There are numerous approaches to discovering a systematic theory of personality in order to answer some of the bothersome questions that have developed in psychology regarding this phenomenon. Personality theory must answer first what makes people behave alike, and also what makes people behave differently. The first question tries to find the conditions, factors, and variables that can account for the reactions the members of the human species show in common, and the second tries to explain the observed differences in the behavior of different individuals in response to the same situation. Another problem in personality theory is to explain what remains the same throughout the lifetime of the individual, what there is of personality that survives through time.

Personality theories can be grouped under a number of …