Facebook altered the tone of the users' news feed to highlight either positive or negative posts from their friends, which were seen on their news feed.
They then monitored the users' response, to see whether their friends' attitude had an impact on their own.
"The results show emotional contagion," wrote a team of Facebook scientists, in a paper published by the PNAS journal - Proceedings of the National Academy of Scientists of the United States.
"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."