In 1978, the farmers in a small Chinese village called Xiaogang gathered in a mud hut to sign a secret contract. They thought it might get them executed. Instead, it wound up transforming China's economy in ways that are still reverberating today. The contract was so risky — and such a big deal — because it was created at the height of communism in China. Everyone worked on the village's collective farm; there was no personal property. "Back then, even one straw belonged to the group," says Yen Jingchang, who was a farmer in Xiaogang in 1978. "No one owned...