So it goes with the fiction we read, the movies we watch, the music we listen to and, scarily, the ideas we subscribe to. They’re not challenged. They’re validated and reinforced. By bookmarking given blogs and personalizing social-media feeds, we customize the news we consume and the political beliefs we’re exposed to as never before. And this colors our days, or rather bleeds them of color, reducing them to a single hue.

We construct precisely contoured echo chambers of affirmation that turn conviction into zeal, passion into fury, disagreements with the other side into the demonization of it. Then we marvel at the Twitter mobs that swarm in defense of Sanders or the surreal success of Donald Trump’s candidacy, whose historical tagline may well be “All I know is what’s on the Internet.”

Those were his exact words, a blithe excuse for his mistaken assertion that a protester at one of his rallies had ties to Islamic extremists. He’d seen a video somewhere. He’d chosen to take it at face value. His intelligence wasn’t and isn’t vetted but viral — and conveniently suited to his argument and needs. With a creative or credulous enough Google search, a self-serving “truth” can always be found, along with a passel of supposed experts to vouch for it and a clique of fellow disciples.

Carnival barkers, conspiracy theories, willful bias and nasty partisanship aren’t anything new, and they haven’t reached unprecedented heights today. But what’s remarkable and sort of heartbreaking is the way they’re fed by what should be strides in our ability to educate ourselves. The proliferation of cable television networks and growth of the Internet promised to expand our worlds, not shrink them. Instead they’ve enhanced the speed and thoroughness with which we retreat into enclaves of the like-minded.

Eli Pariser parsed all of this in his 2011 book “The Filter Bubble,” noting how every tap, swipe and keystroke warps what comes next, creating a tailored reality that’s closer to fiction. There was subsequent pushback to that analysis, including from scientists at Facebook, who published a peer-reviewed study in the journal Science last year that questioned just how homogeneous a given Facebook user’s news feed really was.

But there’s no argument that in an era that teems with choice, brims with niche marketing and exalts individualism to the extent that ours does, we’re sorting ourselves with a chillingly ruthless efficiency. We’ve surrendered universal points of reference. We’ve lost common ground.

“Technology makes it much easier for us to connect to people who share some single common interest,” said Marc Dunkelman, adding that it also makes it easier for us to avoid “face-to-face interactions with diverse ideas.” He touched on this in an incisive 2014 book, “The Vanishing Neighbor,” which belongs with Haidt’s work and with “Bowling Alone,” “Coming Apart” and “The Fractured Republic” in the literature of modern American fragmentation, a booming genre all its own.