Peer pressure applied via Facebook can significantly boost the number of people who vote in an election, a new study has found.
Researchers found about one third of a million more people showed up at the ballot box in the United States in 2010 because of a single Facebook message on election day.
The experiment confirms that peer pressure helps get out the vote and demonstrates that online social networks can affect important real-world behaviour, they said.
Lead author James Fowler, University of California professor of political science in the Division of Social Sciences, said: "Voter turnout is incredibly important to the democratic process. Without voters, there's no democracy.
"Our study suggests that social influence may be the best way to increase voter turnout. Just as importantly, we show that what happens online matters a lot for the 'real world.'"
According to the US Census Bureau, voter participation was about 53 per cent of the voting-age population for the presidential election in 2008. For the Congressional election in 2010, which the study focused on, the turnout was 37 per cent.
In the study, more than 60 million people on Facebook saw a social, non-partisan "get out the vote" message at the top of their news feeds on Nov 2, 2010.
The message featured a reminder that "Today is Election Day"; a clickable "I Voted" button; a link to local polling places; a counter displaying how many Facebook users had already reported voting; and up to six profile pictures of users' own Facebook friends who had reported voting.
About 600,000 people, or one per cent, were randomly assigned to see a modified "informational message", identical in all respects to the social message except for pictures of friends. An additional 600,000 served as the control group and received no Election Day message from Facebook at all.
Prof Fowler and his colleagues then compared the behaviour of recipients of the social message, recipients of the informational message, and those who saw nothing.
Users who had received the social message were more likely than the others both to look for a polling place and to click on the "I Voted" button.
To estimate how many people actually voted, the team used publicly available voting records, the Daily Telegraph reported.
In their analyses, they developed a technique that prevented Facebook from knowing which users actually voted or registered, but it allowed the researchers to compare rates of turnout between users who saw the message and users who didn't. About four per cent of those who said they had voted had not.
However, rates of actual voting, the researchers discovered, were highest for the group that got the social message.
Users who got the informational message - who didn't see photos of friends - voted at the same rates as those who saw no message at all. Those who saw photos of friends, on the other hand, were more likely to vote.
Prof Fowler said: "Social influence made all the difference in political mobilisation.
"It's not the 'I Voted' button, or the lapel sticker we've all seen, that gets out the vote. It's the person attached to it."
The researchers estimate that the direct effect of the Facebook social message on users who saw it generated an additional 60,000 votes in 2010. But the effects of the social network - of social contagion among friends - they say, yielded another 280,000 more, for a total of 340,000.
In other words, Prof Fowler said, the social network yielded an additional four voters for every one voter that was directly mobilised.
Fowler and colleagues get to the total figure by comparing turnout between the friends of those who saw the social message and the friends of those who saw no message. Friends of social-message recipients, regardless of whether they were themselves recipients, were more likely to vote.
Prof Fowler said most of the increase in actual voting was attributable not to the entire set of a user's Facebook friends but to "close friends," people with whom users were most likely to have a close relationship outside the online network, too.
The researchers established this by asking some users about their closest friends and then measuring how often they interact on Facebook.
Prof Fowler and his team showed that Facebook interactions could be used to predict which Facebook friends were also close friends "in real life," and it was these close relationships that accounted for virtually all the difference in voting.
Research is now continuing on what kinds of messages work best for increasing voter participation and what kinds of people are most influential in the process.
Prof Fowler added: "The main driver of behaviour change is not the message - it's the vast social network.
"Whether we want to get out the vote or improve public health, we should not only focus on the direct effect of an intervention, but also on the indirect effect as it spreads from person to person to person."
The findings were published in the journal Nature.