The defining feature of human brains is the size and complexity of the cortex, which provides the underpinnings of rationality for our actions. But just because we have more developed cortexes doesn’t mean we are always rational decision-makers.

We humans constantly find ourselves loving the wrong person, buying things we don’t have the money for and believing that fad diets consisting of nothing but sundaes will work. To be human is to hope against hope. When it comes to decision-making and risk assessment, we tend to think in an asymmetrical manner that feeds an optimistic outlook and denies discouragement.

This has been shown in recent work by Christina Moutsiana and colleagues at University College London and published in the prestigious Proceedings of the National Academy of Sciences. In their study, the researchers asked test subjects to estimate the likelihood of something desirable happening to them.

“There’s a 10 percent chance,” a subject might answer. If the researchers then told the subject that the odds were actually higher — say that there was actually a 20 percent chance of the desirable outcome — the subject was usually quick to adjust his own chances upward to 20 percent, too.

So far, he’s behaving rationally: The subject has rationally updated his estimate based on feedback. But if, after the subject gave his initial answer of 10 percent, the researcher provided bad news, explaining that actually the odds were more like 5 percent, the test subject was less likely to adjust his original estimate, apparently believing that somehow his own chances were better than average.     Humans believe in luck: in their good-luck charms, their lucky numbers, their lucky underwear, even. We resist updating assessments when the feedback is bad but are eager and adept at making upward adjustments when the feedback is good.

That cognitive asymmetry of hope is so pervasive in humans that its exception is considered a disease: clinical depression. Cognitive-behavioral therapists have long framed depression as a disorder of cognitive distortions. A person irrationally believes that because she once experienced a real trauma of having failed, or of having been rejected or abandoned, she then has trouble believing in the likelihood of ever having success or strong, happy relationships.

In other words, the depressed person is resistant to updating expectations, even when the feedback is good, instead malignantly embracing bad feedback until it marinates every neuron.

Yet despite this picture of depression as pathological distortion, considerable research has shown that individuals suffering from depression are often more accurate in their assessment of the world than are healthy control subjects.

“Sadder but wiser,” as the jargon goes. Depression might also be seen, then, as a failure of the human capacity for denial and self-deception. It is the healthy who are neuropsychologically distortive in this realm.

For those counted among the affectively healthy, hope is sustaining. We are able to ignore the reality of death, believing on some level that we and those who matter most to us will live forever  +— or at least lead happy and healthy lives before dying painlessly at a ripe old age. We believe our love will be requited, our efforts rewarded, and that nothing bad would ever happen to Bambi’s mother in real life.

And then, during a week like this last one, we realize with an aching sadness how much we’d been quietly holding out for the news that somewhere, on an uncharted landing-strip-long island in the south Indian Ocean, 239 people were safely waiting to rejoin their loved ones.