Echo chambers are definitely a thing but there are plenty of places where people from opposite ends of the political spectrum come into contact and the result is more polarization.

The mode of communication on the internet engenders this dynamic. People with radically different educational, racial and class backgrounds are thrust together in one place and they’re interacting in an environment where they can’t see each others’ facial features. Empathy is inhibited and people are more willing to say things they wouldn’t in person. Then there’s that lizard-brained effect where people just get angry and more aggressive and say offensive things.

The internet brings people who hang out on Tumblr into contact with folks who hang out on /pol/, i.e. two groups that are marginalized for different reasons. There’s a dialectical relationship at work. That’s the problem I have with Pinker’s shallow thesis. It’s not one causing the radicalization of the other.