Facebook manipulated the news feeds of almost 2 million American users during the 2012 presidential election without telling them. The manipulation led to a 3 percent increase in voter turnout, according to the company’s own data scientist.

In a stunning revelation, the three months prior to Election Day in 2012 saw Facebook “tweak” the feeds of 1.9 million Americans by sharing their friends’ hard news posts rather than the usual personal posts. The effect was felt most by occasional Facebook users who reported in a survey they paid more attention to the government because of their friends’ hard news feeds. Facebook didn’t tell users about this psychology experiment, but it boosted voter turnout by 3 percent.

The experiment was first shared with the public in two talks given by Facebook’s data scientist, Lada Adamic, in the fall of 2012, and more details were disclosed recently by Mother Jones. In those talks, Adamic said a colleague at Facebook, Solomon Messing, “tweaked” the feeds. Afterwards, Messing surveyed the group and found that voter turnout and political engagement grew from a self-reported 64 percent to more than 67 percent.

Michael Buckley, vice president for global business communication at Facebook, said the Messing study was an “in product” test designed to see how users would react with news feeds that were more prominent.

“This was literally some of the earliest learning we had on news,” Buckley told Mother Jones. “Now, we’ve literally changed News Feed, to reduce spam and increase quality of content.”

READ MORE:Facebook will ‘share’ Americans’ political views with Buzzfeed, ABC News

Buckley said the public will not receive full answers about that experiment until some point in 2015, when the academic papers are expected to be published.

It is not the first time that Facebook has been exposed for conducting psychological experiments on its users without their knowledge. In June, it was revealed that Facebook tried to manipulate users’ emotions when toying with the feelings of 689,003 randomly-selected, English-speaking Facebook users by changing the contents of their news feed. During a week-long period in January 2012, researchers staged two parallel experiments, reducing the number of positive or negative updates in each user's news feed.

READ MORE:Facebook manipulated users' emotions as part of psychological experiment – study

“These results suggest that the emotions expressed by friends, via online social networks, influence our own moods, constituting, to our knowledge, the first experimental evidence for massive-scale emotional contagion via social networks, and providing support for previously contested claims that emotions spread via contagion through a network,” according to a paper published in the June edition of the journal Proceedings of the National Academy of Scientists (PNAS).

The Facebook users were not notified of the experiment. However, according to Facebook's terms of service (to which every person agrees when they register on the social network), user data may be used “for internal operations, including troubleshooting, data analysis, testing, research and service improvement.” The researchers argue that their experiment was consistent with Facebook’s data use policy.

READ MORE:Journal that published Facebook psych study sorry…social network no

These experiments are raising eyebrows over the manipulation of voters, as well as the possibility that big data can eventually be used to “engineer the public” without the public’s knowledge, according to sociologist Zeynep Tufekci.

“At minimum, this environment favors incumbents who already have troves of data, and favors entrenched and moneyed candidates within parties, as well as the data–rich among existing parties. The trends are clear. The selling of politicians — as if they were ‘products’ — will become more expansive and improved, if more expensive,” said Tufekci in a report on the peer-reviewed journal First Monday.

“In this light, it is not a complete coincidence that the ‘chief data scientist’ for the Obama 2012 campaign was previously employed by a supermarket to ‘maximize the efficiency of sales promotions.’ And while the data advantage is held, for the moment, by the Democratic party in the United States, it will likely available to the highest bidder in future campaigns.”