The study of personality is based on the essential insight that all people are similar in some ways, yet different in others. There have been many different definitions of personality proposed. However, many contemporary psychologists agree on the following definition: Personality is that pattern of characteristic thoughts, feelings, and behaviors that distinguishes one person from another and that persists over time and situations.  Trait theories
According to the Diagnostic and Statistical Manual of the American Psychiatric Association, personality traits are "enduring patterns of perceiving, relating to, and thinking about the environment and oneself that are exhibited in a wide range of social and personal contexts." Theorists generally assume that a) traits are relatively stable over time, b) traits differ among individuals, and c) traits influence behavior. They consistently are used in order to help define people as a whole. Traits are relatively constant; they do not usually change. Traits are also bipolar; they vary along a continuum between one extreme and the other (e.g. friendly vs. unfriendly). The most common models of traits incorporate three to five broad dimensions or factors. All trait theories incorporate at least two dimensions, extraversion and neuroticism, which historically featured in Hippocrates' humoral theory. Gordon Allport delineated different kinds of traits, which he also called dispositions. Central traits are basic to an individual's personality, while secondary traits are more peripheral. Common traits are those recognized within a culture and thus may vary from culture to culture. Cardinal traits are those by which an individual may be strongly recognized. In his book, Personality: A Psychological Interpretation, Gordon Allport (1937) both established personality psychology as a legitimate intellectual discipline and introduced the first of the modern trait theories. Raymond Cattell's research propagated a two-tiered personality structure with sixteen "primary factors" (16 Personality Factors) and five "secondary factors." In Cattell's lengthy career, he had written 50 books, 500 journals, and 30 different types of standardized tests. For Cattell, personality itself was deﬁned in terms of behavioral prediction. He deﬁned personality as that which permits a prediction of what a person will do in a given situation. John Gittinger's theory and its applications (the Personality Assessment System (PAS)) uses the Wechsler intelligence tests, which are well standardized and objective instruments rather than self-report tests. PAS factors out personality traits (primitivity) and two additional levels, Basid and Surface, which are adaptations by environmentally induced presses and learning. Gittinger's multivariate personality descriptions exceed 500 data-based outcome descriptions. Hans Eysenck believed just three traits—extraversion, neuroticism and psychoticism—were sufficient to describe human personality. Differences between Cattell and Eysenck emerged due to preferences for different forms of factor analysis, with Cattell using oblique, Eysenck orthogonal rotation to analyze the factors that emerged when personality questionnaires were subjected to statistical analysis. Today, the Big Five factors have the weight of a considerable amount of empirical research behind them, building on the work of Cattell and others. Eysenck, along with another contemporary in trait psychology named J. P. Guilford (1959), believed that the resultant trait factors obtained from factor analysis should be statistically independent of one another —that is, the factors should be arranged (rotated) so that they are uncorrelated or orthogonal (at right angles) to one another. Lewis Goldberg proposed a five-dimension personality model, nicknamed the "Big Five": Openness to Experience: the tendency to be imaginative, independent, and interested in variety vs. practical,...
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