Your Mood Is Contagious, and Your Friends Catch It

Your mood is not entirely your own. It moves between people, and people in the same cluster of friends tend to share a mood.

Nicholas Christakis, a hospice doctor who studies social networks, suggests emotions work as a primitive kind of communication. When people feel something, they show it, others can read that display, and then they copy it.

On a subway, one stranger smiles, and the other smiles back without thinking. That is a brief exchange between two people.

He wondered whether the same copying could run in a more lasting way, over time and across many people, not just two strangers trading smiles. He raised the idea of a "quiet riot" under everyday life, or "emotional stampedes" moving through networks.

To explore the idea, he showed one of the first images his group made to study it. In the picture, each person was colored yellow if happy, blue if sad, and green if in between. The colors fell into patches, with happy people near happy people and sad people near sad people, reaching three degrees of separation.

In that picture, you might also get the sense that sad people hold a different spot. Networks have a middle and an edge, and the blue figures seemed to sit toward the edges.

In his quilt picture, the social fabric is made of cheerful and gloomy patches. In a remark about the picture, he offered the view that landing in a happy patch plays a part in whether you become happy.