Are you good at propositional calculus? How about at making Bobby eat his brussels sprouts?

In the world of propositional calculus, there's absolutely no difference between a rule about traveling to Boston by plane and a rule about eating vegetables to get dessert. But in our brains, there's an enormous difference: the first is a arbitrary rule about the world, and the second is a rule of social exchange. It's of the form "If you take Benefit B, you must first satisfy Requirement R."

Our brains are optimized to detect cheaters in a social exchange.