The middle class is at the center of a shape-changing U.S. economy. Here's what you need to know. What's the shape of the U.S. economy right now? It's an "E," apparently. Forget the K-shaped economy.

A theory that's been around for more than a decade describes a person's subjective well-being—"happiness"—as having a U-shape throughout the course of one's life. If plotted on a graph, the shape ...