The social psychologists were right.

The latest episode of Crazy/Genius, produced by Patricia Yacob and Jesse Brenneman, analyzes the recent wave of internet-inspired violence—from Charlottesville to Christchurch—and asks why the web became such a fecund landscape for extremism. Hate is an ancient offline phenomenon. But something about the design of our social-media platforms—and perhaps something inherent to the internet itself—has amplified the worst angels of our nature. (Subscribe here.)

The psychological roots of online hatred have three levels. At the bottom, there is group polarization and the natural tendency of moderate people to become extremist versions of themselves when they interact with like-minded peers. At the next level, there is what you might call Viral Screaming Syndrome—the natural tendency of web content to veer toward high-arousal emotions, such as outrage and paranoia, to attract attention and promote social sharing. “Video is really expensive to make, and reported video is really, really expensive to make,” says the Atlantic staff writer Alexis Madrigal. “You know what’s not expensive to make? A bunch of random, paranoid opinions to cut through the noise.”

Finally, the largest social-media networks have built algorithms that exacerbate both group polarization and the Viral Screaming Effect. For example, YouTube executives knew that extreme and misleading videos were racking up tens of millions of views, but the company’s executives declined to intervene, because they were “focused on increasing viewing time and other measures of engagement,” according to a Bloomberg report in April.

“The No. 1 thing, though, that happens when you look into the failings of the various platforms, YouTube and Facebook specifically, is you see this kind of fractal irresponsibility,” Madrigal says. “They were launching in countries where they literally would have no idea what people were saying. And now they’re being asked to defend elections, they’re being asked to, like, understand deeply the social dynamics of every place in which they are.”

If you think the chief driver of online extremism is algorithmic, then your preferred solutions are likely to be algorithmic tweaks. That’s not enough. After all, the internet’s failures exist only because of the natural tendencies of group behavior. To fix social media’s problems, you have to address them at the level of the group.

For Whitney Phillips, an assistant professor of communication and rhetoric at Syracuse University, the ecosystem of online hate speech resembles a biomass pyramid—with apex predators, such as Alex Jones, at the top, and the rest of us playing the role of worms and fungi at the bottom. “The reason that Alex Jones and other abusers and bigots have that platform is because other people engage with them,” Phillips says. “These messages are able to spread way further and way more quickly than they ever would have been able to do on their own because we share them.” The virus of hateful extremism grows because so many people share it—even if they’re just trying to point out how terrible it is.