Over the weekend, an 18-year-old man livestreamed himself shooting 13 people and killing 10. Within hours it became clear that the shooter’s intent was to kill as many Black people as possible. The suspect wrote online that he was motivated by replacement theory — a racist idea that white people are deliberately being replaced by people of color in places like America and Europe. What are the origins of this theory, and how has it become simultaneously more extreme and more mainstream? Guest: Nicholas Confessore, a political and investigative reporter for The New York Times.