The rise of social media has profoundly affected how people acquire and process information. Using Twitter data on the Brexit referendum and the 2016 US presidential election, this column studies how social media bots shape public opinion and voting outcomes. Bots have a tangible effect on the tweeting activity of humans, but the degree of their influence depends on whether they provide information consistent with humans’ priors. The findings suggest that effect of bots was likely marginal, but possibly large enough to affect voting outcomes in the two elections.

The Chinese government has invested heavily in surveillance systems that exploit information on social media. This column shows that these systems are very effective, even in their simplest form. From the government’s point of view, social media, although unattractive as a potential outlet for organised social protest, is useful as a method to surveil protests, monitor local officials, and disseminate propaganda.