The zero-sum nature of the medieval economy was reinforced by a Christian ideology that was hostile to any commercial practice or technological innovation that might eke more wealth out of a given stock of physical resources. As Tuchman explains: The Christian attitude toward commerce . . . held that money was evil, that according to St. Augustine “Business is in itself an evil,” that profit beyond a minimum necessary to support the dealer was avarice, that to make money out of money by charging interest on a loan was the sin of usury, that buying goods wholesale and selling them unchanged at a higher retail price was immoral and condemned by canon law, that, in short, St. Jerome’s dictum was final: “A man who is a merchant can seldom if ever please God.” As my grandfather would have put it, “Goyische kopp!”—gentile head. Jews were brought in as moneylenders and middlemen but were just as often persecuted and expelled.

The era’s economic backwardness was enforced by laws which decreed that prices should be fixed at a “just” level reflecting the cost of the raw material and the value of the labor added to it. “To ensure that no one gained an advantage over anyone else,” Tuchman explains, “commercial law prohibited innovation in tools or techniques, underselling below a fixed price, working late by artificial light, employing extra apprentices or wife and under-age children, and advertising of wares or praising them to the detriment of others.” This is a recipe for a zero-sum game, and leaves predation as the only way people could add to their wealth.

A positive-sum game is a scenario in which agents have choices that can improve the lots of both of them at the same time. A classic positive-sum game in everyday life is the exchange of favors, where each person can confer a large benefit to another at a small cost to himself or herself. Examples include primates who remove ticks from each other’s backs, hunters who share meat whenever one of them has felled an animal that is too big for him to consume on the spot, and parents who take turns keeping each other’s children out of trouble. As we shall see in chapter 8, a key insight of evolutionary psychology is that human cooperation and the social emotions that support it, such as sympathy, trust, gratitude, guilt, and anger, were selected because they allow people to flourish in positive-sum games. 37 A classic positive-sum game in economic life is the trading of surpluses. If a farmer has more grain than he can eat, and a herder has more milk than he can drink, both of them come out ahead if they trade some wheat for some milk. As they say, everybody wins. Of course, an exchange at a single moment in time only pays when there is a division of labor. There would be no point in one farmer giving a bushel of wheat to another farmer and receiving a bushel of wheat in return.