The second exogenous change during the later Middle Ages was an economic revolution. The economic base of the feudal system was land and the peasants who worked it. As real estate agents like to say, land is the one thing they can’t make more of. In an economy based on land, if someone wants to improve his standard of living, or for that matter maintain it during a Malthusian population expansion, his primary option is to conquer the neighboring lot. In the language of game theory, competition for land is zero-sum: one player’s gain is another player’s loss.
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.” 35

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.”36 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. A fundamental insight of modern economics is that the key to the creation of wealth is a division of labor, in which specialists learn to produce a commodity with increasing cost-effectiveness and have the means to exchange their specialized products efficiently. One infrastructure that allows efficient exchange is transportation, which makes it possible for producers to trade their surpluses even when they are separated by distance. Another is money, interest, and middlemen, which allow producers to exchange many kinds of surpluses with many other producers at many points in time.
Positive-sum games also change the incentives for violence. If you’re trading favors or surpluses with someone, your trading partner suddenly becomes more valuable to you alive than dead. You have an incentive, moreover, to anticipate what he wants, the better to supply it to him in exchange for what you want. Though many intellectuals, following in the footsteps of Saints Augustine and Jerome, hold businesspeople in contempt for their selfishness and greed, in fact a free market puts a premium on empathy. 38 A good businessperson has to keep the customers satisfied or a competitor will woo them away, and the more customers he attracts, the richer he will be. This idea, which came to be called doux commerce (gentle commerce), was expressed by the economist Samuel Ricard in 1704:

Commerce attaches [people] to one another through mutual utility.... Through commerce, man learns to deliberate, to be honest, to acquire manners, to be prudent and reserved in both talk and action. Sensing the necessity to be wise and honest in order to succeed, he flees vice, or at least his demeanor exhibits decency and seriousness so as not to arouse any adverse judgment on the part of present and future acquaintances.39

And this brings us to the second exogenous change. Elias noted that in the late Middle Ages people began to unmire themselves from technological and economic stagnation. Money increasingly replaced barter, aided by the larger national territories in which a currency could be recognized. The building of roads, neglected since Roman times, resumed, allowing the transport of goods to the hinterlands of the country and not just along its coasts and navigable rivers. Horse transport became more efficient with the use of horseshoes that protected hooves from paving stones and yokes that didn’t choke the poor horse when it pulled a heavy load. Wheeled carts, compasses, clocks, spinning wheels, treadle looms, windmills, and water mills were also perfected in the later Middle Ages. And the specialized expertise needed to implement these technologies was cultivated in an expanding stratum of craftsmen. The advances encouraged the division of labor, increased surpluses, and lubricated the machinery of exchange. Life presented people with more positive-sum games and reduced the attractiveness of zero-sum plunder. To take advantage of the opportunities, people had to plan for the future, control their impulses, take other people’s perspectives, and exercise the other social and cognitive skills needed to prosper in social networks.