Gordon Allport, often regarded as the father of American personality psychology, developed a theory that emphasized the importance of understanding people in their present lives rather than focusing on their past, as psychoanalysis did. Allport believed that personality should be studied in healthy, well-adjusted individuals rather than those with psychological problems. He was particularly interested in defining traits, which he saw as fundamental mental structures that guide behavior across different situations. According to Allport, traits are enduring characteristics that shape a person's reactions in various contexts.

Allport's approach to personality emphasized uniqueness, believing that each individual possesses a distinct combination of traits. He identified traits as the core units for understanding personality, classifying them into cardinal, central, and secondary categories. Cardinal traits are rare and dominate a person's entire life; for example, Mother Teresa's altruism serves as a cardinal trait that defines her life and actions. Central traits are common and form the basic foundation of personality. These are the traits people commonly use to describe others, such as intelligent, kind, or honest. Secondary traits are more variable and situation-specific, exerting less influence over behavior. For example, someone might have a secondary trait of preferring to sleep on a particular side of the bed or displaying specific attitudes that vary with the environment.

Allport's work contributed significantly to the lexical approach to understanding traits. This approach assumes that language's most significant traits are embedded, as people naturally describe each other using trait terms. In collaboration with H.S. Odbert, Allport began by analyzing thousands of words from dictionaries to describe personality traits, eventually narrowing this list to 4,500 traits. This led to the development of the factor analysis technique, which helped researchers identify which traits tend to cluster together. His emphasis on individuality and the present remains influential in understanding personality today.