Of Tribalism and Irrationality

So I've just started an excellent (so far) little book called Reefer Madness. In the small bit I've read, there something about where the current Marijuana laws in the United States came from. Basically, there was a large influx of mexican illegal immigrants whse drug of choice was old Mary Jane, and so the logical legal backlash was to make the drug illegal so that they could persecute this new minority. It reminded me of a recent post I saw somewhere, possibly Pharyngula or Dispatches from the Culture Wars that pointed out that a big part of the reason so many people dislike evolution is racism- they don't want to believe they're related to Blacks, the arguement 'My Granpa weren't no Monkey' is actually racist, not just normal-stupid.
Which leads me to wonder at how many of our other current social problems can be traced back to simple racism dressed up. The 'War on Terror' certainly has some racist underpinnings, though there is a stream of legitamet concern in there, a bigger part of the emotional appeal for many people has got to come from 'Given those a-rabs what they had commin'. Of course the Gay marriage thing is pure bigotry.
Tribalism is by nessesity irrational- it allowed our ancestors to have a coherent sense of morals within their own social groups while permitting them to commit atrocities on other groups, which was a nessesary capacity to have in a primative world where there were limited resources and only one way to compete for them. Few other things require us to be so deeply irrational, so maybe we can trace most of the more mindless acts of large groups to this particular part of our psychology.