Trait theory in psychology, is an approach to the study of human personality. Trait theorists are primarily interested in the measurement of traits, which can be defined as habitual patterns of behavior, thought, and emotion. According to this perspective, traits are relatively stable over time, differ across individuals (e.g. some people are outgoing whereas others are shy), and influence behavior.

Gordon Allport was an early pioneer in the study of traits, which he sometimes referred to as dispositions. In his approach, central traits are basic to an individual's personality, whereas secondary traits are more peripheral. Common traits are those recognized within a culture and may vary between cultures. Cardinal traits are those by which an individual may be strongly recognized. Since Allport's time, trait theorists have focused more on group statistics than on single individuals. Allport called these two emphases "nomothetic" and "idiographic," respectively.

There is a nearly unlimited number of potential traits that could be used to describe personality. The statistical technique of factor analysis, however, has demonstrated that particular clusters of traits reliably correlate together. Hans Eysenck has suggested that personality is reducible to three major traits. Other researchers argue that more factors are needed to adequately describe human personality including humor, wealth and beauty. Many psychologists currently believe that five factors are sufficient.

Gordon Willard Allport (November 11, 1897 – October 9, 1967) was an American psychologist. Allport was one of the first psychologists to focus on the study of the personality, and is often referred to as one of the founding figures of personality psychology. He contributed to the formation of Values Scales and rejected both a psychoanalytic approach to personality, which he thought often went too deep, and a behavioral approach, which he thought often did not go deep enough. He emphasized the uniqueness of each individual, and the importance of the present context, as opposed to past history, for understanding the personality.

Allport had a profound and lasting influence on the field of psychology, even though his work is cited much less often than that of other well-known figures. Part of his influence stemmed from his knack for attacking and broadly conceptualizing important and interesting topics (e.g. rumor, prejudice, religion, traits). Part of his influence was a result of the deep and lasting impression he made on his students during his long teaching career, many of whom went on to have important psychological careers. Among his many students were Jerome S. Bruner, Anthony Greenwald, Stanley Milgram, Leo Postman, Thomas Pettigrew, and M. Brewster Smith. His brother Floyd Henry Allport, was professor of social psychology and political psychology at Syracuse University's Maxwell School of Citizenship and Public Affairs (in Syracuse, New York, USA) from 1924 until 1956, and visiting professor at University of California, Berkeley