Facebook's data scientists conducted a massive experiment where it messed with people's feeds and proved that longer-lasting moods, like happiness or depression, can be transferred across the social network. 

The company tweaked the Newsfeed algorithms of689,003 unwitting Facebook users, so that people were seeing an abnormally low number of either positive or negative posts. 

In a recently published study, the scientists say they found that when people saw fewer positive posts on their feeds, they produced fewer positive posts and instead wrote more negative posts. On the flip side, when scientists reduced the number of negative posts on a person's newsfeed, those individuals became more positive themselves.

"Emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness,"