The idea that humans are unrealistically optimistic has become so entrenched over the last 30 years that it is even used to help decide UK Government policy.

A healthy mind, so the theory goes, is one that, unfortunately, is also prone to smoking, eating fatty foods and not caring very much about climate change in the mistaken belief that cancer, heart disease and heatwaves, floods and so on ‘won’t happen to me’.

But a new study suggests the concept of ‘optimism bias’ may stem from little more than a statistical quirk.

Researcher Punit Shah, of King’s College London, said: “There is ample evidence for optimism bias in various real-world situations – England football fans for example.

“But these instances simply show that certain people might be optimistic in certain situations, not that they are generally optimistic.”

In a paper in the journal Cognitive Psychology, the academics wrote they had used a number of tests to show that evidence of optimism bias put forward by a number of other researchers was based on a “statistical artefact”.

“These results clarify the difficulties involved in studying human ‘bias’ and cast additional doubt over the status of optimism as a fundamental characteristic of healthy cognition,” they added.

For example, it has been claimed that people fail to learn from bad news, such as being told their chance of getting cancer.

But the researchers said they also failed to learn from good news, such as being told they had a higher chance of living to 90 than they had thought.

They also created a computer program designed to behave in a completely rational way in psychological tests that had been used to apparently show evidence of innate human optimism. This program responded in much the same way that humans did.

Dr Adam Harris, of University College London, who worked on the research, said: “Previous studies, which have used flawed methodologies to claim that people are optimistic across all situations and that this bias is 'normal', are now in serious doubt.

“We need to look for new ways of studying optimism bias to establish whether it is a universal feature of human cognition or not.

“This assumption that people are optimistically biased is being used to guide large infrastructure projects, with the aim of managing expectations around how much projects will cost and how long they will take to complete.