Eysenck was one of the first psychologists to study personality with the method of factor analysis. Eysenck's results suggested two main personality factors. The first factor was the tendency to experience negative emotions, and Eysenck referred to it as neuroticism. The second factor was the tendency to enjoy positive events, especially social events, and Eysenck named it Extraversion. The two personality dimensions were described in his 1947 book Dimensions of Personality. It is common practice in personality psychology to refer to the dimensions by the first letters, E and N.

E and N provided a 2-dimensional space to describe individual differences in behaviour. An analogy can be made to how latitude and longitude describe a point on the face of the earth. Also, Eysenck noted how these two dimensions were similar to the four personality types first proposed by the Greek physician Hippocrates.
High N and High E = Choleric type
High N and Low E = Melancholic type
Low N and High E = Sanguine type
Low N and Low E = Phlegmatic type

The third dimension, psychoticism, was added to the model in the late 1970s, based upon collaborations between Eysenck and his wife, Sybil B. G. Eysenck, who is the current editor of Personality and Individual Differences.

The major strength of Eysenck's model was to provide detailed theory of the causes of personality. For example, 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". While it seems counterintuitive to suppose that introverts are more aroused than extraverts, the putative effect this has on behaviour is such that the introvert seeks lower levels of stimulation. Conversely, the extravert seeks to heighten his or her arousal to a more optimal level (as predicted by the Yerkes-Dodson Law) by increased activity, social engagement and other stimulation-seeking behaviors.