Take a close look at that glass of water. Half empty? Half full? What you see could make a difference, not only in your daily health, but in how long you live.

So say the results of a new Mayo Clinic study that tracked 839 people over 30 years. In the 1960s, study participants took a standardized test to determine whether they were optimistic, pessimistic or somewhere in between. Those who scored high on the pessimism scale turned out to have a 19% greater chance of premature death than those who scored more optimistically.

The Power of Optimism

"I believe we have compelling evidence that optimists and pessimists differ markedly in how long they will live," says psychologist Martin Seligman of the University of Pennsylvania in his editorial accompanying the study. "It is not clear if pessimism shortens life, optimism prolongs life, or both."

Seligman says there are at least four ways that optimism can affect longevity:

  • Optimists tend to be less passive than pessimists and less likely to develop learned helplessness or negative and debilitating responses to things that happen to them.
  • Optimists tend to be more likely to practice preventive health measures because they believe their actions make a difference.
  • Optimists suffer depression at a markedly lower rate than pessimists; depression is associated with mortality.
  • Optimists' immune systems have been shown to function more effectively than those of pessimists.

Learning to See the Bright Side

For decades, psychologists have studied the link between positive thinking and physical and mental health. According to Seligman, author of Learned Optimism: How to Change Your Mind and Your Life , it's more important to change negative thought patterns into positive ones than to worry about being optimistic. The picture of optimism he paints is not one of Pollyanna-like blindness to reality, but of a learned optimism grounded in accuracy and non-negative thinking.

Based on the results of several large-scale, long-term, carefully controlled experiments, Seligman discovered that optimists are more successful than pessimists. Optimistic politicians win more elections, optimistic students get better grades, optimistic athletes win more contests, and optimistic salespeople make more money.

Why would this be so? In his book Self-help Stuff That Works , Adam Kahn says it is "Because optimism and pessimism both tend to be self-fulfilling prophecies. If you think a setback is permanent, why would you try to change it? Pessimistic explanations tend to make you feel defeated, making you less likely to take constructive action. Optimistic explanations, on the other hand, make you more likely to act. If you think the setback is only temporary, you're apt to try to do something about it."

Optimist vs. Non-optimist

How can you determine whether you think more optimistically or pessimistically?

"I don't like to use the word pessimistic because most people would never consider themselves pessimistic," says Khan, "but many people are willing to admit they aren't optimistic."

Khan, like Seligman and other experts on motivation, defines optimists and non-optimists by how they explain events in their lives. Optimists see setbacks as specific, temporary and changeable, and are therefore motivated to take action. Non-optimists tend to look at setbacks as general, permanent and hopeless, symptoms of widespread failure that cannot be changed.

For example, an optimist who didn't follow through on an exercise routine for a week might say, "I had a lot going on this week. I didn't plan my time too well. I'll have to do better next week." A pessimist in the same situation might say, "I have no self-discipline. I obviously won't be able to meet my goals. Exercise just isn't for me."

A Matter of Degree

Dr. Pierce Howard, author of The Owner's Manual for the Brain, contends that the line between optimism and pessimism is far from clear-cut.

"You're not just an optimist or a pessimist, it's a matter of degree," Dr. Howard says. "You can be successful in life anywhere along the continuum." He points out that pessimistic thinkers make great tax accountants, while optimists are more suited for careers in sales.

Getting into a Good Mood

Mood also has an influence on whether optimistic or pessimistic thoughts dominate your brain, according to Dr. Susan Vaughan, psychiatrist, psychoanalyst and researcher whose latest book, Half Empty, Half Full, explores how working to gain control over moods can result in more positive thinking.

"Mood is a powerful filter on how we see things," maintains Vaughan, who sees most people as a blend of optimism and pessimism, depending on the situation with which they are faced.

She points to three methods optimistic people tend to use to lift their moods:

  • Alternative thinking—When bad things happen, optimists tend to take them less personally and come up with multiple alternatives for why they might have happened, then work actively to fix the situation.
  • Downward comparison—Though it sounds unkind, optimists compare themselves to others who are in worse situations as a way to brighten their own spirits.
  • Relaxation—Optimists tend to use exercise, yoga, and even "putting on a happy face" as ways to relax and thereby improve their moods.

Optimism Not Always the Answer

"The idea that optimists are healthier than pessimists is overly simplistic," says Dr. Howard Friedman, a psychology professor at the University of California, Riverside. "Many times, excessive optimism can be harmful to one's health. This is especially evident among teenagers, who take many risks."

Friedman contends it can be damaging to think optimistically when it comes to difficult health choices like quitting cigarettes, using condoms, or wearing seatbelts.

"I do not agree that in general we could try to make everyone more optimistic. There is absolutely no evidence that trying to do so will improve the general health of the population," Friedman says.

Choosing the Right Strategy

Seligman concurs that there are times when it pays not to be optimistic, such as when planning for a risky future, when advising those with poor chances for the future and when trying to be sympathetic to others' problems. When the cost of failure is high, he advises, optimism is the wrong strategy.

Still, there are times when optimism can be a powerful ally. When achievement is the goal, use optimism. If you're fighting off depression, optimistic thoughts can boost your morale.

Changing From Negative to Positive

Seligman argues that optimism, like other interpersonal skills, can be learned.

"The way you explain setbacks to yourself is as much a habit as the way you tie your shoes," agrees Khan. "It is no harder or easier to change a thought habit than it is to change a physical habit." He recommends writing about setbacks and practicing arguing with your less optimistic thoughts until a more realistic vision of what has happened and what is likely to happen in the future emerges.

"It takes work, discipline, and focus," Khan says. "But if you don't think you have these things, those are the first non-optimistic thoughts to tear apart."