Facebook conducted a massive experiment on the emotional response of users by manipulating what type of posts appeared in their News Feed.
The social network was able to make users feel either more positive or more negative, according to a new study by academics from Cornell and the University of California.
"This tested whether exposure to emotions led people to change their own posting behaviors," researchers said.
For one week in January 2012, Facebook filtered the content seen by about 689,000 people.
Researchers found users exposed to more negative comments tended to post more gloomy messages, and users exposed to more positive content posted more upbeat updates.
The phenomenon is known as "emotional contagion."
The study concluded: "Emotions expressed by friends, via online social networks, influence our own moods, constituting, to our knowledge, the first experimental evidence for massive-scale emotional contagion via social networks."
The change was only observed in as little as one-tenth of a percent of the users -- but authors point out "…in early 2013, this would have corresponded to hundreds of thousands of emotion expressions in status updates per day."
Researchers emphasized the content omitted from the News Feed was always available by viewing a friend’s content directly by going to that friend’s "Wall" or "Timeline."
Now many are questioning whether the study was legal and/or ethical. Some legal experts say Facebook did not secure informed consent from the users involved.
The study was partially funded by the Army Research Office -- an agency within the U.S. Army that funds basic research in the military’s interest - according to a press release from Cornell University.