President Trump’s proposal to “red flag” potential mass shooters is well-intentioned. If we could prevent even one mass killing by identifying and disarming the potential perpetrator beforehand, it would be worthwhile. But do we have the tools to do it, and at what cost to our constitutional rights?

I have studied, taught and written for half a century about the difficulties of predicting violence. My first scholarly article, in 1970, was titled “The Law of Dangerousness: Some Fictions About Predictions,” and a subsequent...