Individual differences in human motivation and emotion that appear early in life.

Psychologists have long argued about what causes differences in personality. In earlier centuries, theorists like Galen have invoked nature, claiming that a difference in the humours or fluids in the bodies was responsible for personality. During most of the twentieth century, political ideology, discoveries about the learning or conditioning capabilities of infants, and the emergence of psychoanalytic theory, which emphasized the importance of early experience, all combined to discredit biological explanations for human motivation and emotion. Nurture and socialization became the explanations of favor.

In the latter half of this century, there has been a resurgence of interest in the contribution of temperament to children's development. Although a number of theorists have their own distinctive definitions, temperament is generally agreed to be a source of individual differences in emotions or motivations (i.e., not cognitive or...