Principle: As Adam Smith explained, businesses thrive when workers specialize and do what they do best. Its offshoot, comparative advantage – in which nations produce what they're best at making – is the foundation of free trade.

Payoff: Think of your marriage as an economy and you and your spouse as trading partners exchanging goods and services in the form of household chores. Just as Japan makes quality electronics and Italy stitches the best suits, so too can each spouse take on the chores he or she does relatively better or faster. "Everyone is better off if they go their separate ways and do what each does best," says Professor Fisman. "I cook more efficiently than my wife," but his wife is more effective at cleaning, he adds.