Feeling blue? Strangely, it might mean that you're actually better at judging your performance—and reality in general—than when you're not.

It's called "depressive realism," and it seems to suggest that in our normal state, we tend to operate under happy delusions that lift away when we're depressed. The idea blows apart the theory that depressed people have too negative an outlook on the world: They may actually just be seeing it how it is.

According to the National Institute of Mental Health, depression is one of the most common mental illnesses in the United States, affecting over 16 million people (or about 6.7 percent of the population) in 2015. So does that mean that over 90 percent of the population is walking around with rose-colored glasses?

Some research seems to suggest so. The concept of depressive realism first entered the scene via a 1979 paper published by L.B. Alloy and L.Y. Abramson. In it, the researchers presented both depressed and non-depressed participants with a button and a green light. They then asked the participants to figure out to what degree their responses (pushing the button) controlled that light. Depressed participants were much better at judging the degree of their control, while participants who weren't depressed tended to assume that they had more control over the light than they actually did.

Depressive realism is "still regarded as a fruitful hypothesis by many, but not all, psychologists," says Colin Feltham, professor emeritus of critical counseling studies at Sheffield Hallam University and author of the book Depressive Realism. A number of studies have investigated the theory with mixed results, he says.

It may be tied to certain other psychological theories, like the terror management theory, Feltham says. Terror Management suggests that human nature is actually wired towards self-deception: In order to avoid facing terrifying concepts like death, most of us live in a state of self-delusion. And maybe, when we're depressed, we're just less likely to be deceived.

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In fact, "some psychologists concede that an element of self-deception may be necessary for well-being," Feltham says. Depressed people may just be lacking that crucial optimism that helps us power through a life full of heartache, meaninglessness, and death. So while they can see things more realistically, we all know that reality sometimes straight-up bites.

The people most likely to experience depressive realism? Introverts, males, and people with high IQs, Feltham says, adding that it's most likely to occur in mildly depressed people—those suffering from major depression, by contrast, are more likely to suffer from larger distortions in their thinking.

Not everybody is convinced of the effects of depressive realism as shown in literature, though. Psychologist Michael T. Moore, a professor at Adelphi University, led a survey of 75 studies on depressive realism that included over 7000 participants. His conclusion? "There's certainly evidence for it, but the evidence that's there shows it occurs in a very narrow range of stimulus conditions," he says. "A broken clock is going to be right twice a day."

Moore believes that it's not that depressed participants have a more accurate view of reality in general. Instead, the controlled experimental setup of many of the studies may have artificially led to such results.

"If you set up a circumstance where there's no relationship between the button and the light, depressed patients might be able to better pick that out, because that set of circumstances happens to conform to their somewhat biased view of the universe, that bad things happen for no reason," Moore explains. "That doesn't necessarily mean that they're more accurate broadly, but under that very narrow set of stimulus conditions, they come out looking more accurate."

What we don't know, Moore emphasizes, is if depressed patients, across a broad range of human experiences, actually perceive the world more accurately. Our research is currently too homogenous, he says.

Whether depressive realism is actually a widely applicable phenomenon is therefore still an open question, Moore says. But it's an important one, because it could have a major impact on how we both prevent and treat depression.

The current leading treatment for depression is cognitive therapy, which operates under the assumption that the depressed patient is sad because he is misperceiving his environment, Moore says. That patient is only remembering negative things and failing to perceive the positive. Cognitive therapy presumably helps patients become more accurate and realistic—thus helping them get better.

But if depressive realism proves to be true, it begs the question: Is cognitive therapy really helping patients see reality more accurately? Or is it just offering patients a set of rose-colored glasses?

For now, the evidence suggests we have some reason to believe that mildly depressed individuals are better at perceiving certain aspects of reality. Whether that holds true for a wider set of circumstances remains to be seen. But in the meantime, enjoy those rose-colored glasses—they may just be a sign of sound mental health.