It's the attitude adjustment that could save your life: A new study finds that happy people live longer -- which means a smile might turn out to be the cheapest, safest, and easiest longevity-booster on the planet.

Can you think of any drug or supplement that can slash your risk of a premature death by 35 percent? I can't -- but the study of 3,800 people between the ages of 52 and 79 found that happiness did just that, even after adjusting for age, gender, depression and other health and lifestyle risks.

All told, just 3.6 percent of the happiest people died during the five-year study -- versus 4.6 percent of those who had average levels of happiness and 7.3 percent of those who were unhappy.

The volunteers also answered questions about fear, anxiety and worry -- but none of those other attitudes seemed to have any effect on who lived and who died.

Just happiness -- although it could also be that the very things that make us happy also help us to live longer.

Married people, for example, live longer... and people who've been married a long time tend to be happier than those who are alone (even if we might joke otherwise).

Happier people also have a tighter circle of friends -- something that's also known to boost both longevity and happiness.

On the other hand, attitude alone really can have a direct and measurable impact on health, like a study a couple of years back that found happy people have a 22 percent lower risk of heart disease.

Another study I told you about over the summer found that an attitude closely linked to happiness -- optimism -- slashed the risk of a stroke.

And last year, researchers found that the most disagreeable people were more likely to have thicker carotid arteries -- which would explain that increase in stroke risk, not to mention the thick bulging neck veins seen on angry cartoon characters.

I know changing your attitude is easier said than done -- especially if you have years of experience in the grouch department. But while it's difficult, it's not impossible -- and if you can pull it off, it might just save your life.