What makes citizens obey the law is not always their sterling character. Instead, fear of punishment -- the shame of arrest, fines or imprisonment -- more often makes us comply with laws. Law enforcement is not just a way to deal with individual violators but also a way to remind society at large that there can be no civilization without legality. Or, as 17th-century British statesman George Savile famously put it: "Men are not hanged for stealing horses, but that horses may not be stolen." In the modern world, we call such prompt, uniform and guaranteed law enforcement "deterrence," from...