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Facebook has 1.28 billion worldwide active users. It has the most data set suitable for studying human behavior. The company's data scientists conducted experiments recently, by manipulating the News Feeds of 689,003 users to study whether online emotions can be contagious.
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
This isn't new at Facebook. They've been conducting scientific studies for years already. MIT Technology Review reported back in 2012 that Mark Zuckerberg himself was using the network's influence to conduct personal experiments:
Influenced in part by conversations over dinner with his med-student girlfriend (now his wife), Zuckerberg decided that he should use social influence within Facebook to increase organ donor registrations. Users were given an opportunity to click a box on their Timeline pages to signal that they were registered donors, which triggered a notification to their friends. The new feature started a cascade of social pressure, and organ donor enrollment increased by a factor of 23 across 44 states.
Obviously, no one likes to be part of a study without consent or knowing about it. Adam Kramer, the Facebook data scientist who co-authored the study posted a response to the study’s backlash on his Facebook page noting, “In hindsight, the research benefits of the paper may not have justified all of this anxiety.”