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Hundreds of thousands of Facebook users were stunned to learn the social network had been manipulating their emotional state without their knowledge as part of a massive secret psychological experiment.

Scientists working for the social network made users into lab rats for a week in January 2012, to see if meddling with the posts they saw when they logged in would change their emotions.

The study found that by messing with the news feeds of 689,003 users, researchers could change the kind of status updates they'd make.

If they were shown more negative posts from friends, the users would make more negative status posts, and the other way around.

The research paper, released in the PNAS journal this week, reads: "emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness."

It's not a huge change - just one-tenth of a percent difference - but that could mean hundreds of thousands of status updates a day.

Facebook's terms and conditions, which all users agree to, allow them to carry out research of this kind without telling users they're being experimented on.

The paper says the automated testing: "“was consistent with Facebook’s Data Use Policy, to which all users agree prior to creating an account on Facebook, constituting informed consent for this research.”

The researchers say they weren't able to see individual users posts at any time, and the positive and negative posts were selected automatically.

In a statement, the social network defended the research.

"This research was conducted for a single week in 2012 and none of the data used was associated with a specific person's Facebook account," they said.

"We do research to improve our services and to make the content people see on Facebook as relevant and engaging as possible. A big part of this is understanding how people respond to different types of content, whether it's positive or negative in tone, news from friends, or information from pages they follow. We carefully consider what research we do and have a strong internal review process."