But journalistic groupthink is a symptom, not a cause. And when it comes to the cause, there's another, blunter way to think about the question than screaming "bias" and "conspiracy," or counting D's and R's. That's to ask a simple question about the map. Where do journalists work, and how much has that changed in recent years? To determine this, my colleague Tucker Doherty excavated labor statistics and cross-referenced them against voting patterns and Census data to figure out just what the American media landscape looks like, and how much it has changed.
The results read like a revelation. The national media really does work in a bubble, something that wasn't true as recently as 2008. And the bubble is growing more extreme. Concentrated heavily along the coasts, the bubble is both geographic and political. If you're a working journalist, odds aren't just that you work in a pro-Clinton county—odds are that you reside in one of the nation's most pro-Clinton counties. And you've got company: If you're a typical reader of Politico, chances are you're a citizen of bubbleville, too.
Resist—if you can—the conservative reflex to absorb this data and conclude that the media deliberately twists the news in favor of Democrats. Instead, take it the way a social scientist would take it: The people who report, edit, produce and publish news can't help being affected—deeply affected—by the environment around them.