Are you a Facebook lab rat?

Surprise! While you have been posting witty statuses and photos of your weekend escapades, Facebook has been using you as a lab rat.

If you use Facebook, you signed off on the site’s term and use agreement, which, like most people, you probably didn’t read. What you might not have realized is that by checking the box at the end of those terms, you allowed Facebook to use you as a guinea pig.

Facebook account holders may have been a subject in a psychological experiment without even knowing it. A paper published in the Proceedings of the National Academy of Scientists (PNAS) journal revealed the results of a study of some 600, 000 unwitting Facebook users who were part of a psychological study for one week in January 2012.

In an experiment organized by Facebook, scientists altered the tone of users’ news feeds to highlight either positive or negative posts from their friends. The scientists then monitored the users’ responses to see if a person’s attitude was affected by the attitudes of those around them. Their results showed that that positive posts tended to breed positive replies, while negative posts led to more negativity, an effect they deemed “emotional contagion.”

More surprising than the results was the fact that the entire experiment was perfectly legal. All users of the site must verify their agreement to the site’s terms and conditions, which include “internal operations, including troubleshooting, data analysis, testing, research and service improvement.” Effectively, the terms allow Facebook to experiment on its users, adding to the already lengthy list of privacy concerns connected to the social media site and others like it. Lead researcher Adam Kramer even said in an interview that he accepted a position at Facebook in part because, “Facebook data constitutes the largest field study in the history of the world.”

Facebook officials seemed surprised at the backlash from account holders who feel manipulated. They apologized for those feelings Sunday, but contended that the experiment was helpful for the company.

“Having written and designed this experiment myself, I can tell you that our goal was never to upset anyone,” Kramer wrote in a statement. “We were concerned that exposure to friends’ negativity might lead people to avoid visiting Facebook.”

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