Without categories, the world would be a bewildering jumble of unrecognizable objects. When we encounter something new—a chair, a tree, a dangerous situation—we know what it is because it looks like something we’ve seen before. As it is with chairs, so it is with people. We plot their extroversion, their compassion, their neuroticism in relation to others—a million tiny signals that coalesce into the thing we label personality.

Today, personality tests have simplified, and monetized, this complex calculation. At interviews, assessment centers and team bonding events, testing is ubiquitous. Matchmakers and their online equivalents entice us with the promise that every question answered will bring us one step closer to unlocking our perfect partner. Some devotees even turn to pers...