Personality is partly determined by the shape of our brains, a new study has found.
In neurotic people prone to mood changes and psychiatric disorders, the outer layer of the brain is thicker and less wrinkly, according to the research.
Individuals with "open" personalities linked to curiosity and creativity show an opposite pattern. The outer layer, or cortex, of their brains is thinner and more folded in certain regions with a greater surface area, say scientists.
Psychologists break down human personality into the "big five" traits - neuroticism, openness, extraversion (which determines how enthusiastic a person is) agreeableness (a measure of altruism) and conscientiousness (a measure of self-control).
The new findings show that personality may be the result of the brain's general structure as well as its internal circuitry.
More than 500 volunteers took part in the international study and had brain imaging scans.
Researchers focused on the anatomy of the cortex, where the higher functions that make us human are centred.
Dr Luca Passamonti, a member of the British, US and Italian team from Cambridge University, said: "Evolution has shaped our brain anatomy in a way that maximizes its area and folding at the expense of reduced thickness of the cortex.
"It's like stretching and folding a rubber sheet - this increases the surface area, but at the same time the sheet itself becomes thinner. We refer to this as the 'cortical stretching hypothesis'."
Cortical stretching has allowed the human brain to expand rapidly without becoming too big for our skulls, the scientists pointed out.
The process was said to begin in the womb and continue throughout childhood and adolescence into adulthood.
At the same time as we get older neuroticism decreases and we become better at handling emotions, said the researchers.
In contrast, the traits of conscientiousness and agreeableness grow more pronounced with age. As a result, people tend to become more responsible and less antagonistic as they get older.
Dr Roberta Riccelli, from Magna Graecia University in Catanzaro, Italy, said: "Our work supports the notion that personality is, to some degree, associated with brain maturation, a developmental process that is strongly influenced by genetic factors."
The volunteers, participants in the US-led Human Connectome Project, were all healthy individuals aged 22 to 36 with no history of neuro-psychiatric or other major medical problems.
Brain structure may have an even stronger impact on personality in people likely to experience neuro-psychiatric illnesses, the scientists believe.
Dr Passamonti added: "Linking how brain structure is related to basic personality traits is a crucial step to improving our understanding of the link between the brain morphology and particular mood, cognitive, or behavioural disorders.
"We also need to have a better understanding of the relation between brain structure and function in healthy people to figure out what is different in people with neuropsychiatric disorders."
The research is published in the journal Social Cognitive and Affective Neuroscience.