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The shape of your brain could say a lot about your personality ― and your risk for developing mental illness.

Neurotic people have a thicker cortex than those who are more open-minded, according to new research. The study, led by a team of international researchers and published this week in the journal Social Cognitive and Affective Neuroscience, is the first to clearly link personality traits to differences in brain shape.

For the first time, researchers investigated the link between brain shape and the “big five” personality traits: neuroticism, agreeableness, extraversion, conscientiousness and openness to experience. Psychologists consider these traits to comprise the major dimensions of personality.

For the study, 507 healthy young volunteers underwent brain scans. The researchers then analyzed the brain scans, focusing specifically on the cortex ― the outermost layer of the brain, which is composed of grey matter. They looked for variations in cortical thickness, surface area and number of folds.

The analysis revealed a strong association between three measures of the cortex ― thickness, area and folding ― and different personality traits.

“This is a clear pattern with thickness going in the opposite direction than area and folding, as a function of different personality traits,” Dr. Luca Passamonti, a Cambridge neuroscientist and lead author of the study, told The Huffington Post.

The most extreme differences were observed between neurotic and open-minded people. Neurotic people had a thicker cortex, with reduced surface area and folding. On the other side of the spectrum, those high in openness to experience had a thinner cortex with increased surface area and folding.

Folding is the evolutionary solution to fitting an increasingly large brain into a small skull, the researchers explain.

Neuroticism has been associated with mental illnesses like anxiety and depression, while openness to experience is clearly linked with creativity, intellectual curiosity and well-being.

Cambridge

“We are continually shaped by our experiences and environment, but the fact that we see clear differences in brain structure which are linked with differences in personality traits suggests that there will almost certainly be an element of genetics involved,” study co-author Dr. Nicola Toschi said in a statement. “This is also in keeping with the notion that differences in personality traits can be detected early on during development, for example in toddlers or infants.”

It’s likely that these types of brain differences are even more pronounced in individuals with mental health conditions.

“Understanding the ‘healthy’ brain and more specifically how variability across personality traits relate to brain structural measures as thickness, area, and folding may help us to develop better model to understand ― and consequently treat ― the diseased brain,” Passamonti said.

It’s difficult to draw meaningful conclusions from the study, which raises as many questions as it provides answers. It’s not clear, for one, whether personality traits determine brain shape, or vice versa. It’s also unclear how these brain differences might factor into mental illness.

“We don’t know whether what we see is driven by genes, environment or both,” he said. “What we know it the most likely possibility is that a complex interaction between genetic predispositions and bad environmental factors are risk factors for many neuropsychiatric disorders.”