Here’s some news that, at least in theory, should make everyone smile.

Happiness is catching.

From the Los Angeles Times:

They say misery loves company, but the same may be even more true of happiness.

In a study published online today by the British Medical Journal, scientists from Harvard University and UC San Diego showed that happiness spreads readily through social networks of family members, friends and neighbors.

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.

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.

The new study “has serious implications for our understanding of the determinants of health and for the design of policies and interventions,” wrote psychologist Andrew Steptoe of University College London and epidemiologist Ana Diez Roux of the University of Michigan in an accompanying editorial.

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 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.

“We know people who are most susceptible to HIV are people who have lots of partners,” Fowler said. “This is the same thing.”

This isn’t the first evidence that emotions can spread like a virus. Studies have found that waiters who offer service with a smile are rewarded with bigger tips. On the flip side, having a mildly depressed roommate made college freshmen increasingly depressed themselves.

There’s much more happy news at the link.