Happiness spreads readily through the social networks of family members, friends and neighbors, U.S. researchers say in a study published Thursday.

Knowing someone who is happy makes you 15.3 percent more likely to be happy yourself, according to the study conducted by scientists from Harvard University and the University of California at San Diego.

A happy friend of a friend increases your odds of happiness by 9. 8 percent, and even your neighbor's sister's friend can give you a 5.6 percent boost, said the study appearing online in the British Medical Journal.

The researchers 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.

The researchers based their conclusion on a study of the relationships of nearly 5,000 people tracked for decades.

The results showed that a happy friend who lives within 0.8 km makes you 42 percent more likely to be happy yourself. If that same friend lives 3.2 km away, his impact drops to 22 percent. Happy friends who are more distant have no discernible impact.

Similarly, happy siblings make you 14 percent more likely to be happy yourself, but only if they live within 1.6 km. Happy spouses provide an 8 percent boost -- if they live under the same roof. Next-door neighbors who are happy make you 34 percent more likely to be happy too, but no other neighbors have an effect, even if they live on the same block.

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

"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.

The research is part of a growing trend to measure well-being as a crucial component of public health. Scientists have documented that people who describe themselves as happy are likely to live longer, even if they have a chronic illness.