It's no big secret. It's simple and helps you live really long. Smile - and make sure you mean it, says a new research.
Pro baseball players in the 1950s who genuinely beamed in their official photographs tended to outlive more sullen-looking sportsmen and those who put on fake smiles.
Players from the US major league with honest grins lived an average of seven years longer than players who didn't smile for the camera and five years longer than players who smiled unconvincingly, found Ernest Abel and Michael Kruger at Wayne State University in Detroit, Michigan.
While it is known that happy people tend to be healthy too, the researchers wondered if this relationship would be reflected in the smiles and longevity of baseball players, reports New Scientist.
Genuine smiles are known as Duchenne smiles after the 19th-century neurologist who defined them in detail.
They engage muscles both near the corners of the mouth and around the eyes - the zygomatic major and the orbicularis oculi respectively. Fake, "non-Duchenne" smiles exercise only mouth muscles.
With training, these muscles are easy to recognise in photographs.
Thus, the researchers who were not aware of what the study was investigating, but were trained to analyse smiles, looked at vintage photographs of 230 major leaguers who played in the 1952 season.
The researchers classified them as non-smilers, Duchenne smilers or non-Duchenne smilers. Then they looked up the lifespans of the 184 players who had already died.
They found that out of the dead players, Duchenne smilers had tended to live the longest, followed by non-Duchenne smilers.
The researchers also accounted for other factors that tend to predispose people to longevity, such as a university education and good health.
They found an even firmer link between strength of smile and length of life.
People who didn't smile had just a 50 per cent chance of surviving to 80, all other things being equal, whereas those with Duchenne smiles had about a 70 per cent chance of surviving to this age.
Overall, 35 per cent of the differences in lifespan correlated with smile intensity.
Abel and Kruger conclude that people who smile genuinely in photographs "may be basically happier than those with less intense smiles", making them more likely to experience the health benefits of happiness, which has been linked with lower levels of stress hormones and a protein implicated in heart disease.
The study has been published in the journal Psychological Science.