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.