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(Pocket-lint) - Facebook secretly tried to manipulate 689,000 users as part of a study to see if it could make them depressed.

Facebook is now apologising for the experiment which it, and two US universities, carried out without consent in 2012.  It was done to test if "exposure to emotions led people to change their own posting behaviours". Specifically sad emotions as it held back positive posts to see how that affected users. The same was done with negative posts.

While Facebook has apologised it also says there was "no unnecessary collection of people's data". The manipulation of emotions, though, was not denied.

Cornell University and UC San Francisco were also involved in this ethically questionable experiment.

Labour MP Jim Sheridan, a member of the Commons media select committee has called for an investigation in the matter. He told The Guardian, "This is extraordinarily powerful stuff and if there is not already legislation on this, then there should be to protect people".

He went on: "They are manipulating material from people's personal lives and I am worried about the ability of Facebook and others to manipulate people's thoughts in politics or other areas. If people are being thought-controlled in this kind of way there needs to be protection and they at least need to know about it."

However, Katherine Sledge Moore, a psychology professor at Elmhurst College, Illinois, said: "Based on what Facebook does with their newsfeed all of the time and based on what we've agreed to by joining Facebook, this study really isn't that out of the ordinary."

Adam Kramer of Facebook, who co-authored the report said: "I can understand why some people have concerns about it, and my co-authors and I are very sorry for the way the paper described the research and any anxiety it caused."

READ: Facebook News Feed redesign now finally rolling out after a year, with new UI changes

Writing by Luke Edwards.