Personality refers to the relatively enduring and distinctive ways of thinking, feeling, and acting that characterize a person’s responses to life situations.

Freud believed strongly in the ideas of unconscious processes and psychic energy as motivators of behavior. Freud’s structural theory of personality suggests that three interacting structures (id, ego, and superego) form the core of personality. The id operates according to the pleasure principle, seeking immediate gratification for its sexual and aggressive impulses. The ego, operating primarily at a conscious level, operates according to the reality principle, finding ways that the id can safely discharge its impulses. The superego is the moral arm of personality and strives to check the desires of the id. Freud believed that the interaction between all three structures of personality, along with the release of psychic energy not only motivated behavior but also could result in anxiety if the three structures of personality don’t work together in harmony. Defense mechanisms such as repression are used by the ego to deal with anxiety. Freud also proposed a series of psychosexual stages through which children develop. Sexual pleasure is focused on different parts of the body during these stages (oral, anal, phallic, latency, and genital). Needless to say, Freud’s theory has been controversial in the field of psychology. Much research does not support the basic suppositions of the theory, but some research does support the importance of the unconscious in motivating behavior. Former Freudian disciples who grew disenchanted with Freudian theory developed their own theories, and, as a group, they became known as the neoanalysts. These theorists suggested that social and cultural factors play a far more important role in the development of personality than Freud had believed and argued that Freud had placed too much importance on childhood events in his theory. Important neoanalysts include Jung and the object relations theorists, including Melanie Klein.

Humanistic psychologists stress people’s striving for self-actualization, the realization of one’s potential. One of the most important theorists, Carl Rogers, believed that we have needs for self-consistency (in self-perceptions) and congruence (between self-perceptions and experience). Inconsistency creates threat and anxiety. Rogers believed that we are born with a need for positive regard from others and that conditional positive regard, which means that others give us approval only if we behave or think in ways that they approve of, can hurt us in a number of ways. Research on the self has pointed to the importance of self-esteem for healthy functioning. People try to self-verify their perceptions and engage in self-enhancement behaviors to develop their self-esteem.

Two of the more prominent theories of traits are Cattell’s sixteen personality factors theory and the five factor model, which, not surprisingly, argue that there are sixteen different personality factors and five different personality factors, respectively. Biological explanations for traits focus on three different causes: activity of the nervous system, genes, and evolution. Studies of brain activity have indicated some differences between introverts and extraverts. Twin studies have indicated that monozygotic twins are far more alike in personality than are dizygotic twins, suggesting a genetic basis of personality traits. Evolutionary personality theory suggests that traits developed throughout human history because they helped us to physically survive and because they aid in reproduction.

Social cognitive theories, as the name would indicate, combine behavioral and cognitive theories in an attempt to understand behavior. According to Bandura’s principle of reciprocal determinism, person factors (including personality and cognitive processes), the environment, and behavior all affect each other. Julian Rotter argued that the likelihood that we will engage in a certain behavior is governed by expectancy and how much we desire or dread the expected outcome of our behavior. Locus of control is a term that Rotter devised to refer to the degree to which we believe that internal or external factors control our behavior. Bandura has suggested that self-efficacy strongly affects how people regulate their lives. Performance experiences, observational learning, emotional arousal, and verbal persuasion all affect one’s sense of self-efficacy. Mischel and Shoda have described a five person variable cognitive-affective personality system (CAPS). The interactions of these five factors result in distinctive behavioral signatures, which are consistent ways of responding to situations. These five factors are encoding strategies, expectancies and beliefs, goals and values, affects (emotions), and self-regulatory processes and competencies.

Personality assessment is accomplished through a number of means, including interviews, behavioral assessment, personality scales, and projective tests. In behavioral assessment, elaborate coding systems are used by psychologists in observing behaviors of interest. Objective personality scales, such as the Minnesota Multiphasic Personality Inventory follow the empirical approach to the study of personality. Projective tests, such as the Rorschach inkblot test and the Thematic Apperception Test are tests of interpretations of ambiguous stimuli. People’s responses to the stimuli are thought by some psychologists to be "projections" of inner needs, feelings, and ways of viewing the world.