The widely held view that people naturally look on the bright side of life may be wrong, say psychologists.
For decades experts have believed it is normal to expect good things to happen in the future and under-estimate the possibility of bad outcomes - a trait known as "irrational optimism bias".
But a new study suggests this assumption may be based on flawed research. After re-assessing the evidence scientists concluded there was no basis for the claim that optimism bias is fundamental to human psychology.
The findings are important because belief in optimism bias can affect the way policy-makers deal with issues ranging from financial crises to obesity and climate change.
Optimism bias is even taken into account by the Government when planning and funding large infrastructure projects, it is claimed.
Study author Dr Adam Harris, from University College London, 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. Our research supports a re-examination of optimism bias before allowing it to guide clinical research and policy."
Research has suggested that people fail to learn from bad news when told the actual chance of experiencing a negative life event, such as cancer. This is said to provide support for optimism bias.
For the new study, computer simulations were designed to behave in a completely rational way when faced with the psychological test of learning from good versus bad news.
The programs were not capable of optimism and unable to show bias. Yet they produced the same data patterns that have previously been interpreted as showing evidence of optimism bias.
The research showed how apparent optimism can arise from purely statistical processes. Optimism bias is merely a statistical artefact that arises because of the relative rarity of negative events, according to the findings published in the journal Cognitive Psychology.
Co-author Punit Shah, from 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."