NEW YORK (CBSNewYork) – An experiment on Facebook users without their knowledge has the Internet buzzing.

“In an experiment with people who use Facebook, we test whether emotional contagion occurs outside of in-person interaction between individuals by reducing the amount of emotional content in the News Feed,” wrote the authors of a study published in the Proceedings of the National Academy of Sciences.

“The experiment manipulated the extent to which [689,003 people] were exposed to emotional expressions in their News Feed,” the authors wrote.

The experiment took place in 2012 without the knowledge of the impacted users.

As part of the experiment, the social network changed its algorithms for the news feed of those users, so they would see either fewer positive or negative posts.

“When positive expressions were reduced, people produced fewer positive posts and more negative posts; when negative expressions were reduced, the opposite pattern occurred,” the authors wrote.

In a subsequent Facebook post, the study’s author apologized for causing a stir.

“At the end of the day, the actual impact on people in the experiment was the minimal amount to statistically detect it – the result that people produced an average of one fewer emotional word, per thousand words, over the following week,” wrote Adam Kramer. “Having written and designed this experiment myself, I can tell you that our goal was never to upset anyone. I can understand why some people have concerns about it, and my coauthors and I are very sorry for the way the paper described the research and any anxiety it caused. In hindsight, the research benefits of the paper may not have justified all of this anxiety.”

Lane Ulanoff, with the digital news site, said that the experiment could raise legal and ethical questions for Facebook.

“The problem is they didn’t tell anyone they were running this study. So, when people found out about it they kinda freaked out,” he told CBS 2’s Kathryn Brown.

Many Facebook users at Kave espresso bar and internet cafe in Bushwick were unpleasantly surprised.

“Well how could they get away with it? How could they do it? How is this even allowed? It’s sort of like a social experiment that Facebook has sort of taken upon themselves to conduct on the masses,” Dean Brown said.

Facebook defended the study and pointed out that users are supposed to read and have to agree to a data use policy which gives Facebook the right to use information for certain purposes including research.

The revelation has users, and privacy activists, wondering whether the website should be more explicit about how it is using information, and why.

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