Only one part of our political system has led to the current polarization, but it was enabled by a series of media developments going back to the 1980s with the abolition of the fairness doctrine, the subsequent rise of right wing AM talk radio, the invention of cable TV which catered to niche markets for the sake of advertising, the creation of Fox News --by an Australian who'd made his fortune in British tabloids-- and then finally the rise of social media which in turn fragmented media audience --what advertising companies like Google and Facebook sell to advertisers-- into ever tinier and increasingly homogeneous target groups with the result that cultural identity is now more important to how citizens make decisions than are actual policy issues.

To see that this is true, one need only look at the fact that people almost never change their minds about anything on the basis of facts or evidence for the very good reason that they don't form their opinions on the basis of facts and evidence in the first place and instead rely on cultural identity as a guide.