Sigmund Freud was the founder of psychoanalysis, which was the first psychodynamic theory, emphasizing unconscious processes and a belief in the formative role of childhood experiences and early unconscious conflicts.

To Freud, the personality consists of the id, the ego, and the superego. Defense mechanisms protect the ego from unconscious anxiety, and include repression, projection, displacement (one form of which is sublimation), regression, and denial. Freud believed that personality develops in a series of psychosexual stages, with the phallic (Oedipal) stage most crucial.

Several thinkers splintered from Freud's original psychodynamic views. Carl Jung believed that people share a collective unconscious that contains universal memories and images, called archetypes. The object-relations school emphasizes the importance of the first 2 years of life rather than the Oedipal phase; the infant's representations of important figures, especially the mother, rather than sexual needs and drives; and the problem in male development of breaking away from the mother.

Psychodynamic approaches have been criticized for violating the principle of falsifiability; for overgeneralizing from atypical patients to everyone; and for basing theories on the unreliable memories and retrospective accounts of adults, which can create an illusion of causality. However, some psychodynamic ideas have received empirical support, including the existence of nonconscious processes and defenses.

Most popular tests that divide personality into “types” are not valid or reliable. In research, psychologists typically rely on objective tests (inventories) to identify and study personality traits and disorders.

Gordon Allport argued that people have a few central traits that are key to their personalities and a greater number of secondary traits that are less fundamental. Raymond Cattell used factor analysis to identify clusters of traits that he considered the basic components of personality. Studies around the world provide strong evidence for the Big Five dimensions of personality: extroversion versus introversion, neuroticism (negative emotionality) versus emotional stability, agreeableness versus antagonism, conscientiousness versus impulsiveness, and openness to experience versus resistance to new experience. Although these dimensions are quite stable, some of them do change over the lifespan, reflecting maturational development, societal events, and adult responsibilities.

In human beings, individual differences in temperaments, such as reactivity, soothability, and positive or negative emotionality, emerge at birth or early in life and influence subsequent personality development.

Behavioral-genetic data from twin and adoption studies suggest that the heritability of many adult personality traits is about .50. Genetic influences create dispositions and set limits on the expression of specific traits.

Even traits that are highly heritable are often modified throughout life by circumstances, chance, and learning. Conclusions that “biology is destiny” or our fates are controlled by our genes are not warranted.

People often behave inconsistently in different circumstances when behaviors that are rewarded in one situation are punished or ignored in another. According to social-cognitive learning theory, personality results from the interaction of the environment and aspects of the individual, in a pattern of reciprocal determinism.

Three lines of evidence challenge the popular assumption that parents have the greatest impact on their children's personalities and behavior: (1) Behavioral-genetic studies find that the major environmental influence is from the nonshared environment; (2) few parents have a consistent childrearing style over time and with all their children; and (3) even when parents try to be consistent, there may be little relation between what they do and how the children turn out. However, parents can modify their children's temperaments, prevent children at risk of delinquency from choosing a path of antisocial behavior, and influence many of their children's values and attitudes.

One major environmental influence on personality comes from a person's peer groups, which can be more powerful than parents. Most children and teenagers behave differently with their parents than with their peers.

Many qualities that Western psychologists treat as individual personality traits are heavily influenced by culture. People from individualist cultures define themselves in different terms than those from collectivist cultures, and they perceive their “selves” as more stable across situations. Cultures vary in their norms for many behaviors, such as cleanliness and notions of time.

Cultural theories of personality face the problem of describing broad cultural differences and their influences on personality without promoting stereotypes or overlooking universal human needs.

Humanist psychologists focus on a person's subjective sense of self and the free will to change. They emphasize human potential and the strengths of human nature, as in Abraham Maslow's concepts of peak experiences and self-actualization. Carl Rogers stressed the importance of unconditional positive regard in creating a fully functioning person. Rollo May brought existentialism into psychology, emphasizing some of the inherent challenges of human existence that result from having free will, such as the search for meaning in life.

As another way of understanding personality from the “inside,” some personality psychologists study life narratives, the stories people create to explain themselves and make sense of their lives. These stories may serve to suppress changes in our lives or encourage them.

Some ideas from humanist psychology are subjective and difficult to measure, but others have fostered research on positive aspects of personality, such as optimism and resilience under adversity, and the importance of life narratives.