Although he was a behaviorist who considered learned habits of great importance, he believed that personality differences are determined by genetic inheritance.
In devising a temperament-based theory, Eysenck did not exclude the possibility that some aspects of personality are learned, but left the consideration of these to other researchers.
Eysenck initially conceptualized personality as two biologically based independent dimensions of temperament, E and N, measured on a continuum, but then extending this to include a third, P. E – Extraversion/Introversion: Extraversion is characterized by being outgoing, talkative, high on positive affect (feeling good), and in need of external stimulation.
Extraverts, according to Eysenck's theory, are chronically under-aroused and bored and are therefore in need of external stimulation to bring them UP to an optimal level of performance.
Introverts, on the other hand, (also about 16 percent of the population) are chronically over-aroused and jittery and are therefore in need of peace and quietness to bring them DOWN to an optimal level of performance.
Neurotic people — who have low activation thresholds, and unable to inhibit or control their emotional reactions, experience negative affect (fight-or-flight) in the face of very minor stressors — are easily nervous or upset.