A while ago, this article about Toxoplasma Gondii was picked up by Slashdot. There's a somewhat better article at The Economist Toxo seems endlessly fascinating to the Slashdot crowd, and to myself as well. Much of the world's population carries a parasite that may affect their personality and behavior, typically with no physical effects (although, rarely, an infection causes intense physical sickness). According to studies, Toxo increases testosterone in men and correlates with high aggression, dogmatism, and rebellion against authority; women with Toxo tend to be more outgoing and men find them more attractive. Although the parasite reproduces in cat stomachs and is present in cat feces it's almost never contracted through contact with cats. Rather, humans usually pick it up by eating raw or undercooked meat. I've seen different sources quote vastly different numbers for national infection rates, but all cite relatively low rates in Japan, South Korea, the US, and the UK, relatively high rates in France and Germany, and often very high rates in countries with serious public health problems. Because infection rates vary so much among different countries (and probably among cultural groups within them, since people are typically infected through food), Toxo infection could be a simple explanation for many cultural differences.
It reminds me of the day, in my intro-level Psych class in college, that the professor explained why stimulants can be an effective prescription for hyperactivity. The underlying theory is that people's brains are always active, spinning idly, keeping themselves entertained. People that are hyperactive spin less than usual, and require more external stimulation to avoid boredom. The right stimulant makes them spin more, require less external stimulation, and function better in classrooms and offices. What fascinated me more was the other side of the coin. People that "spin" more than usual become very quickly overloaded when there's a lot going on around them. That sounds a lot like me. It could be a simple explanation for a pretty big part of my personality.
Then again, there are lots of simple explanations out there. Our personalities are the result of an unknowable number of factors, and so are national and cultural characteristics. To me, Toxo is fascinating because it raises the question not just of who we are, individually and collectively, but also of what we are. It's a reminder that I have lots of living stuff in me that doesn't carry my DNA, and that it's as much a part of me as the parts that do carry my DNA. But I have to remember that any one of these things is just a small component of the whole and probably can't, by itself, explain very much.