Considering how vast are differences in political views, how tiny are differences in underlying biology, I'm extremely skeptical to any biological explanations.

Political distance between American left 2009 and American right 2009 is far smaller than political distance between average American 2009 and average American 1809, or than political distance between average North Korean 2009 and average Somalian 2009.

If those tiny American differences corresponded to big differences in biology, then people from different countries and times would have to be pretty much a different species. That's obviously not true, so I find such explanations extremely unlikely.