Perhaps you are familiar with the phenomenon: You're bummed out when a friend comes waltzing in, all smiles and cheer, and soon enough, you're in a better mood.

It's this simple logic that led scientists to compare the way we spread emotions like happiness and sadness with the way we spread infectious disease.

We already have scientific models to track the spread of disease. Allison Hill, a biophysicist at Harvard University, used this same type of model to track the spread of emotion. Her team used social and medical data from a large population in Massachusetts that has been collected since 1948.

Basic study of infection is conducted by putting people into clusters of infected and uninfected. Emotion can be tracked in the same way.

People who showed strong signs of happiness increased the chances of others around them becoming happy by 11%. That should put a smile on our faces!

Unfortunately, the study found that sadness if even more contagious; one sad person can double others' chances of becoming sad.

The study also showed that the more friends you have, the more likely you are to get infected, both by their emotions and their head colds.

Read More:

• Happiness and Sadness Spread Like Emotion (WiredScience)
• Emotions as Infectious Diseases in a Large Social Network: the SISa Model (TheRoyalSociety)

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