A randomised trial shows that the power of the press is real

MALCOLM X, an American political activist, described the media as the most powerful entity on Earth, “because they control the minds of the masses”. Some journalists may find this proposition flattering, but though those who study such things agree newspapers exert some influence over their readers, the effect has proved devilishly difficult to quantify. Now, Gary King of Harvard University and his colleagues have measured the impact of stories from almost three dozen different news sources on the American public, as judged by the content of posts on Twitter, a microblogging service. Their study, published this week in Science, found that even stories from the news sites that formed part of the study, which were small compared with, say, the New York Times or the Washington Post, increased Twitter discussion of the issues in those stories by about 60%. They also shifted the nature of the views expressed in those tweets towards those of the published pieces.

Many researchers have looked in the past at the question of media influence. They have done so by, for example, comparing places that had a radio signal with those that did not. These studies, however, ran into a common problem, namely distinguishing (often small) effects that arise because of exposure to the media from those that stem from innate differences between the two groups being studied. In medical research, the tool for overcoming such problems is the randomised controlled trial. This is a type of experiment used to assess the efficacy of medical interventions by assigning patients to one of two groups at random, and giving members of one the drug or treatment in question, while those in the other group, acting as a control, receive a placebo with no known therapeutic effects. Dr King applied this approach to try to determine the effects of reading the news.