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.
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%.
If the patient thought their risk was 40%, then at the end of the experiment they downgraded their own risk to about 31%, she said.
However, if the patient originally thought their risk was 10%, they only marginally increased their risk - they "leaned a little bit, but not a lot".
When the news was positive, all people had more activity in the brain's frontal lobes, which are associated with processing errors. With negative information, the most optimistic people had the least activity in the frontal lobes, while the least optimistic had the most.
It suggests the brain is picking and choosing which evidence to listen to.
Dr Sharot said: "Smoking kills messages don't work as people think their chances of cancer are low. The divorce rate is 50%, but people don't think it's the same for them. There is a very fundamental bias in the brain."
