London - Smile and the world smiles with you, the old saying goes. And it may be more relevant than ever for the internet age.

It is well known that good moods can be contagious, but researchers now think that happiness can even spread through the world wide web.

American researchers studied social networking interactions over three years and found that positive messages spread through contacts.

They analysed more than a billion Facebook status updates, among more than 100 million users, and found that positive statements generated similar messages among friends.

The researchers, from the University of California, analysed status updates on Facebook in the top 100 most populous cities in the US between January 2009 and March 2012, using automated text analysis.

And they found that happy messages spread further than miserable ones.

The study, published in the journal PLOS ONE, examined posts by people in cities where it was raining - known to increase the number of negative posts and reduce the number of positive ones - and the impact this had on the status updates of people in cities where it was not raining.

To make sure there was no “topic contagion” all weather-related status updates were removed from the analysis.

Each additional positive post generated an estimated 1.75 positive posts amongst friends while each additional negative post yielded 1.29 more negative posts.

James Fowler, professor of political science at University of California, said: “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. “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 added that he believed the study probably under estimated how much emotion spreads through digital social media.

“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 said the findings raised the prospect of “greater spikes” in global emotion in the future with the growth in online communication, causing volatility in all aspects of life from political systems to financial markets.

It was possible that social media could be harnessed to create an “epidemic of well-being”, they suggested.

“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,” Prof Fowler said.

“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 wellbeing.” - Daily Mail