Experiments don’t just take place in laboratories; they also happen on Facebook.

On November 2^nd, 2010, more than 61 million adults visited Facebook’s website, and every single one of them unwittingly took part in a massive experiment. It was a randomised controlled trial, of the sort used to conclusively test the worth of new medicines. But rather than drugs or vaccines, this trial looked at the effectiveness of political messages, and the influence of our friends, in swaying our actions. And unlike most medical trials, this one had a sample size in the millions.

It was the day of the US congressional elections. The vast majority of the users aged 18 and over (98 percent of them) saw a “social message” at the top of their News Feed, encouraging them to vote. It gave them a link to local polling places, and clickable button that said “I voted”. They could see how many people had clicked the button on a counter, and which of their friends had done so through a set of randomly selected profile pictures.

But the remaining 2 percent saw something different, thanks to a team of scientists, led by James Fowler from the University of California, San Diego. Half of them saw the same box, wording, button and counter, but without the pictures of their friends—this was the “informational message” group. The other half saw nothing—they were the “no message” group.

By comparing the three groups, Fowler’s team showed that the messages mobilised people to express their desire to vote by clicking the button, and the social ones even spurred some to vote. These effects rippled through the network, affecting not just friends, but friends of friends. By linking the accounts to actual voting records, Fowler estimated that tens of thousands of votes eventually cast during the election were generated by this single Facebook message.

The effects appear to be small but could still be influential when multiplied through large social networks.

I suspect we’ll continue to see more and more of this in the future. Platforms like Facebook or Google or Amazon have access to millions of users and can run experiments that don’t change a user’s experience of the website much.