Knowing someone who is happy makes you 15.3% more likely to be happy yourself, the study found. A happy friend of a friend increases your odds of happiness by 9.8%, and even your neighbor's sister's friend can give you a 5.6% boost.

"Your emotional state depends not just on actions and choices that you make, but also on actions and choices of other people, many of which you don't even know," said Dr. Nicholas A. Christakis, a physician and medical sociologist at Harvard who co-wrote the study.

Christakis and UCSD political scientist James H. Fowler examined the relationships of nearly 5,000 people who were tracked for decades as part of the landmark Framingham Heart Study.

They discovered that happy people in close geographic proximity were most effective in spreading their good cheer. They also found the happiest people were at the center of large social networks.

In many regards, they concluded, happiness is like a contagious disease.