This campaign season, Barack Obama has branded himself as the candidate of hope. And many voters and pundits have swooned, enchanted by his message of positivity and his faith that better times are ahead for the nation and its citizens.

Perhaps his rhetoric resonates because, as it turns out, humans have a natural bias toward such a positive outlook. A majority of people, for example, think they will live longer than average - a mathematical impossibility - and most people underestimate their chances of getting a divorce or cancer.

"The general population is optimistic," said Tali Sharot, a neuroscientist at University College London. Depending on how the trait is measured, studies show that as many as 80 percent of people display characteristics of optimism.

Its prevalence suggests that optimism has a biological basis, which scientists are beginning to identify in the brain, and is beneficial from an evolutionary standpoint.

Optimism - and pessimism - are considered stable personality traits, and it is unclear whether people can change their natural tendencies. Pessimists need not fret, however, because early research shows that gloomier outlooks may be advantageous in their own way.

Sharot, who normally studies emotion and memory, became curious about optimism when she noticed that her test subjects were more prone to imagine positive events than negative ones in the future.

In a follow-up study, she found further evidence of this optimism bias. When she asked volunteers to imagine the future, "they were more likely to imagine positive events near in the future and negative events as distant in the future," she said.

Sharot and her colleagues then scanned subjects' brains as they thought about the future, and observed that imagining happy occurrences, such as winning an award, triggered more activity in regions of the brain involved with processing emotions than imagining negative experiences.

The findings, she said, point toward the neurological basis of optimism.

Other studies, of identical and fraternal twins, have revealed that optimism is partly inherited. The other contributing factors are unknown.

Sharot thinks that optimism makes good evolutionary sense.

"It gives people motivation to get up in the morning," she said. "If they have optimistic projections, they're more likely to be able to work and function."

Of course, some people are naturally disposed toward pessimism, meaning that they tend to expect bad outcomes.

Pessimism can be useful for some situations and people, said Wellesley psychologist Julie Norem, who studies a coping strategy that she calls defensive pessimism. When defensive pessimists think about the future, they imagine all the things that could go wrong and then plan for the worst-case scenario. Norem has found that, for people who are naturally anxious, defensive pessimism is actually a better coping strategy than optimism.

"My evidence shows that taking anxious people and making them optimistic doesn't make them do better," she said. "In fact, they do less well and aren't as happy."

The biggest problem with pessimism, as Norem sees it, is merely how it is judged. "American culture highly values optimism," she said. "There are potential costs if others see you as pessimistic."

Other researchers, however, say that there are plenty of reasons to prefer optimism. Many studies have shown that people with rosy outlooks are healthier, with a reduced risk of dying from cardiovascular disease and better lung function, for example.

"Folks who do have a more optimistic outlook seem to have a more adaptive immune response to the environment," said Rosalind Wright, a physician at Brigham and Women's Hospital and an assistant professor at Harvard Medical School, who has studied optimism and pulmonary health.

Though the details are still unclear, Wright believes that positivity may affect the levels of hormones such as cortisol and adrenaline that the body releases during times of stress, weakening the immune system.

Some researchers remain skeptical of such ideas, pointing out that optimists may smoke less and sleep more, for example, or be healthier for other indirect reasons.

"There's still a lot to be sorted out," Norem said. "When we find a relationship between some aspect of immune system functioning and some kind of optimism, that doesn't mean that if you change one, you change the other."

Since optimism has a clear effect on how people think about the future, researchers have also become interested in exploring how positivity might influence economic decisions. Indeed, economists at Duke found that compared to pessimists, optimists work more hours per week, save more money, are more likely to own stock, and are more likely to say that they're never going to retire.

But being overly optimistic can pave the way to disaster.

"The moderate optimists are prudent people," said David Robinson, who conducted the Duke study. "They pay their credit cards on time. They tell you that they save because saving is a good thing to do. ... Extreme optimists are just the opposite. They have short planning horizons, they don't pay their credit cards off on time. As you get extremely optimistic, the good behaviors drop off."

Extreme optimists, he theorizes, don't think they have to plan for the future.

Other economists have also found that positive thinking can be a hindrance when running a business. In a 2005 article in the Harvard Business Review, Dan Lovallo of the University of Western Australia and Princeton economist Daniel Kahneman found that business executives are often overly optimistic in deciding to launch new ventures, leading them to underestimate the costs and risks.

The importance of positivity can vary by profession. University of Pennsylvania psychologist Martin Seligman, a leading researcher on optimism, has found that pessimistic law students are the most successful. Optimistic sales agents, on the other hand, significantly outsell pessimistic ones.

As for presidential candidates? According to Seligman's analysis of presidential elections between 1948 and 1984, optimists usually win. Pessimists lost 9 of those 10 elections.