Saturday, 18 October 2008

Could more caesarian sections mean bigger heads?

That's the position adopted by a certain Joseph Walsh, of the American Biology Teacher. I haven't been able to find the original article online, but there's a nice review of it over at Pharyngula here.

The basic outline of his argument is as follows:

Many causes have been given for the increased Cesarean section rate in developed countries, but biologic evolution has not been one of them. The C-section rate will continue to rise, because the ability to perform a safe C-section has liberated human childbirth from natural selection directed against too small a maternal pelvis and too large a fetal head. Babies will get bigger and pelvises will get smaller because there is nothing to prevent it.

That is, the only thing keeping the heads of babies small is the mortality associated getting them stuck in their mothers' bony pelvises on the way out. With the availability of C-sections, evolution is now free to enlarge the head size of babies (although not too much, naturally, at least for engineering reasons!).

Walsh marshals together several strains of circumstantial evidence in support of his case, which I actually find more persuasive than P.Z. Myers does. Anyway, make up your own mind!

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