# Peak blood, a real crisis

As I've mentioned before, a world with a lot of vampires is a world with a blood supply problem. I recently watched Daybreakers, which takes this seriously; nearly everyone in the world is a vampire and the vampires farm most of the remaining humans for blood while sending out undeath squads to round up the rest. Obviously, this isn't a scalable proposition and sure enough the vampires are frantically trying to develop some kind of substitute for human blood before supplies run out.

In a world where synthetic blood isn't possible, there's some maximum stable fraction of vampires, dictated by the maximum amount of blood that a non-vampire can produce divided by the amount of blood that a vampire needs to survive. According to Wikipedia blood donations are typically around 500ml and you can donate every two months or so. This works out to about 3 liters of blood per donor per year. Presumably, if you didn't mind doing some harm to the donors (e.g., if it's involuntary), you could get a bit more, but this still gives us a back of the envelope estimate. I have no idea what vampires need, but if it's say a liter a day, then this tells you that any more than about 1% of the population being vampires is unstable. This is of course a classic externality problem, since being a vampire is cool, but not everyone can be a vampire. If we wish to avoid over-bleeding, they will need some sort of system to avoid creating new vampires.

Luckily, this is a relatively well understood economics problem with a well-known solution: we simply set a hard limit on the number of vampires and then auction off the rights (cap-and-trade won't work well unless we have some way of turning vampires back into ordinary humans). I'd expect this to raise a lot of money which we can then plow into synthetic research to hasten the day when everyone can be a vampire; either that or research into better farming methods the better to hasten the red revolution.

That will teach you to rely on Wikipedia. You can donate at least every week, possibly with some side-effects. The Red Cross, being conservative and not having good data, picked the two-month waiting period in the 30's (or earlier), and has never changed it.

True story: My grandmother gave every week during WWII. She knew that she was supposed to wait 8 weeks (or 7, maybe), so she rotated between 8 different donation centers around Boston. When my mother (then a nurse) found out, she had a fit. My saintly grandmother's response was "it seemed like they needed it". I assume she was talking about the extra blood, not the better tracking of donors that would come later...

Paul, when I was a kid growing up in Canada, the donation interval was three months. At some point (I'm guessing some time during the 1970s), they shortened it to eight weeks--I recall this because my father, a regular donor, picked up his pace accordingly. I would assume that the change was based on the evidence of red blood cell recovery time cited in the Wikipedia article. But regardless, the donation rate certainly hasn't been universally static since the 1930s.

I used to give 6 times a year, but came down with lymphoma, and am now not allowed to give blood.

So in addition to vampires and blood suppliers there are those that are neither.