Mathematics can be beguilingly elegant. It can also be dangerous when people mistake its elegance for truth.

Albert Einstein¡¯s theory of general relativity might be the best example of elegant math, capturing a wide range of subtle and surprising phenomena with remarkable simplicity. Step toward the practical, though, and physics moves quickly away from elegance to makeshift usefulness. There¡¯s no pretty expression for the operation of a nuclear reactor, or for how air flows past the swept wings of an aircraft. Understanding demands ugly approximations, or brute-force simulation on a large computer.

In one very practical and consequential area, though, the allure of elegance has exercised a perverse and lasting influence. For several decades, economists have sought to express the way millions of people and companies interact in a handful of pretty equations.