Gordon Willard Allport, one of the most influential of American psychologists in the 1900s, was the youngest of four brothers. He was born in Montezuma, Indiana in 1897. One of his elder brothers, Floyd Henry Allport, was also an influential psychologist, and it is said inspired him (Hall & Lindzey). Allport, who graduated from Harvard with a Ph.D. In 1922, was a long time member of the faculty at Harvard University from 1930 until his death in 1967. He produced a number of influential books and professional works over his career such as the influential book The Nature of Prejudice. Allport was initially exposed to Freudian notions of behavior as a graduate student, but he rejected the notions of Freudian psychology and later notions of behaviorism (in fact there is the famous story of his meeting with Freud that often used to explain the development of his own theories). Allport became very interested in the study of the personality, and is often referred to as one of the founding figures of personality psychology. He put emphasis on the uniqueness of the individual and the importance of the context to understand behavior instead of focusing on past history. Allport's work has a deep influence on modern psychological theories (Hall & Lindzey, 1985). Allport most often studied " bigger" topics over his career such as prejudice, religion, and traits. He also left a lasting impression on his students during his long teaching career, many of whom went on to have important careers themselves. In essence the majority of Allport's concepts regarding personality have to do with motivational issues; with what drives the person. Allport was also adamant on obtaining rational guidelines in his approach to the study of personality. He diligently studied definitions by other experts including definitions of temperament and character before arriving at his own definition of personality. He grouped these definitions into different categories. Temperament referred to biological dispositions and character referred to code of behavior that is evaluated by others (Allport, 1937a). Allport found that what he termed "mask definitions" of personality focused on the "external stimulus value" of the person as they present themselves in the world (Allport, 1968). Behaviorism's focus on observable behavior only qualifies here. Essence definitions of personality focused on an essential inner quality or thing that makes people human. Psychoanalytic constructs such as the id, ego, and superego, etc. are an example of this approach. Essence definitions purport that there is something inside people that makes their personality what it is. Omnibus definitions of personality approach defining personality by summing up all that there is to know about one's past, present, and future. These definitions suggest that everyone is unique so that personality science would find it difficult to develop a universally applicable theory of personality that would apply to all people Allport, 1966).

Allport combined what he believed to be the best elements of other definitions of personality into his now famous definition:

"Personality is the dynamic organization within the individual of those psychophysical systems that determine his unique adjustment to his environment" (Allport, 1937a, p. 48).

Allport carefully selected his words when defining personality. We can understand personality as having a type of central organization the holds the components of personality together but is also developing and changing ("dynamic organization"). Personality is also a real entity and not just some explanation or categorization formed by an observer based on someone's actions, but it is something real comprised of mental and neural units ("psychophysical systems"). A finally, personality has a function or it does something for people ("determine his unique adjustment to his environment"). This definition has endured for years and really helps understand the rest of Allport's notions of personality and the aspects of his theories. According to Allport, the basic units of personality are traits, personal dispositions, and the proprium.

One of the major components of Allport's personality theory is the notion of traits and personal dispositions. Allport defined a trait as a "neuropsychic structure having the capacity to render many stimuli functionally equivalent..." And operated by initiating and guiding behaviors (Allport, 1961, p. 347). So again for Allport, who had spent hours going through the dictionary categorizing trait descriptions, a trait was not just a subjective label applied to a behavior by an outsider, but is real entity. Traits function by predisposing people to perceive various groups of stimuli as having similar meanings and to respond to these stimuli with similar behaviors.

In order to describe the concept of trait Allport compared traits to habits...

A habit can function as a trait; however, a trait is not always a habit. A habit can become a trait later in life and Allport discussed the example of the young child brushing his teeth. At first it seems like a habit, but later, as the habit persists, the child can be said to possess personal cleanliness as a trait (Allport, 1955). Allport described a trait as a "fusion of habit and endowment rather than a colligation or chain of habits alone" (Allport 1937a, p. 293). The transformation of habit to trait occurs when the motivation shifts from being conditioned response to one of pure gratification of the activity as motivation. At this point the trait becomes autonomous (Allport, 1937b, 1968). However, one cannot generalize much about a person from habits such as humming to music or hanging ones car keys when coming inside. Habits are responses to specific situations; traits are more generalized (Allport, 1961).
A trait can also function as an attitude. Attitudes are more general than habits, but less general than traits. As attitudes guide behavior, so can traits. However, Allport argues that even though the concepts of attitude and trait are very similar, they also differ in three ways: First, an attitude refers to something either material or conceptual and is more specific in its focus than a trait. Second, traits are more general dispositions directed towards many similar things or similar situations. Where traits are more extended, an attitude can still be situational. Third, attitudes are usually favorable or unfavorable opinions towards something, a characteristic that a trait does not necessarily possess. So while attitudes and traits share similarities and often function in concert, Allport stressed that it is important to distinguish the two concepts and keep them separate even when they overlap (Allport, 1961).

Allport was careful to distinguish between common traits, which permit comparisons across a wide range of individuals, and personal dispositions (personal traits), which are distinctive to the individual (Allport, 1961). For example, a person can be labeled as an "aggressive" person, but to describe any one person's aggressiveness would result in disparate portrayals for each person consisting of much more than the single word "aggressive." Personal dispositions are always unique to the individual and require an elaborate explanation. This idea of common and personal traits brings up the difference between nomothetic approaches to personality which attempt to look at common traits in large numbers of people and idiographic approaches that look at the uniqueness of the individual (Allport & Allport, 1921). Allport was concerned with the uniqueness of the individual.

In his study of the uniqueness of the person, Allport reasoned that some traits have less personal significance than do others. This led Allport to describe three types of personal dispositions: cardinal dispositions, central dispositions, and secondary dispositions (Allport, 1968). A cardinal disposition is one that is so pervasive that the majority of one's behaviors and activities can be traced to or are motivated by this particular trait. Only few people possess a cardinal trait, but for the ones who do possess one, this trait may rule their personality.

Central dispositions are easily detected characteristics within a particular person, but are not overarching in the way that cardinal dispositions are. Most people have a number of these, perhaps five to ten central dispositions on average according to Allport (Schultz, 1976). A person can be accurately described by knowing their central dispositions as central dispositions are highly characteristic of a person. These dispositions are the ones Allport believed were important to measure and compare and he emphasizes the central traits throughout his writings (Allport, 1937a; 1968).

Finally, Allport described the secondary dispositions. These are less important, more focalized, and more difficult to detect dispositions. For instance, a normally agreeable person may become aggressive when they see someone abusing an animal. The dispositions are "aroused by a narrower range of equivalent stimuli and they issue into a narrower range of equivalent responses" (Allport, 1937a, p. 338). As such, other people may not notice a person's secondary traits unless they are very close acquaintances.

Although Allport strived to name and identify different traits and dispositions in people, he did not believe that these dispositions existed independent of each other within the person. Allport regarded traits and dispositions as highly inter-connected and often related to each other. No one trait or disposition works alone. Those that are activated depend on the situation or environment. For example, guilt-behavior and feelings of ambivalence that could sometimes be due to…