That's the title of an October 2012 paper (PDF) by Marcella Alsan and David Cutler, and a longstanding, much-debated question in global health circles . Here's the abstract:
Uganda is widely viewed as a public health success for curtailing its HIV/AIDS epidemic in the early 1990s. To investigate the factors contributing to this decline, we build a model of HIV transmission. Calibration of the model indicates that reduced pre-marital sexual activity among young women was the most important factor in the decline. We next explore what led young women to change their behavior. The period of rapid HIV decline coincided with a dramatic rise in girls' secondary school enrollment. We instrument for this enrollment with distance to school, conditional on a rich set of demographic and locational controls, including distance to market center. We find that girls' enrollment in secondary education significantly increased the likelihood of abstaining from sex. Using a triple-difference estimator, we find that some of the schooling increase among young women was in response to a 1990 affirmative action policy giving women an advantage over men on University applications. Our findings suggest that one-third of the 14 percentage point decline in HIV among young women and approximately one-fifth of the overall HIV decline can be attributed to this gender-targeted education policy.
This paper won't settle the debate over why HIV prevalence declined in Uganda, but I think it's interesting both for its results and the methodology. I particularly like the bit on using distance from schools and from market center in this way, the idea being that they're trying to measure the effect of proximity to schools while controlling for the fact that schools are likely to be closer to the center of town in the first place.
The same paper was previously published as an NBER working paper in 2010, and it looks to me as though the addition of those distance-to-market controls was the main change since then. [Pro nerd tip: to figure out what changed between two PDFs, convert them to Word via pdftoword.com, save the files, and use the 'Compare > two versions of a document' feature in the Review pane in Word.]
Also, a tip of the hat to Chris Blattman, who earlier highlighted Alsan's fascinating paper (PDF) on TseTse flies. I was impressed by the amount of biology in the tsetse fly paper; a level of engagement with non-economic literature that I thought was both welcome and unusual for an economics paper. Then I realized it makes sense given that the author has an MD, an MPH, and a PhD in economics. Now I feel inadequate.