For most of our evolutionary history, humans lived in tribes. This was critical for our survival. Breaking away from the tribe, or getting cast out, would render a life that was (to wrench Hobbes out of context) "solitary, poor, nasty, brutish, and short." We are tribal in nature.
For millennia, a tribal environment increased the odds of overcoming environmental challenges, including competition with other tribes. In such a setting, loyalty -- commitment to the tribe -- strengthened the cohesion of the group, making success more likely. Through social cooperation (made possible within the security of their community) humans increase their prospects for survival and for enjoying an environment in which they can flourish.
Humans have evolved as a tribal species. Tribal loyalty has been selected for. It has been bred into us, providing many advantages. But of course there are downsides as well. Close knit groups feature sharp divisions between those inside and those outside. This dichotomy almost inevitably generates, in some measure, suspicion, hostility, fear, or hatred toward the outsiders. These attitudes may have no reasonable basis. Tribalism has colored the insiders' cognitive judgments: "Although tribal loyalties inspire many noble behaviors, they can impel humans to sacrifice sound reasoning and judgmental accuracy for group belonging and commitment …. In other words, tribal loyalties can lead to tribal biases."
These emotional eruptions render cooperation with others (outsiders) impossible. And, when political tribes face off, that's a problem for democracy.
Tribalism is often referenced in discussion of political polarization (Red vs. Blue America), as it does in today's Washington Post. Without further ado, here is the quote of the day:
“The evolution of cooperation required out-group hatred. Which is really sad,” said Nicholas Christakis, a Yale sociologist and author of “Blueprint: The Evolutionary Origins of a Good Society.”