content: Exactly, based on no other information, we organize into troops like primates based on the most apparent similarity we can find.  First, humans will cling to humans, then we go by what we can see or is readily apparent (dress, skin color, language) to create a unit for protection (one human is food, 7 humans are a vicious killing machine). Take those same self-organized groups and add more information (for instance, what happens if 20 of the 100 people are all Marines, or some other heavy cultural bonding) and you'll find vastly different groupings with no regard for skin color or 'ethnicity'.  Because we're not inherently racists, we're inherently scared animals trying to form into groups form protection with little to no information about who we're grouping up with.  So we find any similarity we can in the absence of any other info, and then we use that to form a group.  It's ingrained into us and how we're built because it's served us well for thousands of years.  We make the best decision we can based on the available information, and when that information is minimal, we don't make very good decisions. I think the cultural split that we see is very artificial.  We're all Americans, we just allow politicians to paint our basic responses (as stated above around self organizing in the absence of good information) as something that is more useful to them, like racism.  And when we believe them (despite the vast majority of what we see with our own eyes and in our own interactions) they win.