LONDON - When you're smiling, the whole world really does smile with you.

A paper being published today in the British Medical Journal concludes that happiness is contagious and that people pass on their good cheer even to strangers.

American researchers who tracked more than 4,700 people in Framingham, Mass., as part of a 20-year heart study also found the transferred happiness is good for up to a year.

The study is another sign of the power of social networks. It ran through 2003, just before the rise of social-networking Web sites like Friendster, MySpace and Facebook. Nicholas Christakis, a professor in Harvard University's sociology department and co-author of the study, said he couldn't say for sure whether the effect works online.

Christakis estimates that each happy friend boosts your own chances of being happy by 9 percent. Having grumpy friends decreases it by about 7 percent.