For one week in 2012 - and without the explicit consent or knowledge of users - Facebook tampered with the algorithm used to place posts into their news feeds to study how this affected their mood.Share
The researchers wanted to see if the number of positive or negative words in messages they read affected whether users then posted positive or negative content in their status updates.
The study, conducted by researchers affiliated with Facebook at Cornell University and the University of California at San Francisco, appeared in the June 17 edition of the Proceedings of the National Academy of Sciences.
Results of the study spread when the online magazine Slate and The Atlantic website wrote about it on Saturday.
"Emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness," the study's authors wrote.
"These results indicate that emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks." (Read more.)