Here's a depressing thought: what if being depressed, at least a little bit, is actually a good thing? And if it is - if being generally pessimistic is a useful personality trait to have - then isn't that a cause for optimism? In which case, is it really a depressing thought after all? Shouldn't it make you happy about being depressed, in fact, and therefore not depressed? Recently, I have been attempting to resolve this paradox, but my brain just locks up, rendering all further thought or action impossible, like whenever I try to use those self-service checkouts at Sainsbury's.

The cause of all this trauma was discovering "depressive realism" - the theory that people suffering from depression might have a less distorted picture of the world than the non-depressed. This has been controversial ever since it was first proposed in the 1970s, when two psychologists, Lyn Abramson and Lauren Alloy, recruited groups of non-depressed and mildly depressed people and sat them in front of a light bulb and a button. The subject pressed the button, and the bulb either came on or it didn't. In fact, the button didn't control the bulb at all, but the non-depressed people were much more likely to believe they were in charge of events. The non-depressed people, it seemed, were too caught up in protecting their self-esteem to make accurate judgments.

Recent research has thrown doubt on some aspects of this downbeat conclusion, but not on the general point that happiness may be largely a matter of delusion. We're rubbish, for example, at predicting what will make us happy in the future, as Daniel Gilbert points out in Stumbling On Happiness, which became a bestseller this year, presumably because people thought reading it would make them happy. (Presumably it didn't.) We treat our future selves like beloved children, Gilbert writes, dedicating our lives to making them happy - and they respond like rebellious teenagers, throwing it back in our faces.

"Shouldn't we know the tastes, preferences, needs, and desires of the people we will be next year - or at least later this afternoon?" Gilbert asks. "Shouldn't we understand our future selves well enough to shape their lives - to find careers and lovers whom they will cherish, to buy slipcovers for the sofa that they will treasure for years to come? So why do they end up with attics and lives that are full of stuff that we considered indispensable and that they consider painful, embarrassing or useless?"

And guess what? Depressed people turn out to be less prone to these errors of thinking. One of Gilbert's conclusions is that if we experienced the world as it truly is, we wouldn't be able to get out of bed in the morning. And, of course, some seriously depressed people can't. So maybe they're the best people to be running the world, managing our national finances, assessing the threat from climate change, determining foreign policy - as opposed to upbeat politicians, who pride themselves on their optimistic (and therefore delusional) visions of a bright new future. But they can't. Because they're in bed. It's a pretty hopeless situation, really.

oliver.burkeman@theguardian.com