Facebook conducts a psychological study of 600,000 users, who are not informed about the test. It tests whether positive or negative posts by people’s friends affect their own emotional response to a topic. The paper, published in the Proceedings of the National Academy of Sciences, finds that it does have an effect:
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.