One of the things the Internet has fundamentally altered is the personal ad. Rather than a mystery of short text descriptions and a long wait for mail to be exchanged, personal ads now provide photos, profiles, and near-instant gratification. There's been a blossoming of services that promise to match people with their best prospect for long-term happiness, as well as apps that offer a quick hookup.

A new study focused on the role of something that's a bit of a hybrid between old and new school: Craigslist classifieds, which have also become a popular way of arranging hookups. By comparing areas before and after the arrival of a local Craigslist, Jason Chan and Anindya Ghose found that the availability of these classifieds are associated with a 16 percent increase in new HIV infections.

The study was prompted by a couple of well-described phenomena. One is that people are using the convenience and relative anonymity of the Internet to find partners; interviews with users of various services show that they post ads not only for what the authors term "no-strings-attached relationships," but they're also looking for more diverse sexual experiences, and part of that includes having multiple partners (not necessarily at once).

The other thing that intrigued the authors is the fact that HIV rates in most of the US had either been steady or falling through the early part of this century. But starting in 2005, that trend reversed, and rates have continued to climb since. This trend has often been attributed to the development of effective viral control strategies, which have reduced the fear of infection.

Conveniently, the spread of Craigslist provides a natural experiment: local sites were rolled out gradually over time, allowing the comparison of infection rates before and after their arrival. For the most part, the authors had to look at state-level data, which is inexact (some sites are focused on cities that sit on state borders), but they were also able to do a county-by-county analysis for a few areas, including New York City.

The data suggests that there was no rise in infection rate in advance of Craigslist opening shop, and there appears to be a bit of lag after a local site opens. This makes sense given that each city will take some time before users discover and start using the service, and the authors looked into this in more detail by tracking the number of ads present in each local site. Within a year of the opening of a local Craigslist, as activity picked up, HIV infections began to rise. The effect remained after the authors controlled for a variety of other demographic factors in each area.

The authors checked the rates of a couple of diseases that aren't spread through human contact and found that they weren't changed after the opening of new local Craigslist sites. They also showed that there was no change in testing rates after Craigslist opened, so this seems to be a real increase in infections rather than simply better screening.

How large is the effect? The authors calculate that "a 10 percent increase in the daily number of personal ads leads to a 0.7 percent increase in HIV cases annually in a state." That may sound small, but it works out to roughly a 16 percent increase in infection rates, or roughly 6,000 additional cases a year. The authors calculate that those cases add $60 million annually to our nation's healthcare costs.

The authors' claim that Craigslist can drive infectious disease rates seems rather surprising, so we asked Roheeni Saxena of Columbia University's Mailman School of Public Health to look over the study. "Exploring the relationship between technology and epidemiology is important to understanding the spread of infection in a technology-driven world," Saxena told Ars. But she went on to say that the paper makes conclusions about causality that the data doesn't fully support: "The data does support the existence of some correlation between Craigslist introduction to a metropolitan area and an increase in HIV prevalence, but one of the central tenets of epidemiology is that correlation does not equal causation."

In addition, Craigslist isn't the only service that offers the possibility of relatively easy hookups—beyond the possibility that the correlation is spurious, many other newly launched services could be contributing to this trend (and even competing with each other for the same population of users). It's simply that Craigslist provides the cleanest before-and-after view of the problem. And given that we haven't come much closer to figuring out how to eliminate an HIV infection, it really is a problem.

MIS Quarterly, 2014. DOI unavailable. (About DOIs).