New research from the University of California in San Diego, and published in the journal PLOS One, suggests that happy status updates encourage happy updates from other users.

Previous studies have shown that emotion spreads among people in direct, person-to-person contact. This "emotional contagion" has been documented among friends, acquaintances, and even among strangers.

But how successfully this contagion is mediated through online relationships is less well known.

The researchers behind the new study analyzed over a billion anonymized status updates from more than 100 million Facebook users. The users were drawn from the top 10 most populous US cities, and their Facebook updating occurred during a period of 1,180 days between January 2009 and March 2012.

However, the researchers did not read any of these social media messages. Instead, they used a piece of software called the Linguistic Inquiry Word Count that assessed the emotional content of each post for them.

Using the randomness of weather to test cause and effect

As any regular users of Facebook will know, the state of the day's weather can have an across-board effect on the mood of your contacts. The researchers noticed that rainy weather increased the number of negative posts by 1.16% and decreased the number of positive posts by 1.19%.

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The researchers behind the new study analyzed over a billion anonymized status updates from more than 100 million Facebook users.

But the researchers were less interested in people's dislike of rain in general, and more in how one person's experience of rain could cause a ripple of negative feeling among people who were not currently being rained on.

A random variable such as the weather would allow the researchers to measure how the emotional tenor of one person's status updates negatively or positively influences the posts of their friends.

To make sure that it was the posts that were influencing mood, and that all the Facebook users were being rained on, the researchers restricted analysis to friends who were in different cities where there was no rain. And to make sure it was not the topic of rain that was the contagious element, they removed all weather-related status updates from their analysis.

According to the study, the negative posts by the people in rainy cities influenced the posts of their finds in non-rainy cities. Each negative post would prompt another 1.29 negative posts from a user's friends.

Happy status updates are more 'contagious' than unhappy updates

If this sounds depressing, then there is an optimistic twist in the results. Happy status updates had a more powerful influence than unhappy posts. Each happy post would encourage an additional 1.75 happy updates from a user's friends.

"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 in the Division of Social Sciences and of medical genetics in the School of Medicine 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," he adds.

In fact, Fowler and his team believe that emotion is even more contagious online than their study was capable of measuring.

"For our analysis," he says, "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."

Fowler believes that the study's findings are significant in understanding - and therefore being able to manipulate - public well-being:

"If an emotional change in one person spreads and causes a change in many, then we may be dramatically underestimating the effectiveness of efforts to improve mental and physical health. We should be doing everything we can to measure the effects of social networks and to learn how to magnify them so that we can create an epidemic of well-being."

Recently, Medical News Today reported on a study examining the psychological effects of meeting Facebook or other social media contacts face to face for the first time.