A connection between brain region size and personality was found for four out of the five traits (all except openness/intellect).
Those who scored high on neuroticism — which indicates a tendency to experience negative emotions, including anxiety and self-consciousness — was associated with a larger mid-cingulate cortex, a region thought to be involved in the detection of errors and response to emotional and physical pain. Neurotics also had a smaller dorsomedial prefrontal cortex, a region implicated in the regulation of emotions.
Extroverts, those who are sociable, outgoing and assertive, had a larger medial orbitofrontal cortex, a region involved in processing rewards. This goes along with the idea that extroverts are sensitive to rewards, which in our society often involve social interactions and status.
Conscientious people, who tend to be orderly, industrious and self-disciplined, had a larger middle frontal gyrus, a region involved in memory and planning.
The researchers note however, that a bigger brain region does not necessarily mean the region has better functioning, although extensive evidence supports this assumption.