People are overconfident, on the whole. Most of us believe we are more intelligent, more skilful or better drivers than the average person. But we can’t all be right.

Such beliefs are not harmless. Overconfidence – and the self-deception that drives this excessive self-belief – can contribute to extreme risk-taking, failed businesses, disastrous investments, dangerous driving and political miscalculations. Why then have so many of us retained this trait?

According to the late evolutionary biologist Robert Trivers, self-deception evolved in humans because it also helps us deceive others more effectively. He argued that when people genuinely believe in a lie or falsehood, they suppress the subtle cues that betray deception – such as nervousness, hesitation, vocal strain or a forced expression.

But we think something is missing from Trivers’s account. When overconfidence is such a common trait, why should anyone believe your hype simply because you believe it yourself? This is the question our latest research set out to answer.

Our approach was based on the handicap principle developed by Israeli biologists Amotz and Avishag Zahavi. The classic example is the peacock’s tail, which demands enormous metabolic energy to grow and maintain, and makes flying, running and escaping from predators harder.

Only an unusually healthy, capable bird can grow such plumage and survive while dragging it around. Because the tail is so costly, it works as an “honest signal” of these attributes. Our claim is that self-belief is a peacock’s tail for people, signalling attributes that are hard to observe directly.

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