The free will one is one we've discussed before, and I see many other people have already mentioned it. But that's a bit of a cheat, because I already believe it not to be true.

There are a few things that I tend to act as though they were true, while not really believing them. Karma is one; I don't really believe it, but it provides a handy narrative for me to follow. I could be persuaded that that's wrong-headed thinking, and that it's better to be purely rational on that one, but so far I haven't been.

If it turns out that one particular strand of the human race (blacks, straight people, blue-eyed people) is statistically a genetic liability, I'm not sure I want to know about it. That's possibly more of an ethical question than one of fact, though; I might acknowledge the fact, but take the view that I'm not going to let it affect my decisions.