"It’s what we call an over-exclusion bias," Mina Cikara, a Harvard psychologist who studies intergroup conflict, said. When you start fearing others "your circle of who you counted as friends is going to shrink. And that means those people outside of the bounds get less empathy, get fewer resources." It also means you become more vigilant and obsessed with marking who is an insider and who is not. "You want to draw those boundaries brighter, so you don’t make any mistakes about who you want to share your resources with or who you want to trust," she says.
Brian Resnick writing in Vox