Human behaviour is thought to spread through face-to-face social networks, but it is difficult to identify social influence effects in observational studies [9] [10] [11] [12] [13] , and it is unknown whether online social networks operate in the same way [14] [15] [16] [17] [18] [19] . Here we report results from a randomized controlled trial of political mobilization messages delivered to 61 million Facebook users during the 2010 US congressional elections. The results show that the messages directly influenced political self-expression, information seeking and realworld voting behaviour of millions of people. Furthermore, the messages not only influenced the users who received them but also the users' friends, and friends of friends. The effect of social transmission on real-world voting was greater than the direct effect of the messages themselves, and nearly all the transmission occurred between 'close friends' who were more likely to have a face-to-face relationship. These results suggest that strong ties are instrumental for spreading both online and real-world behaviour in human social networks. Recent experimental studies 6,14-16 have attempted to measure the causal effect of social influence online. At the same time, there is increasing interest in the ability to use online social networks to study and influence real-world behaviour [17] [18] [19] . However, online social networks are also made up of many 'weak-tie' relationships 20 that may not facilitate social influence 21 , and some studies suggest that online communication may not be an effective medium for influence 22 . An open question is whether online networks, which harness social information from face-to-face networks, can be used effectively to increase the likelihood of behaviour change and social contagion. One behaviour that has been proposed to spread through networks is the act of voting in national elections. Voter turnout is significantly correlated among friends, family members and co-workers in observational studies 23, 24 . Voter mobilization efforts are effective at increasing turnout 25 , particularly those conducted face-to-face and those that appeal to social pressure 26 and social identity . There is also evidence from one face-to-face field experiment that voting is 'contagious', in the sense that mobilization can spread from person to person within two-person households 28 . Although anecdotal accounts suggest that online mobilization has made a big difference in recent elections 21 , a meta-analysis of email experiments suggests that online appeals to vote are ineffective 24 . Voter mobilization experiments [26] [27] [28] have shown that most methods of contacting potential voters have small effects (if any) on turnout rates, ranging from 1% to 10%. However, the ability to reach large populations online means that even small effects could yield behaviour changes for millions of people. Furthermore, as many elections are competitive, these changes could affect electoral outcomes. For example, in the 2000 US presidential election, George Bush beat Al Gore in Florida by 537 votes (less than 0.01% of votes cast in Florida). Had Gore won Florida, he would have won the election. To test the hypothesis that political behaviour can spread through an online social network, we conducted a randomized controlled trial with all users of at least 18 years of age in the United States who accessed the Facebook website on 2 November 2010, the day of the US congressional elections. Users were randomly assigned to a 'social message' group, an 'informational message' group or a control group. The social message group (n 5 60,055,176) was shown a statement at the top of their 'News Feed'. This message encouraged the user to vote, provided a link to find local polling places, showed a clickable button reading 'I Voted', showed a counter indicating how many other Facebook users had previously reported voting, and displayed up to six small randomly selected 'profile pictures' of the user's Facebook friends who had already clicked the I Voted button (Fig. 1) . The informational message group (n 5 611,044) was shown the message, poll information, counter and button, but they were not shown any faces of friends. The control group (n 5 613,096) did not receive any message at the top of their News Feed. The design of the experiment allowed us to assess the impact that the treatments had on three user actions; clicking the I Voted button, clicking the polling-place link and voting in the election. Clicking the I Voted button is similar to traditional measures of self-reported voting, but here users reported their vote to their social community rather than to a researcher. We therefore use this action to measure political self-expression, as it is likely to be affected by the extent to which a user desires to be seen as a voter by others. In contrast, social desirability should not affect other user actions in the same way. Clicking the polling-place link took users to a separate website that helped them to find a polling location, and this action was not reported to the user's social community. We therefore use this action to measure a user's desire to seek information about the election. Finally, we used a group-level process to study the validated voting behaviour of 6.3 million users matched to publicly available voter records (see Supplementary Information) . We first analyse direct effects. We cannot compare the treatment groups with the control group to assess the effect of the treatment on self-expression and information seeking, because the control group did not have the option to click an I Voted button or click on a pollingplace link. However, we can compare the proportion of users between the two treatment groups to estimate the causal effect of seeing the faces of friends who have identified themselves as voters (Fig. 1) . Users who received the social message were 2.08% (s.e.m., 0.05%; t-test, P , 0.01) more likely to click on the I Voted button than those who received the informational message (20.04% in the social message group versus 17.96% in the informational message group). Users who received the social message were also 0.26% (s.e.m., 0.02%; P , 0.01) more likely to click the polling-place information link than users who received the informational message (Fig. 1 ). Although acts of political self-expression and information seeking are important in their own right, they do not necessarily guarantee that a particular user will actually vote. As such, we also measured the effect that the experimental treatment had on validated voting, through examination of public voting records. The results show that users
The use of social media for political mobilization has been studied in REF , and the authors analyze the effect of political messages on political self-expression and interactions.
A 61-million-person experiment in social influence and political mobilization
Nature
Nature
Computer Science Sociology Medicine