Perceptions, memories, fears, hopes and wants are all blended by your mind to create a map of reality. One of the most important parts of that map is identity: your sense of who belongs where, in a world made up of races, ethnic groups, nations, religions, cultures, political parties, and countless other categories of person.

But how does the mind come up with these labels? How does it sort their overlapping claims -- the demands of being, at the same time, for instance, a Muslim, an American, a soccer mom'' and a Democrat? How does it decide which labels are for fun, like Red Sox fan,'' and which labels are serious (likeRed State voter''). Why can the same religious or political identity mean life or death in one place, but count for little in another? And, most of all, why do people the world over care so much about these groupings? Why are we willing to kill, or to die, for a nation, a religion, or a football team?