PhotoA study finds that Facebook feelings are contagious, with positive posts begetting more positive posts and negative posts begetting more negative ones.

The study of more than 100 million U.S. Facebook users, conducted by researchers at the University of California, San Diego, finds that positive posts are a bit more influential than negative ones. Published in PLOS ONE, the study analyzes over a billion anonymized status updates.

"Our study suggests that people are not just choosing other people like themselves to associate with but actually causing their friends' emotional expressions to change," said lead author James Fowler, professor of political science at UC San Diego. "We have enough power in this data set to show that emotional expressions spread online and also that positive expressions spread more than negative."

There is abundant scientific literature on how emotion can spread among people – through direct contact, in person – not only among friends but also among strangers or near-strangers. Little is known, though, about emotional contagion in online social networks. 

Rainy days and Sundays

The researchers analyzed anonymous English-language status updates on Facebook in the top 100 most populous cities in the U.S. over 1,180 days. Researchers did not view any names of users or even the words posted by users. They relied on automated text analysis, through a software program called the Linguistic Inquiry Word Count, to measure the emotional content of each post.

To find if there's a causal relationship, the researchers needed to run an experiment. They found a natural one in rain. Rainy weather, it turns out, reliably changes the tenor of posts – increasing the number of negative posts by 1.16 percent and depressing the number of positive by 1.19 percent.

To make sure that rain was not affecting the friends directly, they restricted their analysis to friends who were in different cities where it was not raining, and to make sure it was not topic contagion, they removed from their analysis all weather-related status updates.

So, did the change in emotional expression by the people being rained on induce a change in their friends that stayed dry? Yes. According to the study, each additional negative post yields 1.29 more negative posts among one's friends, while each additional positive post yields an additional 1.75 positive posts among friends.

Fowler said the study probably underestimates how much emotion spreads through a digital social network.

"It is possible that emotional contagion online is even stronger than we were able to measure," he said. "For our analysis, to get away from measuring the effect of the rain itself, we had to exclude the effects of posts on friends who live in the same cities. But we have a pretty good sense from other studies that people who live near each other have stronger relationships and influence each other even more. If we could measure those relationships, we would probably find even more contagion."

The researchers believe their findings have widespread implications. Emotions, they write, "might ripple through social networks to generate large-scale synchrony that gives rise to clusters of happy and unhappy individuals." And with ever more avenues for expression in a digitally connected world, they write, "we may see greater spikes in global emotion that could generate increased volatility in everything from political systems to financial markets."

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