One reason optimists retain a positive outlook even in the face of evidence to the contrary has been discovered, say researchers.
A study, published in Nature Neuroscience, suggests the brain is very good at processing good news about the future.
However, in some people, anything negative is practically ignored - with them retaining a positive world view.
The authors said optimism did have important health benefits.
Scientists at University College London said about 80% of people were optimists, even if they would not label themselves as such.
They rated 14 people for their level of optimism and tested them in a brain scanner.
“There is a very fundamental bias in the brain”
Dr Tali Sharot
University College London
Each was asked how likely 80 different "bad events" - including a divorce or having cancer - were to happen.
They were then told how likely this was in reality. At the end of the session, the participants were asked to rate the probabilities again.
There was a marked difference in the updated scores of optimists depending on whether the reality was good or bad news.
Dr Tali Sharot, lead researcher, gave the example of the risk of cancer being set at 30%.
from Nature Neuroscience: "Unrealistic optimism is a pervasive human trait that influences domains ranging from personal relationships to politics and finance. How people maintain unrealistic optimism, despite frequently encountering information that challenges those biased beliefs, is unknown. We examined this question and found a marked asymmetry in belief updating. Participants updated their beliefs more in response to information that was better than expected than to information that was worse. This selectivity was mediated by a relative failure to code for errors that should reduce optimism. Distinct regions of the prefrontal cortex tracked estimation errors when those called for positive update, both in individuals who scored high and low on trait optimism. However, highly optimistic individuals exhibited reduced tracking of estimation errors that called for negative update in right inferior prefrontal gyrus. These findings indicate that optimism is tied to a selective update failure and diminished neural coding of undesirable information regarding the future."