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Human behaviour is thought to spread through face-to-face social networks, but it is difficult to identify social influence effects in observational studies9-13, and it is unknown whether online social networks operate in the same way14-19. 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 studies6,14-16 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 behaviour17-19.However, online social networks are also made up of many 'weak-tie' relationships20 that may not facilitate social influence21, and some studies suggest that online communication may not be an effective medium for influence22. 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 studies23,24. Voter mobilization efforts are effective at increasing turnout25, particularly those conducted face-to-face and those that appeal to social pressure26 and social identity27. 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 households28. Although anecdotal accounts suggest that online mobilization has made a big difference in recent elections21, a meta-analysis of email experiments suggests that online appeals to vote are ineffective24.
Voter mobilization experiments26-28 have shown that most methods of contacting potential voters have small...