If you've ever wondered if your mood really does affect others, a new study reported in the British journal BMJ brings some scientific evidence to the idea that, for better or for worse, mood is actually contagious.

... People who rated themselves the happiest tended to be at the center of their social networks.

Researchers James Fowler at the University of California, San Diego and Nicholas Christakis of Harvard Medical School combed the questionnaires of over 4,700 individuals, from 1983-2003. Participants were the descents of the original subjects in the Framingham Heart Study, which started in 1948 and continues today. Fowler and Christakis wondered if, by studying the social networks of the participants, some pattern of happiness and grumpiness could be traced among acquaintances.

Indeed, the researchers found just that. There were distinct clusters of happy and unhappy people, beyond what would be expected by chance. Fowler and Christakis call what they found the "cascade" effect: people who rated themselves the happiest tended to be at the center of their social networks. Their friends also tended to be quite happy — in fact, the friends' odds of being happy increased by 25%. Friends of those friends had an almost 10% higher chance of being happy. The cascade reached outward from the central individual for three degrees (to his or her friends' friends' friends).

Interestingly, it isn't necessary for all people in the network to know one another in order to be affected by the cascade. Fowler points out that the people in the middle of the social network may be people "you have never met. But their mood can have a profound effect on your own mood."

Although grumpiness also tended to spread in a similar cascade pattern, the effect wasn't quite as pronounced. The authors estimate that having unhappy friends lowers your odds of being happy only by about 7%.

Fowler says that the findings of the study affected his own outlook on the world. "To think about the way we're connected to one another has caused me to take more responsibility for my own actions," he said. "If I head home in a happy mood, I'm not just making my son happy, I'm potentially making my son's friend happy. I'm not just making my wife happy, I'm making my wife's mother happy."