In Hans Eysenck’s book, The Biological Basis of Personality (1967), he begins to establish the relationship between the RAS in the brain (molecular structure) and the personality trait of extraversion (molar behavior). (Howard, Pierce J., PhD. The Owner’s Manual for the Brain. p 25-26, 726. GA: Bard Press, 1994, 2000.)

The RAS controls the general level of arousal (e.g., one’s position on the extroversion-introversion continuum). (Ornstein, Robert, PhD. The Roots of the Self. p 51-52. NY: HarperCollins, 1995.)

The RAS is housed in the brainstem. Its functions are essential to the alert conscious state. (Guiffre, Kenneth. The Care and Feeding of Your Brain. Kenneth Guiffre, MD. p 22-23. NJ: Career Press, 1999.)

The reticular activating system is tuned differently in low gainers (extraverts) versus high gainers (introverts). In general the introvert’s RAS is set higher, making them more highly aroused to begin with, so they require less stimulation than extraverts. (Ornstein, Robert, PhD. The Roots of the Self. p 54-55. NY: HarperCollins, 1995.)