In order to investigate whether the emotions of other social media users could lead "people to experience the same emotions without their awareness," researchers manipulated 689,003 users' News Feeds to show statuses that were especially positive or negative.
The results were published in the Proceedings of the National Academy of Sciences in an article called "Experimental evidence of massive-scale emotional contagion through social networks."
"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," according to the authors.
It turned out that users exposed to negative content were a fraction of a percent more likely to produce negative content, and those shown more positive content were more likely to produce positive content -- though a fraction of a percent on the scale of millions of users is not so small.
Users were never told that the tweaking of the News Feeds would be taking place, but the manipulation was legal according to the terms listed in the user agreement, which specify that user data can be used for "research."
One of the authors of the study, Adam Kramer, who is also a Facebook employee, attempted to explain the experiment and apologize for any harm it may have caused in a post on his personal Facebook page.
"The goal of all of our research at Facebook is to learn how to provide a better service. Having written and designed this experiment myself, I can tell you that our goal was never to upset anyone," Kramer wrote. "I can understand why some people have concerns about it, and my coauthors and I are very sorry for the way the paper described the research and any anxiety it caused. In hindsight, the research benefits of the paper may not have justified all of this anxiety."