This giggly little girl may live a long life.

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Since at least 3500 B.C., people have been looking for ways to reverse aging and prolong life. From Ponce de Leon and the Fountain of Youth to Indiana Jones and the Holy Grail, each generation has eagerly sought out the path to immortality. While humanity continues its fruitless search for an anti-aging panacea, sensible doctors admonish patients to quit smoking, exercise regularly and follow a low fat diet. But who really wants to live forever if you can't have any fun doing it?

Yet, imagine those same doctors sending you out the door with a stack of Monty Python DVDs and knock-knock jokes instead of an Rx for a daily jog and a diet of brussels sprouts.

Based on mounting research, such a premise isn't too far off. Happiness, loosely defined as satisfaction and appreciation for one's life, appears to be the psychological equivalent of Vitamin C. In ways that aren't yet completely understood, a positive outlook on life helps to protect against the negative effects of stress, safeguarding people from becoming sick.

Not only are happy people less likely to send out surges of stress hormones like cortisol that contribute to heart disease, but they're also more likely to follow good self-care and have better social networks -- both reliable indicators of good physical health. Chronic unhappiness, on the other hand, leads to increased blood pressure and decreased immunity. Unhappy people lacking emotional support also don't tend to take very good care of themselves.

All of this means that a Pollyanna personality not only makes life more enjoyable, it could also extend that life by a significant number of years. Find out just how many on the next page.