This is going to be one of those "Now that you see it, you can't unsee it" situations. It sounds ridiculous at first, then starts to become grossly obvious the more you look around. We referenced a study a while back in which scientists could get people to be less racist merely by washing their hands first. At the time, that seemed to me like one of those oddball results, like the one that found you can make a person smarter by having them wear a lab coat. But no, it turns out it was but a glimpse into the dark, swirling demon lurking within the soul of humanity: our primitive fear of germs.
The theory says that over the centuries, certain people and groups evolved with a higher paranoia toward infections, due to living in regions/climates where that sort of thing was more of a threat. Meeting another tribe thus meant encountering diseases you had no immunity to, and to this day, their descendants will instinctively be more untrustworthy of other cultures and tightly regulate "unclean" behavior. In groups, they form societies that are fiercely nationalistic and insist on flamboyant outward displays of such (like, say, Confederate flags on pickup trucks) to signal to one another that they're "safe." They also enforce strict sexual morality (to prevent the spread of STDs). Over time, they tend to gravitate toward dictatorships, submitting to the strongman promising to protect them from the contaminated outside world.
Hey, did you know Hitler's rise occurred right after the Spanish flu ravaged Europe? And that he was comparing the Jews to disease pretty much from Day One?
Experts say that the rise of democracy and progressive ideas in general can be attributed to science conquering many of the infectious diseases that were dominating our decision-making up to then. Still, those habits are passed down through both genes and culture (particularly in warmer climates), and it's easy to see it today. You can hook conservatives and liberals up to a brain scan, and the conservatives react more strongly to disgusting images, even if they insist that stuff doesn't bother them. Other studies show that right-wingers tend to be more obsessive-compulsive, feeling a unending urge to purge their surroundings of disorder.