The mechanism appears to be an inherited variation in DNA that affects serotonin transport. The less efficient one's serotonin transporter the higher one's level of neurotic traits like anxiety. Since traits like anxiety are associated with the autonomic nervous system, it may be that serotonin plays a role in the emotional activation system suggested by Eysenck. Siever and Frucht have proposed a two-factor model relating neurotransmitters to behavioral tendencies. This model looks at the interaction effects of norepinephrine and serotonin .
Norepinephrine stimulates the focus of attention toward the external environment.
This effect is exaggerated when one is over-reactive to norepinephrine. If one is under-reactive to norepinephrine the focus of attention is engaged with self or on internal states. Norepinephrine appears to be a possible contributor to any explanation for the extroversion trait. Serotonin suppresses impulses and responses to stimuli. When one is under-reactive to serotonin, impulses and behavior are disinhibited and more difficult to control.