Apparently, we can't help ourselves. As a species, we believe we will get that raise, win that poker game, build that dam, and successfully woo that special someone.

A propensity for optimism has long been a defining element of human culture, from Alexander the Great just saying "yes" to conquering much of the ancient world to a Harvard dropout named Bill becoming the richest man in the world. "A pessimist sees difficulty in every opportunity," said Winston Churchill. "An optimist sees opportunity in every difficulty."

Just ask your brain. In a recent study conducted at New York University, cognitive neuroscientist Elizabeth Phelps and her former graduate student Tali Sharot used functional magnetic resonance imaging to reveal that our brains are far more active when we sugarcoat the future than when we are negative about what's going to happen. The study appeared last month in the scientific journal Nature.

Scientists have long known that people tend to be overly optimistic about future events. Couples getting married believe they will avoid divorce, and contractors insist they will replace the roof on your house faster than often actually happens.

In Britain, the Department for Transport recently issued a "Supplementary Green Book Guidance on Optimism Bias," which mandates adjustments, based on distortions caused by optimism, to estimates of project costs, benefits, and duration. The guidelines are necessary, it said, because "there is a demonstrated, systematic tendency for project appraisers to be overly optimistic."

In eons past, this optimism may explain why a relatively frail, slow-footed creature with small teeth overcame so much to dominate the earth. Saber-toothed tigers? Build a better spear. Freezing caves in the winter? Build a bonfire. Trekking thousands of miles across tundra and land bridges to North America during the last ice age? Let's go!

The N.Y.U. team scanned the brains of 15 volunteers who were told to envision various possible life events, positive and negative-from winning a prize to losing a lover. Subjects were asked to envisage some events as having happened in the past, and some in the future.

The question posed by the team: Does our brain interpret bad and good outcomes differently?

One way to assess this is to see what areas of the brain light up when a person thinks optimistically or pessimistically-that is, which regions surge with blood flows that signal brain activity that the fMRI can detect.

Vivid memories "lit up" the amygdala and the rostral anterior cingulated cortex. Negative reminiscences caused much less energetic activity in these areas of the brain, which are important to regulating emotions.

Curiously, the same thing happened when subjects contemplated future actions, suggesting that the brain in most people may mute the effect of anticipating potentially upsetting events.

When subjects who were scanned filled out questionnaires after the tests, they reported feeling optimistic about future events, and that they imagined them happening more vividly and sooner than potentially negative events.

This happened even for seemingly banal events. "When we asked people about projecting a haircut in the future, this neutral event became the best haircut ever-never looked better, and met a cute guy at the hairdressing salon," according to the study.

One possible explanation is that optimism became an evolutionary advantage for humans, though Phelps cautions that the amygdala and rACC developed in evolution long before humans appeared. This means that other animals may share some aspects of this rose-tinting of future activities.

No one has yet compared humans' optimism to that of other animals that I know of, although it is possible that the two regions of the brain involved in this phenomenon are modulated in humans by the more recently evolved frontal-cortex centers of the brain responsible for high reasoning, motivation, and decisionmaking.

Other fMRI scans for emotions such as fear and anxiety-which cause a flurry of activity in the amygdala-have shown that the reasoning centers of the brain in the frontal lobe activate and appear to interact with these older regions to influence and modulate behavior.

As with many fMRI studies that focus on small groups of volunteers in universities, it's possible that the subjects at N.Y.U. were biased toward optimistic souls, and that there is an equally pervasive tendency among some people to be pessimistic. Researchers have found that some people suffering from depression have malfunctioning amygdalas and rACCs.

Phelps told me that being overly optimistic is also dangerous-which may explain how we humans sometimes have willfully ignored or diminished the impact of our own actionseven when they cause us harm in the future. This includes the rise of carbon dioxide in the atmosphere linked to global warming. "Pessimistic" scientists raised the alarm about this as early as the 1970s, but society only recently has accepted the reality.

Then again, sitting here in Northern California, the optimists in Silicon Valley are already proclaiming—with their amygdalas and rACCs undoubtedly raging—that human ingenuity can fix the global-warming problem as surely as we created the fossil-burning contraptions spewing greenhouse gases in the first place.