Multiple studies have shown that how much a person smiles in childhood photographs can predict their future happiness. And now, thanks to baseball playing cards, we know smiling actually makes you live longer.
Of course, it isn't actually the smiling itself that makes people live longer, but rather the positive emotions and outlook one can infer from how much a person smiles in his or her photographs. Smile intensity in childhood photographs has been shown to be correlated with future marriage stability and satisfaction. However, researchers have generally been limited in what correlations they can test, because it's hard to put together large groups for which you've got both photos and detailed biographical information.
That's where baseball players enter the picture. Baseball cards go back to the nineteenth century, and professional baseball players just might be the most extensively researched group of humans in history, so there's no shortage of biographical data. Researchers at Michigan's Wayne State University put smile intensity to the test by examining how well the players' smiles predicted their longevity, then controlled for all the other possible variables that might affect lifespan.
The researchers removed a ton of potentially confounding variables, including "college attendance, marital status, birth year, career length, age at debut year, and BMI." Once they had controlled for all those factors, they found smile intensity really did do a good job predicting longevity - it accounted for 35% of the total variability in players' survival chances. Not bad for smiling once a year when your picture is taken.
There were other factors the researchers found that helped longevity - players who attended college had a "44% reduced risk of dying in any particular year", and for every addition season a player stayed in baseball, he enjoyed a 4% increased chance of surviving in a given year. Those two seem to make sense - players who are able to keep playing longer are likely to, in general, be fitter than those who have shorter careers, and they probably have more accrued income to spend on healthcare down the road. As for college attendance, more recent players are more likely to have attended college, so they also enjoy the generally increased life expectancy in the overall population.
Now, if you'll forgive me for willfully misinterpreting this study, I present to you a foolproof plan for immortality. To wit:
1. Become a professional baseball player.
2. Never, ever retire.
3. For the love of god, SMILE in your baseball cards!
And, hey presto, you should live forever. What could be simpler? As an added bonus, you should make many millions of dollars in the process. But be careful to only try this in professional baseball. The health benefits of playing professional football forever are rather less certain.