In the wake of the horrific school shootings in Parkland, Fla., President Trump has called repeatedly for building or reopening mental institutions. Strangely, perhaps, he has echoed an argument made by some experts who study the mental health care system. It’s not that they believe that having more institutions would somehow prevent spree killings, as Mr. Trump apparently does. The majority of these murderers appear to be angry, antisocial individuals — with access to guns — whom the mental health system probably could not have spotted in advance. The proposal to bring back asylums — in a modern, transparent form...