Franz Boas’s description of Inuit (因纽特人) life in the 19th century illustrates the probable moral code of early humans. Here, norms (规范) were unwritten and rarely expressed clearly, but were well understood and taken to heart. Dishonest and violent behaviours were disapproved of; leadership, marriage and interactions with other groups were loosely governed by traditions. Conflict was often resolved in musical battles. Because arguing angrily leads to chaos, it was strongly discouraged. With life in the unforgiving Northern Canada being so demanding, the Inuit’s practical approach to morality made good sense.

The similarity of moral virtues across cultures is striking, even though the relative ranking of the virtues may vary with a social group’s history and environment. Typically, cruelty and cheating are discouraged, while cooperation, humbleness and courage are praised. These universal norms far pre-date the concept of any moralising religion or written law. Instead, they are rooted in the similarity of basic human needs and our shared mechanisms for learning and problem solving. Our social instincts (本能) include the intense desire to belong. The approval of others is rewarding, while their disapproval is strongly disliked. These social emotions prepare our brains to shape our behaviour according to the norms and values of our family and our community. More generally, social instincts motivate us to learn how to behave in a socially complex world.

The mechanism involves a repurposed reward system originally used to develop habits important for self-care. Our brains use the system to acquire behavioural patterns regarding safe routes home, efficient food gathering and dangers to avoid. Good habits save time, energy and sometimes your life. Good social habits do something similar in a social context. We learn to tell the truth, even when lying is self-serving; we help a grandparent even when it is inconvenient. We acquire what we call a sense of right and wrong.

Social benefits are accompanied by social demands: we must get along, but not put up with too much. Hence self-discipline is advantageous. In humans, a greatly enlarged brain boosts self-control, just as it boosts problem-solving skills in the social as well as the physical world. These abilities are strengthened by our capacity for language, which allows social practices to develop in extremely unobvious ways.