In More Sex is Safer Sex Steven Landsburg famously argued (based on work by Michael Kremer) that if more people, especially more sexually conservative people, had sex the AIDS epidemic could be reduced.' Landsburg wrote:

Imagine a country where almost all women are monogamous, while all mendemand two female partners per year. Under those circumstances, a fewprostitutes end up servicing all the men. Before long, the prostitutesare infected; they pass the disease on to the men; the men bring ithome to their monogamous wives. But if each of those monogamous wiveswere willing to take on one extramarital partner, the market forprostitution would die out, and the virus, unable to spread fast enoughto maintain itself, might well die out along with it.

In The Wisdom of Whores (see also my earlier post) Elizabeth Pisani says that such a country exists, it's Thailand, and the results of more sex were safer sex - exactly as Landsburg argued. Here's Pisani's story:

Thailand used to fit the the classic 'virtuous girls, philandering boys' model.' At the start of the 1990s, 57 percent of twenty-one-year-old men in Northern Thailand trooped off to the brothel to do their philandering.' More than half the sex workers who soaked up their excess energy were HIV-infected....

Then...the Thai economy boomed.' Girls were getting better educations than ever before...Educated girls were waiting longer before getting married, but not before having sex.' By the end of the 1990s, 45 percent of girls aged 15-21 in northern Thailand admitted to having sex with boyfriends before marriage, compared to less than a tenth of that in a nationwide survey in 1993.

...So at the end of the decade, we have a lot more premarital sex and not all that much condom use with girlfriends.' But now that these young, cash-strapped guys can have sex without paying, they've stopped handing over cash for sex.' By the end of the 1990s, only 7 percent of young men were paying for sex, and HIV prevalence in sex workers had come down too.

....In short, more women having premarital sex equals less HIV.

Pisani cites neither Landsburg nor Kremer so I believe her account is independent.' Note that Pisani also credits Thailand's successful condom program.

(Via Marginal Revolution.)