War and Peace and War: The Rise and Fall of Empires

by Peter Turchin

WHEN HUMANS EVOLVED THE ABILITY to cooperate with unrelated individuals, they relied on face-to-face interactions and memory to distinguish friends and acquaintances from the enemies or untrustworthy individuals in the group. There must have been an intense selection pressure for what science writer Malcolm Gladwell calls the “social channel capacity,” the ability to handle the complexities of living in large social groups. After all, to ensure cooperation you need to remember not only what each group member did to you, but also what they all did to each other. If Mary cheated Jane, she might also cheat you. When Bob fell asleep on guard duty, but John did not shun him, as he was supposed to do, it means John is failing to cooperate in a collective sanctioning task. Making a society work is a complicated business. But social channel capacity cannot be developed beyond a certain point—we cannot remember everyone on Earth, let alone what they have done to one another. As the size of the group increases, the number of relationships to remember explodes. If you belong to a group of 5 people, you only need to keep track of 10 separate relationships. If the group has 20 people, however, you need to remember 190 two-way relationships. The group size increased fourfold, but the number of relationships almost twenty-fold. Among the primates, humans live in the largest groups, and have the largest brains, but a limit is inescapable. “The figure of 150,” estimates the British anthrop...