Emotions can spread like contagion among users of online social networks, according to a recent study.

Researchers from Cornell University, the University of California, San Francisco (UCSF), and Facebook found out about the so-called "emotional contagion" effect by reducing the amount of either positive or negative stories that appeared in the news feed of 689,003 randomly selected Facebook users.

"People who had positive content experimentally reduced on their Facebook news feed, for one week, used more negative words in their status updates," Jeff Hancock, professor of communication at Cornell's College of Agriculture and Life Sciences and co-director of its Social Media Lab, said in a statement. "When news feed negativity was reduced, the opposite pattern occurred: Significantly more positive words were used in peoples' status updates."

For the study, researchers never saw the content of actual posts, per Facebook's data use policy; instead, they counted only the occurrence of positive and negative words in more than 3 million posts with a total of 122 million words. They report that 4 million of those words were "positive" and 1.8 million were "negative."

Hancock said peoples' emotional expressions on Facebook predicted their friends' emotional expressions, even days later.

"We also observed a withdrawal effect: People who were exposed to fewer emotional posts in their news feed were less expressive overall on the following days," Hancock wrote in the paper. "This observation, and the fact that people were more emotionally positive in response to positive emotion updates from their friends, stands in contrast to theories that suggest viewing positive posts by friends on Facebook may somehow affect us negatively."

Hancock plans to direct future research into how expressions of positive and negative emotions influence levels of engagement in other online activities, such as liking and commenting on posts.

Researchers said the findings could have implications for public health.

"Online messages influence our experience of emotions, which may affect a variety of offline behaviors," Hancock said.