Researchers from University College London found there is an "optimism bias" in the brain which makes us see "the glass half-full".
According to the study, people "update" their beliefs accurately only when things turn out to be better than expected, not when they are worse than expected.
The scientists reached their findings by presenting people with a list of adverse events – including getting Alzheimer's disease and being robbed – and asking them to estimate how likely it was these things would happen to them.
After giving their answers the volunteers were provided with the actual probability of each event hitting them. They then rated the likelihood of the same events again, in order to measure how information about actual probabilities changes people's estimates of the likelihood of misfortunes befalling them.
The study, published in the journal Nature Neuroscience, found people were far more likely to change their estimates when the feedback indicated that in reality they were much less likely to suffer an adverse event than they had believed.
But when they were told that an adverse event was much more likely than they had originally thought, they still tended to give the original, incorrect estimate.
The study's authors, Tali Sharot, Christoph Korn and Raymond Dolan, wrote: "Our findings offer a mechanistic account of how unrealistic optimism persists in the face of challenging information.
"However, any advantage arising out of unrealistic optimism is likely to come at a cost. For example, an unrealistic assessment of financial risk is widely seen as a contributing factor to the 2008 global economic collapse.
"Dismissing undesirable errors in estimation renders us peculiarly susceptible to view the future through rose-coloured glasses."Reuse content