It is the key to health, wealth and finding true love, according to dozens of self-help manuals and lifestyle gurus. But scientists have discovered that positive thinking and the repetition of stock optimistic phrases such as “I can do it” or “I will succeed” do more harm than good.

Researchers sought to assess how positive thinking affected people with varying levels of self-confidence. They questioned dozens of men and women, measured their self-esteem using the standard psychological methods and then asked them to write down their thoughts and feelings.

In the middle of the exercise, some were assigned to tell themselves: “I am a loveable person” every time a bell was rung. After the exercise, they were asked a series of further questions to measure their self-worth and optimism. The scoring system ranged from 0 to 35.

The confidence of those with high self-esteem appeared to have been boosted further by repeating the phrase. They scored an average of 31 compared with an average of 25 for those with equally high self-esteem who did not.

Those with low self-esteem who repeated the statement scored a dismal average of 10. Their peers with equally low self-esteem who were not asked to do so managed a rather more chirpy average of 17.

The findings were published in this year’s Psychological Science journal.

Joanne Wood, Professor of Psychology at the University of Waterloo in Ontario, Canada, and an author of the report, said it seemed that repeating positive statements worked only if it reinforced what the person already believed.“It appears that positive self-statements, despite their widespread endorsement, may backfire for the very people who need them the most,” she said.

“I think that what happens is that when a low-self-esteem person repeats positive thoughts, all they do is contradict what is there already. So if they’re saying, ‘I’m a loveable person’, they might then think, ‘Well, I’m not always loveable’ or ‘I’m not loveable in this way’. Then these contradictory thoughts may overwhelm the positive thoughts.”

Professor Wood said that positive thinking might be effective when it was used as part of a broader programme of therapy. “But on its own it tends to have the reverse effect of what it is supposed to do.”

She urged the purveyors of self-help books, magazines and TV shows to stop sending a message that simply chanting a positive mantra could transform a life. “It’s frustrating to people when they try it and it doesn’t work for them,” she said.

Professor Wood is now exploring how self-esteem is maintained. Early findings suggest that people with high self-esteem are more likely to try to improve their moods when they are sad, and savour their moods when they are happy. Those with low self-esteem try to reduce feelings of happiness. “Such differences in emotion regulation probably help to maintain self-esteem differences,” she said.

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