Social psychologist Jonathan Haidt in NYTimes.com, a selection from his book The Righteous Mind: Why Good People Are Divided by Politics and Religion (2012):
We evolved to be tribal, and politics is a competition among coalitions of tribes. When people feel that a group they value—be it racial, religious, regional, or ideological—is under attack, they rally to its defense, even at some cost to themselves. The great trick that humans developed at some point in the last few hundred thousand years is the ability to circle around a tree, rock, ancestor, flag, book, or god, and then treat that thing as sacred. People who worship the same idol can trust one another, work as a team, and prevail over less cohesive groups. So if you want to understand politics, and especially our divisive culture wars, you must follow the sacredness.