The widely held view that people naturally look on the bright side of life may be wrong, say UK psychologists.
For decades experts have believed it is normal to expect good things to happen in the future and underestimate 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 reassessing 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 policymakers 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.
"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," study author Dr Adam Harris, from University College London, said.
"This assumption that people are optimistically biased is being used to guide large infrastructure projects. 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.