We all like to think of ourselves as rational creatures who smartly prepare for the worst. We watch our back, weigh the odds and pack an umbrella when the skies look threatening. But although we take such precautions, we generally expect things to turn out pretty well — often better than they actually do.

The belief that the future will probably be much better than the past and present is known as the optimism bias, and most of us have this tendency to overestimate the likelihood of good events happening to us and underestimate the likelihood that bad events will come crashing down.

Take marriage, for example. In the Western world, divorce rates are higher than 40 percent: Two out of five marriages end in divorce. But newlyweds estimate their own likelihood of divorce at zero.

Why is optimism about our personal future so resilient? It starts with what may be the most extraordinary of human talents: the ability to move back and forth through time and space in one’s mind. To think positively about our prospects, it helps to be able to imagine ourselves in the future. Our capacity to envision a different time and place is critical for our survival. It allows us to plan ahead, to save resources for times of scarcity, and to endure hard work in anticipation of a future reward.

While mental time travel has clear survival advantages, conscious foresight came to humans at an enormous price — the understanding that death awaits. The knowledge that old age, sickness, decline of mental power and oblivion are somewhere around the corner can be devastating.

Ajit Varki, a biologist at the University of California at San Diego, argues that the awareness of mortality on its own would have led evolution to a dead end. The despair would have brought the daily activities needed for survival to a stop. The only way that conscious mental time travel could have arisen is if it emerged along with irrational optimism. The knowledge of death had to emerge in parallel with the persistent ability to picture a bright future.

The capacity to envision that future relies partially on the hippocampus, a brain structure that is crucial to memory. (People with damage to the hippocampus are unable to recollect the past; they are also unable to construct detailed images of future scenarios.)

But the human brain doesn’t travel in time randomly. It tends to engage in specific types of thoughts: We consider how well our kids will do in life, how we will obtain that desired job, whether our team will win. We also worry about losing loved ones, failing at our job or dying in a plane crash. But research shows that most of us spend less time mulling negative outcomes than we do positive ones.

Why do we maintain this rosy bias even when information challenging our upbeat forecasts is so readily available? Only recently have we been able to decipher this mystery. My colleagues and I at University College London recently scanned the brains of people as they processed both positive and negative information about the future.

Among other things, we asked them to estimate how likely they were to encounter 80 negative events, including developing cancer, having Alzheimer’s disease and being robbed.

We then told them the likelihood that a person like them would suffer these misfortunes; for example, the lifetime risk of cancer is about 30 percent. Then we asked again: How likely are you to suffer from cancer?