The Red Tribe is most classically typified by conservative political beliefs, strong evangelical religious beliefs, creationism, opposing gay marriage, owning guns, eating steak, drinking Coca-Cola, driving SUVs, watching lots of TV, enjoying American football, getting conspicuously upset about terrorists and commies, marrying early, divorcing early, shouting "USA IS NUMBER ONE!!!", and listening to country music.
The Blue Tribe is most classically typified by liberal political beliefs, vague agnosticism, supporting gay rights, thinking guns are barbaric, eating arugula, drinking fancy bottled water, driving Priuses, reading lots of books, being highly educated, mocking American football, feeling vaguely like they should like soccer but never really being able to get into it, getting conspicuously upset about sexists and bigots, marrying later, constantly pointing out how much more civilized European countries are than America, and listening to "everything except country".
(There is a partly-formed attempt to spin off a Grey Tribe typified by libertarian political beliefs, Dawkins-style atheism, vague annoyance that the question of gay rights even comes up, eating paleo, drinking Soylent, calling in rides on Uber, reading lots of blogs, calling American football ?sportsball?, getting conspicuously upset about the War on Drugs and the NSA, and listening to filk ? but for our current purposes this is a distraction and they can safely be considered part of the Blue Tribe most of the time)
I think these "tribes" will turn out to be even stronger categories than politics. Harvard might skew 80-20 in terms of Democrats vs. Republicans, 90-10 in terms of liberals vs. conservatives, but maybe 99-1 in terms of Blues vs. Reds.
It's the many, many differences between these tribes that explain the strength of the filter bubble - which have I mentioned segregates people at a strength of 1/10^45? Even in something as seemingly politically uncharged as going to California Pizza Kitchen or Sushi House for dinner, I?m restricting myself to the set of people who like cute artisanal pizzas or sophsticated foreign foods, which are classically Blue Tribe characteristics.
Are these tribes based on geography? Are they based on race, ethnic origin, religion, IQ, what TV channels you watched as a kid? I don't know.