Social behavior boils down to the “Morris Theorem”: “Change is caused by lazy, greedy, frightened people looking for easier, more profitable, and safer ways to do things.” These people are much the same everywhere. Their societies develop along similar paths. Geography explains different outcomes. “Maps, not chaps,” as Morris likes to say.

“The agency of individuals actually matters much less than historians tend to assume,” Morris tells me. “It’s hard to find any examples of decisions made by single individuals that ­really changed the big story very much—until you get into the 20th century, when you’ve got nuclear weapons.”