The part of the brain responsible for self-generated thought is highly active in neuroticism, which yields both of the trait's positives (eg creativity) and negatives (eg misery), researchers said.
People who score high on neuroticism in personality tests tend to have negative thoughts and feelings of all types, struggle to cope with dangerous jobs, and are more likely to experience psychiatric disorders within their lifetime.
Isaac Newton was a classic neurotic. He was a brooder and a worrier, prone to dwelling on the scientific problems before him as well as his childhood sins, researchers said.
The most popular explanation for why people are neurotic comes from British psychologist Jeffrey Gray, who proposed in the 1970s that such individuals have a heightened sensitivity to threat.
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A previous study showed that individuals in an MRI scanner who spontaneously have particularly negative thoughts (a key marker of neuroticism) displayed greater activity in the regions of the medial prefrontal cortex that are associated with conscious perception of threat.
Researchers said that individual differences in the activity of these brain circuits that govern self-generated thought could be a causal explanation for neuroticism.
Mobbs had also shown that this switch from anxiety to panic is controlled by circuits in the basolateral nuclei of the amygdala - the brain's emotional centre.
If you "have a tendency to switch to panic sooner than average people, due to possessing especially high reactivity in the basolateral nuclei of the amygdale, then that means you can experience intense negative emotions even when there's no threat present," said lead author Adam Perkins, a personality researcher at King's College London.
The over-thinking hypothesis also explains the positives of neuroticism, researchers said.
The creativity of Newton and other neurotics may simply be the result of their tendency to dwell on problems far longer than average people, they said.
The study was published in the journal Trends in Cognitive Sciences Opinion.