One sign of an innate predisposition, a part of human nature: When people do things that make no logical or emotional sense. For example, part of the evidence of a built-in language ``module'' is the absence of logic in grammar. The past tense of most English verbs is formed by adding ``-ed'' (he departed) but sometimes it is not (he went). Fact and reason can't tell which ones are which, any more than they can tell you why in French an automobile is a ``she'' and Christmas is a ``he.''
Similarly, I think, people's propensity to see and care about ``our'' people is independent of reason and even of deep emotions (like self-preservation). This is not apparent when a political entity fights to prevent itself from being conquered by another. Faced with an enemy that wants to prevent you from speaking your language, practicing your religion, honoring your ancestors, you might well feel that the nation's cause aligned with reason and your urge for self-preservation.
People also fight for other kinds of ``us,'' however. Kinds that involve no real political, economic or cultural stakes. When people die because Sunni and Shiites are fighting in the Middle East, onlookers might say, ``well, that's religion's effect.'' But when someone is killed over soccer? When 70 other people are injured? That's a sign that people's love of teaming up is capable of sparking over anything. Like grammar, our map of human kinds -- which tells us where we belong, and who ``we'' don't like -- is deducing, and obeying, its own independent rules.
