Evolutionary psychology
Theoretical foundations

The theories on which evolutionary psychology is based originated with Charles Darwin's work, including his speculations about the evolutionary origins of social instincts in humans. Modern evolutionary psychology, however, is possible only because of advances in evolutionary theory in the 20th century.

Evolutionary psychologists say that natural selection has provided humans with many psychological adaptations, in much the same way that it generated humans' anatomical and physiological adaptations. As with adaptations in general, psychological adaptations are said to be specialized for the environment in which an organism evolved, the environment of evolutionary adaptedness. Sexual selection provides organisms with adaptations related to mating. For male mammals, which have a relatively high maximal potential reproduction rate, sexual selection leads to adaptations that help them compete for females. For female mammals, with a relatively low maximal potential reproduction rate, sexual selection leads to choosiness, which helps females select higher quality mates. Charles Darwin described both natural selection and sexual selection, and he relied on group selection to explain the evolution of altruistic (self-sacrificing) behavior. But group selection was considered a weak explanation, because in any group the less altruistic individuals will be more likely to survive, and the group will become less self-sacrificing as a whole.

In 1964, the evolutionary biologist William D. Hamilton proposed inclusive fitness theory, emphasizing a gene-centered view of evolution. Hamilton noted that genes can increase the replication of copies of themselves into the next generation by influencing the organism's social traits in such a way that (statistically) results in helping the survival and reproduction of other copies of the same genes (most simply, identical copies in the organism's close relatives). According to Hamilton's rule, self-sacrificing behaviors (and the genes influencing them) can evolve if they typically help the organism's close relatives so much that it more than compensates for the individual animal's sacrifice. Inclusive fitness theory resolved the issue of how altruism can evolve. Other theories also help explain the evolution of altruistic behavior, including evolutionary game theory, tit-for-tat reciprocity, and generalized reciprocity. These theories help to explain the development of altruistic behavior, and account for hostility toward cheaters (individuals that take advantage of others' altruism).

Several mid-level evolutionary theories inform evolutionary psychology. The r/K selection theory proposes that some species prosper by having many offspring, while others follow the strategy of having fewer offspring but investing much more in each one. Humans follow the second strategy. Parental investment theory explains how parents invest more or less in individual offspring based on how successful those offspring are likely to be, and thus how much they might improve the parents' inclusive fitness. According to the Trivers–Willard hypothesis, parents in good conditions tend to invest more in sons (who are best able to take advantage of good conditions), while parents in poor conditions tend to invest more in daughters (who are best able to have successful offspring even in poor conditions). According to life history theory, animals evolve life histories to match their environments, determining details such as age at first reproduction and number of offspring. Dual inheritance theory posits that genes and human culture have interacted, with genes affecting the development of culture, and culture, in turn, affecting human evolution on a genetic level, in a similar way to the Baldwin effect.
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