Facebook under fire for massive experiment carried out without users' knowledge

Facebook has come under fire after carrying out a psychological experiment on nearly 700,000 people without their knowledge.

The experiment, which was carried out in 2012, manipulated content on users' newsfeeds to see whether it would affect their moods.

In a study with academics from Cornell and the University of California, Facebook filtered users' news feeds – the flow of comments, videos, pictures and web links posted by other people in their social network.

In an introduction to the study, the authors say: "We show, via a massive experiment on Facebook, that emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness.

"We provide experimental evidence that emotional contagion occurs without direct interaction between people (exposure to a friend expressing an emotion is sufficient), and in the complete absence of nonverbal cues."

One test reduced users' exposure to their friends' "positive emotional content", resulting in fewer positive posts from the observed users. Another test reduced users' exposure to "negative emotional content" - and fewer negative posts was the result.

It is claimed that Facebook may have breached ethical and legal guidelines by not telling users they would be involved.