Personality development is psychological term that defines the enduring pattern of thought, feeling, emotion, and behavior exhibited by one particular individual over another, that which goes into making each person unique. Most psychologists believe that personality emerges early in one's development, but changes and develops over the course of a lifetime. Many adult personality traits can be traced to their temperament as an infant, even before the development of language capabilities. As the old adage states, the child is the father of the man.
Personality development is affected by a number of factors. Genetics, for example, contributes somewhat, but is largely overshadowed by one's environment and life events. Thus removing the genetic component, as far as personality development goes, many siblings are no more alike than any random group of strangers. However, identical twins raised apart are as equally similar in personality to identical twins raised together, suggesting that personality development is ultimately a combination of nature and nurture.
Numerous theories of personality development have emerged since the early 20th century. Sigmund Freud's Stages of Psychosexual Development is one of the earliest, and most famous examples. Another, Jean Piaget's Stages of Cognitive Development, has children progressing through four stages of that markedly change how one thinks.