[...] There is however a less obvious, but far more important difference between nationalism and familial favoritism: Despite its mighty evolutionary basis, almost everyone recognizes moral strictures against familial favoritism. Almost everyone knows that “It would help my son” is not a good reason to commit murder, break someone’s arm, or steal. Indeed, almost everyone knows that “It would help my son” is not a good reason for even petty offenses – like judging a Tae Kwon Do tournament unfairly because your son’s a contestant.
Nationalism, in contrast, is widely seen as an acceptable excuse for horrific crimes against outgroups. Do you plan to murder hundreds of thousands of innocent foreign civilians? Just say, “It will save American [German/Japanese/Russian/whatever] lives” – and other members of your tribe will nod their heads. Do you want to deprive millions of foreigners of the basic human rights to sell their labor to willing buyers, rent apartments from willing landlords, and buy groceries from willing merchants? Just say, “It’s necessary to protect American jobs” in a self-righteous tone, then bask in the admiration of your fellow citizens - Bryan Caplan [via Café Hayek]