Contrary to the lefty hypothesis that nationalism activates prejudice toward outgroups, a new study in the Personality and Social Psychology Bulletin finds that subliminal exposure to the American flag reduces hostility toward outgroups (PDF – subscription required):

Three studies examined the implications of nationalistic
ideologies and exposure to the U.S. flag for the activation of egalitarian concepts and outgroup hostility.
Study 1 demonstrated that subliminal exposure to the
U.S. flag activated participants’ egalitarian concepts. In
Study 2, highly nationalistic participants who were
exposed to the U.S. flag reported less hostility than did
those not exposed to the flag, whereas the flag did not
influence the hostility of participants low in national-
ism. Study 3 demonstrated that for participants high in
nationalism, greater activation of egalitarian concepts
on subliminal exposure to the U.S. flag was associated
with less hostility toward Arabs and Muslims in the
presence of the U.S. flag. The theoretical and practical
implications of these findings are discussed.

Update 2/26 7:27 AM: As a commenter points out below, my wording was imprecise. Individuals who are highly nationalistic are, in general, more hostile to outgroups. What I was trying to say is that this perspective would lead many on the left to suggest that exposure to the flag would prime this ingroup bias. Instead, the study above shows that subliminal exposure to the flag reduces hostility toward Arabs and Muslims. This encouraging finding mirrors other recent work distinguishing between nationalism and forms of patriotism that are not linked with outgroup hostility.