The urge to smoke is contagious, but quitting apparently is, too. A team of researchers who showed that obesity can spread person-to-person has found a similar pattern with smoking cessation_ A smoker is more likely to kick the habit if a spouse, friend, co-worker or sibling did.

What's more, smokers tend to quit in groups and those who don't stop puffing increasingly find themselves pushed to the edge of their social circles, the researchers found.

"Your smoking behavior depends upon not just the smoking behavior of the people you know, but also the people who they know" and so on, said Dr. Nicholas Christakis, a medical sociologist at Harvard Medical School and lead author of the new report.