Although to many the study of individual behavior has meant the study of individual differences in behavior, an adequate theory of personality process and structure must also account for similarities in behavior. The classical test theory metaphor used by applied and personnel psychologists and the analysis of variance metaphor used by the interactionists, although compelling, both emphasize sources of variation rather than sources of consistency. Athough it is important to consider the interaction of persons and situations as well as the effects of individual and situational differences, by using either a correlational or an analysis of variance metaphor we are unable to ask questions other than how some people are the same and some are different. A generalization of the analysis of variance metaphor is to consider the other components of the general linear model. Estimates of any particular behavior are expressed in terms of the central tendency across all people, the responsivity to particular situational and person variables, the interaction between the situational and person variables, as well as the reliable within person variance and that associated with unknown sources of variance.
Theories differ in their central focus as well as in their range of generalizability.
Evolutionary personality theory, psychoanalytic theory, behavior theory, and sociology emphasize the commonalties of individual behaviors. Every member of every species needs to meet the challenges of survival and of reproduction. How these challenges are met within a species reflects species typical solutions. By understanding how these problems are answered by humans as a species we can understand the fundamentals of human nature. Trait theorists focus on systematic individual differences and similarities among people.