A tiny part of the brain could play a part in a pessimistic outlook and teach people to expect the worst, scientists believe.
The habenula, an evolutionarily ancient part of the brain half the size of a pea, plays an important role in learning from bad experiences research from University College London's Institute of Cognitive Neuroscience has found.
In some people, they think an over-active habenula may be linked to depression, pessimism and a negative outlook.
Lead researcher Dr Jonathan Roiser said: "The habenula tracks our experiences, responding more the worse something is expected to be."
He added: "In this study we showed that the habenula doesn't just express whether something leads to negative events or not; it signals quite how much bad outcomes are expected."
Dr Roiser said the findings, published in the latest edition of the journal Proceedings Of The National Academy Of Sciences, may point the way towards new treatments.