Is there a person on the planet who has ever been helped by being told not to worry? The slogan "Don't worry, be happy" comes originally from the Indian mystic Meher Baba, but many of us know it best as a knuckle-gnawingly annoying 1988 song by Bobby McFerrin. And it's surely no accident that you only ever hear that song these days in war movies (Jarhead, Welcome To Sarajevo) where it's used as a savagely ironic counterpoint to the horrors on screen. When you stop to think about it, ordering anyone to stop feeling how they're feeling is an enormously thoughtless act, though we do it all the time - the phrase "Cheer up!" being the most obvious example. The ultra-bestselling American pop psychologist Wayne Dyer calls worry a "useless emotion", as if that should be enough for us to drop it. But his observation is itself useless. And, of course, wrong: there's a good reason why we've evolved to be able to map out, and plan for, how bad the future might be. It's just that sometimes it would be nice to be able to stop.

"I am an old man and have known a great many troubles, but most of them never happened," Mark Twain is supposed to have said. Self-help authors, echoing Twain, like to claim that our brains can't distinguish between a real scenario and a vividly imagined one - so that, on a physical and emotional level, we respond to worries about a horrible event as if they were the horrible event. More and more experimental evidence suggests this might be right: according to one recent study, it's possible to suffer the effects of post-traumatic stress disorder from events that were hallucinated. One man in the study, delirious from liver disease, believed hospital staff were beating his son to death; he suffered the long-term psychological effects you'd expect if they actually had. It's not hard to extrapolate from this what the smaller but real effects of our everyday fearful imaginings might be.

But with ordinary worry, unlike extraordinary trauma, there's something else - what the psychologist Edward Hallowell calls "the hidden pleasures of worry". "One of the hidden pleasures is that worriers believe they're not safe unless they're worried - that the deal they make with fate is, if I torture myself by worrying, I won't be punished with bad outcomes," he told me. "The other hidden pleasure is that contentment is too bland; worry is more stimulating. We don't say, 'She was gripped by contentment.' The good news, though, is that worriers tend to be the smartest, most creative people we've got. It takes a lot of imagination to dream up all these worries."

Hallowell's number one prescription is "never worry alone". I asked why, half-expecting some complex neurological explanation. "It's just a fact of human nature. We're better in connection than in isolation." This works with worry, as with any area of life. "If you're in a big room alone in the dark, you feel frightened," Hallowell said. "If you're with someone else, you laugh."

&183; oliver.burkeman@theguardian.com