Smiling probably evolved from displays of social submission. Chimpanzees make a smiling face when they are afraid. In humans this may have later evolved into an expression to diffuse aggressive encounters. Now that smiling indicates a positive mood, it’s also correlated with reduced stress hormones and blood pressure. Numerous studies have linked both of these to your chances of having a heart attack. Marriage also lowers your heart attack risk and a 2009 study found that people who frown in college yearbook photos are five times more likely to get a divorce than those who smile. And overall, smiling correlates with good health. Researchers who looked at photos of 230 baseball professionals from the 1950s found that the players with the most genuine smiles tended to live the longest.