The biological theory, behavioral theory, psychodynamic theory, humanist theory and trait theory are psychological theories of personality. The study of personality includes multiple questions of what makes each human unique and the ways in which each is similar to others. Personality studies include using basic psychometric and assessment techniques.
Biological theorists such as Hans Eysenck assume that genetics are responsible for personality. Research pertaining to heritability has shown evidence of a link between personality and genetics. Behavioral theorists, including B.F. Skinner and John B. Watson, believe that personality is a result of interactions individuals have with their specific environments.
The psychodynamic theory emphasizes that the unconscious mind and childhood experiences are causes of personality. Sigmund Freud and Erik Erikson's psychodynamic theories include the psychosexual stage theory and stages of psychosocial development. Sigmund Freud believed that the id, ego and superego are three components of personality.
Humanist theorists such as Carl Rogers and Abraham Maslow emphasize that free will and individual experiences develop personality. The innate need for personal growth that influences behavior, known as self-actualization, is also emphasized in the humanist theory. The trait theory claims that personality is made up of a number of broad traits. Some of these traits include Eysenck's three-dimension theory and the five-factor theory of personality.