This chapter introduces the concepts of rational choice theory: incentives, rules, and constraints. It defines them and explains the connections between them, drawing on examples from everyday life. These concepts are identified as the key to finding the sense in seemingly senseless social practices.

This chapter uses the economic way of thinking to understand judicial ordeals—trials by fire and water—in medieval Europe. It explains how judges leveraged citizens' superstition through ordeals to find fact in criminal cases. This enabled judges to accurately determine defendants' guilt or innocence where "ordinary" evidence was absent.

This chapter uses the economic way of thinking to understand the sale of wives at public auctions in Industrial Revolution England. It explains how unhappy wives used wife sales to exit marriage where the law effectively gave husbands the right to their wives' marital status and denied wives property rights. This enabled spouses to forge Coasean divorce bargains indirectly when they couldn't do so directly.

This chapter uses the economic way of thinking to understand Vlax Gypsy superstitions: belief in ritual pollution, belief that pollution is contagious, and belief that non-Gypsies are dangerously polluted. It explains how Gypsies use these superstitions to support social ostracism as a means of governing their societies. This enables Gypsies to secure public order in their communities despite their inability to rely on government or ostracism alone for this purpose.

This chapter uses the economic way of thinking to understand monastic maledictions—divine curses—in tenth- through twelfth-century Francia. It explains how clerics used maledictions to protect their communities' property rights against plunder. This enabled clerics to secure their property rights despite government's absence and their inability to rely on physical self-help for this purpose.

This chapter uses the economic way of thinking to understand oracles among the Azande people of Africa. It explains how the Azande leveraged their superstition through a poisoned-chicken oracle—benge—to peacefully resolve petty conflicts with their neighbors. This enabled the Azande to address cooperation-threatening animus that couldn't be addressed through formal legal institutions.

This chapter uses the economic way of thinking to understand the criminal prosecution of insects and rodents by ecclesiastic courts in Renaissance France, Italy, and Switzerland. It explains how ecclesiastics used vermin trials to bolster citizens' waning belief in the validity of ecclesiastics' supernatural sanctions. This enabled ecclesiastics to improve tithe compliance where heretics threatened their tithe revenue.

This chapter uses the economic way of thinking to understand trial by battle in land disputes in Norman England. It explains how judges used judicial combats as "violent auctions" to allocate contested property rights to the litigants who valued them more when judges were unable to identify those rights' true owners. This enabled judges to efficiently allocate contested property rights where a high cost of trading land prevented the Coase theorem from doing so.