Racial prejudice apparently stems from deep evolutionary roots and a universal tendency to form coalitions and favor our own side. And yet what makes a “group” is mercurial: In experiments, people easily form coalitions based on meaningless traits or preferences—and then favor others in their “group.” Researchers have explored these innate biases and begun to ask why such biases exist. What factors in our evolutionary past have shaped our coalitionary present—and what, if anything, can we do about it now? Several avenues of research are probing the origins of what many psychologists call in-group love and out-group hate. Researchers are testing the implicit biases of young children and even primates, and devising experiments to ratchet bias up and down. Evolutionary researchers are trying to parse the group environments of our ancestors and are debating just how big a selective pressure came from out-group male warriors.