Kelly Brownell and Terence Wilson from the University of Pennsylvania and the Rutgers University in the US recently published a review of psychological and medical research on why people relapse when they’re trying to achieve a long-term goal, such as giving up smoking or getting fit.

The paper, published in American Psychologist, distinguishes between a “lapse” and a “relapse”. A lapse, the researchers say, refers to the initial process - a slip or mistake, which may or may not lead on to an actual relapse. For example, you eat a cream bun and then realise you shouldn’t have. This is a lapse and you can take corrective action. If you go on to finish the whole packet as you feel your diet’s ruined anyway, that’s a relapse.

Brownell and Wilson suggest an evocative metaphor for relapses: a person teetering on the edge of a cliff. A dieter is always on the brink of relapse, ready to stumble at the slightest disruption. The first slip creates a momentum, like slipping down a cliff, so that a complete relapse is certain.

But Dr Brownell’s research shows that those who lapse at the beginning of a diet seem to do better in the long run than those who steadfastly stick to it. He believes that strict dieters have trouble recovering from the inevitable slip that their perfectionism merely postpones. In contrast, the initial strugglers may do well later because they cope better with temporary setbacks.

It could be argued, therefore, that a lapse is a necessary step on the path to success. Brownell and colleagues are dubious about this, cautioning that the key to the achievement of any longer-term goal is probably to keep lapses to a minimum. For example, a lapse could strengthen someone’s view that losing weight is beyond them.

But you do have to learn to handle lapses to attain longer-term goals. For instance, the researchers examined successful dieters in several weight-loss programmes and found that most had a “threshold” gain of 3lb, at which point they took self-correcting action. They did not give in. So the key is to take immediate action when a blip looms. The art of success could be learning how to stop lapses turning into relapses.

Dr Raj Persaud is consultant psychiatrist at the Maudsley Hospital in south London and Gresham professor for public understanding of psychiatry. His latest book is The Motivated Mind (Bantam Press). Email: rajpersaud@tes.co.uk