Personality includes those stable psychological characteristics that define each human being as unique. Both children and adults have personality traits (long-term characteristics, such as temperament) and states (changeable characteristics, such as moodiness). While a variety of explanations are possible, most experts agree that whatever the causes, an individual's personality is solidly established by the end of early childhood.

According to Freud, the second year of childhood is the anal stage of psychosexual development, when parents face many new challenges while toilet training their children. Fixations at this stage may give rise to characteristic personality traits that fully emerge in adulthood. These personality traits include anal retention (excessive neatness, organization, and withholding) or anal expulsion (messiness and altruism).

Personality theorists after Freud have attempted to explain early childhood personality development. Learning theorists claim that personality develops as a result of classical conditioning (Ivan Pavlov's learning by association), operant conditioning (B. F. Skinner's learning by reinforcement and punishment), and observational learning (Albert Bandura's learning by imitation). This latter category involves identification, or internalization, whereby children observe and adopt the values, ideas, and standards of their significant others. Cognitive psychologists speculate that personality arises, in part, from the attitudes and biases expressed by the adults around them. Gender theorists claim that personality develops from “gender identification” and “gender socialization”. Geneticists speculate that personality arises from “wired in” genetic and biochemical influences rather than psychosocial ones.

In the final analysis, no perspective alone can adequately explain the complex processes of personality development. A