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.
Species typical behaviors that are the result of genetic selection are proposed by evolutionary psychologists who ask about the origins and reasons for human nature. Behavior geneticists examine the genetic architecture of specific traits as well as the covariances of traits with each other and with different parts of the environment. Behavior genetic analyses demonstrate within family and between family environmental effects. Genes affect particular dimensions of individual differences by modifying biological structures and regulating ongoing processes. Rather than the evolutionary question of why, explanations at the biological level ask how .