Psychologists use theory to categorize and explain different personalities. These explanations have been influenced by the various branches of psychology. Some psychologists study what motivates us and/or our emotional responses to experiences to understand our individual differences. Other psychologists seek to understand personality, including why different personalities exist, how they are developed, and if and how they change. Originating from the psychodynamic perspective, the study of personality involves consideration of behavior and mental processes and how they interact to produce an individual’s personality. A full explanation of personality also involves incorporating humanistic and social-cognitive perspectives from earlier units.