Biological basis of extraversion amd introversion

Eysenck proposed that extraversion was caused by variability in cortical arousal; "introverts are characterized by higher levels of activity than extraverts and so are chronically more cortically aroused than extraverts". Because extraverts are less aroused internally, they require more external stimulation than introverts. This theory may be backed up by evidence that the brains of extraverts are more responsive to dopamine than those of introverts. Other evidence of this “stimulation” hypothesis is that introverts salivate more than extraverts in response to a drop of lemon juice.

One study found that introverts have more blood flow in the frontal lobes of their brain and the anterior or frontal thalamus, which are areas dealing with internal processing, such as planning and problem solving. Extraverts have more blood flow in the anterior cingulate gyrus, temporal lobes, and posterior thalamus, which are involved in sensory and emotional experience. This study and other research indicates that introversion-extraversion is related to individual differences in brain function.

Twin studies of extraversion and introversion