“When unwritten rules are violated over and over, we become overwhelmed — and then desensitized,” Levitsky wrote. “We grow accustomed to what we previously thought to be scandalous.”

Something else happens that’s even more deadly. When people in positions of power use dehumanizing language to describe other groups, atrocities often follow.

This is not ancient history: Consider what happened less than 30 years ago in Rwanda when some 800,000 civilians were slaughtered in a three-month period in 1994. Hutu extremists targeted both the Tutsi minority, who were a majority of those killed, as well as moderate Hutus.

What triggered the violence in part were the messages that came from people in positions of power in Rwanda. Many, like Rogan, had a public megaphone and an audience.

In a New Yorker essay on “How Norms Change,” the author Maria Konnikova described how Hutu leaders took to the radio calling Tutsis “cockroaches,” sanctioning the violence that followed. She said that “norms can shift at the speed of social life” when the wrong leaders command the public’s attention.