Trait theories have arisen in recent years, with the object of determining aspects of personality that compel an individual to respond in a certain way to a given situation. Gordon Allport delineated three kinds of traits with varying degrees of intensity: cardinal traits, central traits, and secondary traits. Raymond Cattell used a group of obvious, surface personality traits to derive a small group of source traits, which he argued were central to personality. Objections to trait theories point out that behavior is largely situation dependent, and that such traits as honesty are not especially helpful in characterizing personality and behavior. Despite such objections, trait theories have been popular models for quantifying personality. Paul Costa has postulated five basic dimensions of personality—introverson-extroversion, friendly compliance–hostile noncompliance, will, neuroticism, and openness to experience—and has developed a test to measure these traits.

Abraham Maslow and Carl Rogers supported a humanistic approach to personality, pointing out that other approaches do not factor in people's basic goodness and the motivational factors that push them toward higher levels of functioning. Researchers offering biological approaches to personality have focused on the action of specific genes and neurotransmitters as determinants.

Psychologists may use psychological tests to determine personality. Well-known personality tests include the Rorschach test, in which an individual is asked to look at ink blots and tell what they bring to mind; the Minnesota Multiphasic Personality Inventory, which uses a true-false questionnaire to delineate normal personality types from variants; and the Thematic Apperception Test, which employs cards featuring provocative but ambiguous scenes, asking the viewer their meaning. The American Psychiatric Association has sought to delineate personality disorders in its periodically revised and updated Diagnostic and Statistical Manual of Mental Disorders.