Epidemiology: Virulent encounters.

Technology makes it easier to find sexual partners, but that may have knock-on effects.

MANY men are keen to have more sex with more people, even if they don’t admit it to their other halves. Before the days of the internet, finding a casual partner meant putting in some effort: chatting up someone in a bar, say, or hanging about in a shady spot known to be frequented by like-minded people. Nowadays, a naughty encounter is but a click away, thanks to smartphone apps such as Tinder and Grindr and listings websites such as Craigslist. But with convenience comes less happy consequences, as a paper just published in MIS Quarterly suggests.

Jason Chan of the University of Minnesota and Anindya Ghose of New York University have looked specifically at what effects a local Craigslist site has on its state's rates of reported HIV cases. Craigslist started life as a round-robin e-mail in the San Francisco area in 1995, but has ballooned to a 700-site network in 70 countries. The sites make their money mainly from listings for jobs or housing, but their free personal ads, which often solicit casual sexual partners, account for a big chunk of the sites' content. These sites were rolled out piecemeal across America over several years, providing a natural laboratory to examine how their arrival affected sexual health.
Drs Chan and Ghose looked at HIV rates in 33 states between 1999 and 2008, mostly in America's central regions (Craigslist's spread to populous cities along coastal regions was much faster, muddying the data there). The arrival of Craigslist, they found, was correlated with an average increase of 15.9% a year in the number of HIV infections compared with what would have been expected had it not been launched; the pair estimate that the listing website was associated with between 6,130 and 6,455 extra infections a year throughout the country. That held true even after controlling for national and local HIV trends (which were often in decline), the level of urbanisation, and changing rates of people getting tested for the virus.

Although the sites also feature a section dedicated to escort services, these did not appear to be correlated with the increased infection rate. Instead, it seems, the majority of new infections result from men seeking other men. Dr Chan suggests that may be because sexual encounters elicited through online channels tend to differ from those arrived at by old-fashioned means. Online services aggregate many different sexually available men, upping a given user's chances and thereby lowering his fear of rejection—and creating a more nuanced market in which users can be more detailed about their proclivities (as a quick visit to a Craigslist personals section bears out). That, in turn, might modify the character of the encounter.
A prior study from the Center for AIDS Intervention Research suggests just that, showing for example that liaisons arranged online are associated with risky sexual behaviours such as unprotected, receptive anal intercourse. Not everyone agrees with that connection, however. Ford Hickson, a lecturer in public health at the London School of Hygiene and Tropical Medicine, says that while sites such as Craigslist have almost certainly increased the overall number of men having casual sex with men, the proportion of those engaged in risky behaviours has probably remained constant.
Either way, the influence that such sites have on public health is an under-researched topic. Dr Chan’s study of Craigslist only considered data to 2008. Since then, mobile phone apps such as Grindr and Manhunt, which use GPS to help men find nearby and amenable users in real time, have become popular. Experts agree that they have probably had a similar effect on infection, but hard figures are lacking. Indeed, the new apps may present new epidemiological concerns. Paige Padgett of the University of Texas in Houston, who is researching how people use dating apps, says that 40% of liaisons are men seeking sex with men. Because of the apps’ immediacy, she says, users don’t screen their potential partners as rigorously as they might once have done.
As researchers seek a better understanding of the way these technologies help to spread HIV, some tough questions will be raised. These may include whether health authorities should track the number of users of a service to prepare better for outbreaks of sexually transmitted diseases. There is also the issue of whether apps should carry sexual-health advice. Some already do by, for example, suggesting how better to screen partners. This removes a quandary for site owners and app developers, who might think that prominent links to HIV testing centres, for example, may put customers off. As ever, having better-informed customers is the best answer. ”It is not about restricting people’s opportunities,” says Dr Padgett, “it is about educating them.”

economist.