Over the past decade, a psychological pattern has spread across American life, crossing class and geography: chronic political anxiety marked by outrage and a constant sense of threat.

A patient paused mid-thought in my Manhattan office the other day and apologized: “I know I’m saying this as a white guy.” The comment felt out of place, so I asked him what he meant. When his explanation still didn’t fit, I asked what being a white guy had to do with the situation at...