Tanya Lewis in Scientific American:
Could reading a cheerful or depressing post on Facebook influence your own mood? Apparently so, according to a new study conducted by the social networking company. When Facebook removed positive posts from the news feeds of more than 680,000 users, those users made fewer positive posts and more negative ones. Similarly, when negative posts were removed, the opposite occurred. The findings provide experimental evidence that emotions can be contagious, even without direct interaction or nonverbal cues, the researchers say. [The Top 10 Golden Rules of Facebook] “These results indicate that emotions expressed by others on Facebook influence our own emotions,” the authors wrote in the study published June 17 in the journal Proceedings of the National Academy of Sciences.
The idea that emotional states can spread among people without their awareness, known as emotional contagion, has been shown before in laboratory experiments. One study found that lasting moods such as depression and happiness can be transferred via a real-world social network, but the findings have been controversial because it was based on correlational evidence and could not rule out other potential variables. Facebook researchers decided to look for evidence of emotional contagion among users of its online social networking site. Facebook users often express emotions on the site, which their friends can view in their personal news feed.