In this section, we discuss two of the founding figures in trait theory, Raymond Cattell and Hans Eysenck, both of whom conducted their seminal research in the 1940's and 1950's. Although they differed in their conclusions, they approached the problem of how to describe personality in a very similar way: by asking people to indicate the extent to which they and others could be described with each of a large number of words.

Raymond Cattell started with approximately 18,000 words for describing personality that had been culled from the English lexicon. He narrowed this unmanageably large number of words down to approximately 170 by choosing those that seemed most representative and largely independent of each other. He then had people describe themselves and others using each of these words, and then subjected the descriptions to factor analysis to determine which groups of descriptions tended to occur together (and which never occurred together). Using this method, Cattell came up with a list of sixteen factors of personality (each a continuum from one extreme to the other), and developed a questionnaire that could be used to measure each of these factors in an individual or a group. The factors identified by Cattell were: sociable-unsociable, intelligent-unintelligent, emotionally stable- unstable, dominant-submissive, cheerful-brooding, conscientious-undependable, bold-timid, sensitive-insensitive, suspicious-trusting, imaginative-practical, shrewd-naïve, guilt proclivity-guilt rejection, radicalism-conservatism, self- sufficiency-group adherence, self-disciplined-uncontrolled will, and tense- relaxed.

Eysenck's early work took place at approximately the same time as Cattell's and used an almost identical method. However, Eysenck used factor analysis slightly differently, and came up with only two factors: extroversion-introversion and neuroticism-stability. Extroversion- introversion refers to a person's tendency to seek stimulation and novelty: a person who is highly extroverted is more likely to take risks, to have many friends, and to be outgoing than someone who is highly introverted. Neuroticism-stability refers to a person's tendency to become emotionally upset. Eysenck believed that these two traits were heavily influenced by biology. In particular, he thought that extroverted people had higher thresholds for stimulation than introverted people. According to his theory, the differences in behavior between extroverts and introverts are due to the tendency to seek out an optimal level of stimulation; introverts are likely to be overstimulated by the kinds of activities that extroverts find most comfortable.

Since Cattell and Eysenck, many researchers have conducted many studies to determine whether there are sixteen traits, as Cattell argued, or two, as Eysenck argued, or whether the truth is somewhere in between. The consensus that has arisen is that across many studies and many situations, five factors appear to account for most of the personality differences between people. These factors include two of Eysencks--extroversion and neuroticism--as well as three new ones: agreeableness, conscientiousness, and openness to experience. Of these, extroversion and neuroticism account for most of the variance. Personality assessments based on the Big Five tend to be relatively stable over time, but they are relatively poor predictors of any specific behavior.