Hans Eysenck

Hans Eysenck was the first psychologist to make this trait or temperament business into something more mathematical: he gave long lists of adjectives to hundreds of thousands of people and used a special statistics called factor analysis to figure out what factors - trait dimensions - carry the most weight. He took the results of this work and created a test called the Eysenck Personality Questionnaire (EPQ).

Instead of making these traits either-or, like Jung did, he saw them as dimensions. His first trait dimension was, like Jung, extraversion-introversion. But rather than say you were one or the other (an I or an E), he gave you a score on extraversion-introversion: A low score meant you were introverted, a high score extraverted. Of course, this meant you could be halfway in-between - as in fact most people are!

His second trait dimension he called neuroticism. If you scored high on this scale, that meant you tended to be a very nervous, emotional sort of person. While it doesn't mean you are necessarily a neurotic, it does mean you are more likely to develop neurotic problems such as phobias, obsessions, compulsions, and depression than someone who scores low. Low neuroticism is nowadays often called emotional stability.

The third dimension is called psychoticism. He added this later in his research, after he had gotten more data from people who were in mental institutions. As the name implies, these are people with tendencies to psychosis, meaning that they are more likely to have problems dealing with reality. Psychotic people sometimes have hallucinations and often have delusions such as odd beliefs about being watched, perhaps by the CIA or even by creatures from other planets. A middle score on psychoticism might mean that you are a bit eccentric or that you take risks that other people aren't as likely to take. A low score means that you are pretty normal in this regard.

Eysenck's research gets a great deal of respect, and most psychologists see his theory as on the right track.