Virulent encounters

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.