Evolutionary psychology is one of many biologically informed approaches to the study of human behavior. Along with cognitive psychologists, evolutionary psychologists propose that much, if not all, of our behavior can be explained by appeal to internal psychological mechanisms.

The theory of natural selection was explored by 19th-century naturalist Charles Darwin. Natural selection explains how genetic traits of a species may change over time. This may lead to speciation, the formation of a distinct new species.

One example of recent natural selection in humans involves the ability to tolerate the sugar, lactose, in milk. In most parts of the world, adults are unable to drink milk because their body switches off the intestinal production of lactase, an enzyme that digests the sugar in the milk, after weaning.

Behaviorism, also known as behavioral psychology, is a theory of learning which states all behaviors are learned through interaction with the environment through a process called conditioning. Thus, behavior is simply a response to environmental stimuli.

A frequent criticism of evolutionary psychology is that its theories and assumptions are not falsifiable. One theory, for example, asserts that human social behavior is guided by specific evolved predispositions that were selected because they enhanced reproductive success during human evolutionary history.

Behavior has been viewed as a pacemaker of evolutionary change because changes in behavior are thought to expose organisms to novel selection pressures and result in rapid evolution of morphological, life history and physiological traits.

To the extent that behaviors are controlled by genes, they may evolve through natural selection. If behaviors increase fitness, they are likely to become more common over time. If they decrease fitness, they are likely to become less common.

Kin selection, a type of natural selection that considers the role relatives play when evaluating the genetic fitness of a given individual. Kin selection occurs when an animal engages in self-sacrificial behaviour that benefits the genetic fitness of its relatives.

Kin selection (commonly referred to as altruism) is an example of an adaptive behavior that directly influences the genetic composition of a population. It involves evolutionary strategies that favor the persistence of an organism’s relatives, often at the cost of the organism’s own survival and reproduction.