Joe Carter has a (perhaps unintentionally, I'm not sure) rather hilarious post about God, vampires, and the anthropic principle. Somehow Joe managed to find an academic paper on this: "Ghosts, Vampires and Zombies: Cinema Fiction vs Physics Reality" by Costas J. Efthimiou and Sohang Gandhi.
If these physicists are right, we can reasonably infer from the feeding habits of vampires in vampire literature, together with the continuing population of non-vampire human beings, that there are at most 512 vampires in the world, assuming there are vampire-slayers. Without vampire-slayers, it seems there just couldn't be vampires at all, according to these numbers.
Now I'm not going to examine the connection with the anthropic principle that Joe is emphasizing, other than to observe that both just seem to be simple cases of inferences to the best explanation, but I do have one philosophical point to make. The argument against vampires leaves out a crucial step. One scenario ought to be considered more carefully before ruling out the possibility of vampires given our evidence. The argument takes the continuing population of humans as a piece of starting evidence. But should we be so sure that the population really is continuing in the way that we think it is? Isn't it possible that the population is just a growing numbers of vampires, and only relatively few of the people who remain are still real humans, with a huge vampire conspiracy going on pretending that humans are still around in large numbers? If the vampires have enough technology to clone humans to provide "offspring" for the vampires masquerading as human couples and a continued food source, then I can't see how this assumption can be ruled out as easily as is being done.
The paper makes some funny points about ghosts and zombies also, but I thought this was just a good example of physicists trying to pass philosophical arguments off as science, which is usually complained about when it's intelligent design but apparently just good science when it's skeptical. Well, it turns out a philosopher might have been able to point out how they're not being skeptical enough. A good skeptic wouldn't rule out the skeptical scenario I proposed without some more careful argumentation.