Charles Darwin wasted no time applying his theory of evolution to human psychology, following On the Origin of Species (1859) with The Descent of Man (1871) and The Expression of the Emotions in Man and Animals (1872). Ever since, the issue hasn’t been whether evolutionary theory can illuminate the study of psychology but how it will do so. Still, a concerted effort to explain how evolution has affected human behavior began only in the 1970s with the emergence of sociobiology. The core idea of sociobiology was simple: behavior has evolved under natural and sexual selection (in response to competition for survival and reproduction, respectively), just as organic form has. Sociobiology thereby extended the study of adaptation to include human behavior.