Vivek Murthy, the US surgeon general, recently warned that “being socially disconnected” has a similar effect on mortality as smoking up to 15 cigarettes a day. This statement was widely reported in the media, including in the Washington Post, the Times and the Daily Mail. But where does this “15 cigarettes a day” figure come from? Dr Murthy is referring to a study published in 2010 that explored social relationships and mortality rates. The researchers combined the data from 148 studies on the topic, in what is known as a “meta-analysis”, to arrive at a more robust statistical answer to...