Although Darwin's 19th century advocates stressed the role of competition in natural selection, Darwin established speculation about cultural evolution in regard to human cooperation: "It must not be forgotten that although a high standard of morality gives but slight or no advantage to each individual man and his children over other men of the tribe, yet that an increase in the number of well-endowed men and an advancement in the standard of morality will certainly give an immense advantage to one tribe over another. A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice for the common good, would be victorious over most other tribes; and t his would be natural selection."

The authors note that human social complexity is far beyond that of other social animals, and that biological mechanisms of kin selection and reciprocity are not adequate to fully explain human social behavior. Culture, defined as "information stored in individual brains (or in books and analogous media) that was acquired by imitation of, or teaching by others," has the properties of transmission forward through time and selection of successful strategies in common with genetic evolution. As Homo sapiens evolved, the mental and social capacity for cooperative work was highly adaptive for groups of small, relatively weak creatures with neither fangs nor claws nor wings. "Social instincts" enabled humans to band together in groups larger than the 50 individuals that our brain size allows in other primates. But that made cultural transmission of learning possible, which over-rode genetic group selection.

Although the Pleistocene era, with its radical climate changes, could have exerted long-term pressure on genetic group selection, the rapid evolution of social complexity over the past 10,000 years has not been long enough for significant genetic selection. Culture is both enabled by genetically-shaped human sociality, and is a means of progressively ratcheting mutually beneficial social cooperation. Once sociality, learning, and symbolic media make it possible to externalize and transmit individual learning, cooperative invention changes the game. Individual innovators can gain advantage through prestige and reputation, but only by displaying what they know, while learning and innovation enable the entire tribe to benefit from their innovations.

Culture harnesses and channels social instincts, enabling the creation of institutions. Norms enable the diffusion of enforcement of altruistic punishment through the population, leveraging emotions such as anger and shame to guard against free-riding, defection, and exploitation.