If you are looking for evidence about how divided our country is, you just have to scroll through your social media feed. Why does it seem so hard for people to talk and understand others outside their political groups? Scientists say that our tendency toward an “us vs. them” mindset is actually an evolutionary trait meant to keep us safe. Yale psychologist Laurie Santos studied monkeys to understand how and why they make fast judgments about each other. And University of Chicago psychologist Sendhil Mullainaithan looked into how humans make similar snap judgments.