/money." Meanwhile, among the sexually-active schoolgirls in the control group, 90% said they received an average of US$6.50 a month in gifts or cash from their partners. Such "gifts" are significant, given the country's GDP per capita was $287.5 in 2008.

After a year, schoolgirls receiving payments from the cash-transfer program seemed to avoid older men, who tend to be wealthier and are much more likely to be HIV positive than schoolboys. The sexual partners were two years older on average than the girls, compared with three years for the control group.