Are Republicans Different?

I generally assume that there's a good deal of symmetry in American politics with a normal distribution curve.  In other words, both right and left have their extremes, their nuts, their excesses, and we all have our blindnesses.  I still think it's a good assumption, good to moderate my tendency to self-righteousness, the feeling I know better.  But,as Kevin Drum notes, over the past 10 years the sorting process whereby political partisans listen to their favorite cable news channel worked more strongly for Republicans than Democrats.  I don't understand why, if my operating assumption is true.