Life seems illogical sometimes. Individuals do risky things: take drugs, have unprotected sex, mug each other. If you’ve ever been in love, you know how crazy it feels, and divorce rarely looks like a rational process. The unfathomable appears on a larger scale: why do some neighborhoods thrive while others become ghettos, some cities live while others die? Why is the United States a democracy while China is not? Thorny questions, and you might think the outcome is impossible to predict. But economists believe that life is logical, and rational patterns often underpin what looks like chaos or confusion.