Cruising for sex on Craigslist personal ads resulted in a 15.9 percent increase in HIV infections after the website entered various U.S. markets, according to a recent study by the University of Minnesota published in MIS Quarterly.

The study estimates that the Craigslist effect boiled down to about 6,000 additional HIV cases a year between 1999 and 2008 in the 33 states studied and additional treatment costs of $62 million to $65.3 million.

Jason Chan, an assistant professor of information and decision sciences at the U’s Carlson School of Management, said he got the idea for the study several years ago while working on his doctorate in management information systems at New York University (NYU). Chan’s dissertation focused on the social and health impacts of Internet platforms. In the course of his studies, he’d read about a doctor who traced sexually transmitted diseases to online chat rooms.

“I went to Craigslist in New York and went through the ads. To my shock, I found out almost all the ads were hookup ads. They were very explicit,” said Chan, who wrote the journal article with Prof. Anindya Ghose of NYU’s Stern School of Business.

Chan explained that because Craigslist entered individual markets without respect to concurrent HIV trends, it provided a “natural experiment” platform for the study.

The econometric method used by Chan and Ghose has the equivalent effect of comparing HIV trends across markets with and without Craigslist, and before and after Craigslist exists in these locations.

They also compared the HIV rate against the number of personal ads in contrast to the number of ads placed by escorts in a separate Craigslist section for professional services.

The study found that HIV incidence began to increase about a year after Craigslist entered a market, then climbed through the study period. Chan said the one-year delay makes sense, because it takes time for the ads to reach a critical mass and for those infected to seek help.

Curiously, the association between Craigslist and HIV occurred only with respect to personal ads — not professional escort services. Chan figures that sex workers took more precautions than people seeking sex in a social context.

Craigslist has since dropped the professional escorts ads, so it’s possible that those have moved surreptitiously into the personal ads. Browsers there are greeted with this notice: “Safer sex greatly reduces the risk of STDs (e.g. HIV). Please report suspected exploitation of minors.”

Not narrowed down

The study was unable to differentiate between the various categories of personal ads, such as men looking for men, women looking for men and other permutations.

“My suspicion is most of this [effect] is coming from the M for M section,” Chan said. “We do know that anal sex is a large reason why HIV is spread. It’s also prevalent in the gay community. It makes sense that this group is the most likely to be affected.”

The study employed a standard methodology that has been used to investigate other questions, such as the effect online book reviews have on sales.

On to phone apps

Paige Padgett, assistant professor of management, policy and community health at the University of Texas Health Science Center at Houston, called the U’s study “amazing,” but noted that some people consider Craigslist to be a bit old school. She’s studying mobile dating apps with geolocation functions that facilitate instant connections when people are out on the town.

“I have some preliminary data I have done with men who have sex with men, looking at which phone apps are the most popular,” Padgett said.

She said about 40 percent of respondents used the apps primarily to meet men for sex, but many also report using them for romance, to make friends or out of boredom.

The problem, Padgett says, is that phone apps accelerate intimacy.

“You’re sharing personal information a lot faster … so then when you actually meet face to face, you feel you really know that person,” she said.