One of Northern Ireland’s most repeated jokes perfectly illustrates the tendency for social group memberships to determine our actions toward one another. A guy walks out of a Belfast bar into a dark alley and is soon backed up against the wall by a threatening man, who asks, “Are you Catholic or Protestant?” The guy replies, disconcertedly, “Neither. I’m Jewish.” The attacker pauses briefly, then presses his potential victim again, “Well . . . are you a Catholic Jew or a Protestant Jew?”