A Facebook data scientist is apologizing for a study that angered many Facebook users by trying to manipulate their emotions based on the posts they saw in their News Feeds.

In the study, published in March in the Proceedings of the National Academy of Sciences, the researchers said they changed the algorithms on News Feeds of almost 700,000 Facebook users for one week in January 2012, to see whether a mostly positive -- or a mostly negative -- News Feed would elicit different types of status updates.

The study concluded that when positive expressions were reduced, people produced fewer positive posts and more negative posts; when negative expressions were reduced, the opposite pattern occurred. These results indicate that emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks.

However, Facebook soon faced a backlash from users who felt that the social network was improperly trying to mess with their emotions -- or at least should have notified them that their News Feeds may have been modified for experimental purposes.

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