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. When one is over-reactive to serotonin, impulses and behavior are inhibited and control is excessive. Serotonin appears to be a possible contributor to an explanation for the neuroticism trait. Eysenck thinks P is mediated by a polygenic temperament source trait.