For most of recorded history, China’s economy dwarfed America’s. Yet by 1900 the roles had flipped. Using Angus Maddison’s long-run GDP estimates (in purchasing power parity terms), the US economy overtook China’s sometime between 1870 and 1900: China’s share of world GDP fell from about 17% to 11% while the US rose from 10% to 18%. That reversal, and today’s renewed race, hinges on two forces: population and income per person.