Also, possibly, because people’s choice of political party is looking less and less like a choice and more like an inborn trait (such as introversion and extroversion). Recent studies show that people prefer the body odor of those who share their political leanings:
Data also seem to show that you can guess a persons political affiliation with something close to 95% accuracy by looking at a brain scan conducted while the subject is viewing a disgusting image (conservatives respond more intensely):
So, it could be that like hair color and eye color, genetically related groups are more likely to share political opinions. It also suggests that no matter how much we think we’ve chosen our politics, we probably haven’t.
