Two terrible tendencies now feed off each other, growing stronger every day: the more smugness, the more satisfying to poke holes in it; the more toxic the trolling, the greater the sense of moral superiority. The result: an odoriferous stew of political rhetoric that is nearly irresistible to those on the inside and confusingly abhorrent to those on the outside.

The explosion of the smugs-vs.-trolls phase of our political discourse is traceable to a now infamous 2004 confrontation between Jon Stewart and Tucker Carlson in the waning days of “Crossfire,” in which Mr. Stewart, a comedian, dropped his jester’s mask and accused Mr. Carlson and his ilk of undermining serious discourse with their partisan feuding and made-for-TV talking points. “Stop hurting America,” was his specific request. Mr. Carlson sputtered and fumed; it was generally agreed that Mr. Stewart won the day.

Right around the same time, New York University psychologist, Jonathan Haidt, was formulating a theory about why liberals and conservatives have such a hard time productively conversing. After mucking around in a lot of survey data, he came up with this basic idea: Liberals and people of the left underpin their politics with moral concerns about harm and fairness; they are driven by the imperative to help the vulnerable and see justice done. Conservatives and people of the right value these things as well but have several additional moral touchstones — loyalty, respect and sanctity. They value in-group solidarity, deference to authority, and the protection of purity in mind and body. To liberals, those sincerely held values can look a lot like, in Dr. Haidt’s words, “xenophobia, authoritarianism and Puritanism.” This asymmetry is the fountainhead of mutual incomprehension and disdain.