I am unconvinced by the argument that H. sapiens can’t be right at a limit on brain size because some people have larger-than-average heads without their mothers being dead.
Presumably head size is partly determined by environmental factors outside genetic control, and presumably having your mother die in childbirth is a really big disadvantage, much worse than being slightly less intelligent. If that’s so, then what should it look like if we, as a species, are hard against that wall? (Which I take to mean that any overall increase in head size would be bad even if being cleverer is a big advantage.) I suggest we’d see head sizes that are far enough away from outright disaster that, even given that random environmental variation, death in childbirth is still pretty rare, but not completely unknown. And, of course, that’s just what we see; death in childbirth is very rare now, in prosperous advanced countries, but if you go back 100 years or look in less fortunate parts of the world it’s not so rare at all.
This could be quantified, at least kinda. We could look at how the frequency of death in childbirth, in places without modern medical care, varies with head size (though controlling this adequately would be hard); we could look at the (rather weak and confounded with other things) relationship between head size and intelligence; and we could say that it looks as if d(IQ)/d(head size) points of IQ are about as valuable, evolutionarily, as a probability -d(maternal mortality)/d(head size) of having your mother die when you’re born.
Without doing that calculation or something like it, I don’t think Eliezer is justified in saying that it doesn’t look as if there’s been strong selective pressure for bigger brains; it seems to me like the pressure could be pretty strong without the picture being qualitatively different from what we actually see.
Alternately, the selection could in reality be applied more strongly to helplessness of the infant or shape of the pelvis rather than head size at birth.
I get the impression that deaths from childbirth don’t, usually, come about because the head is too large, but because there are other complications: Breech births, especially.
I am unconvinced by the argument that H. sapiens can’t be right at a limit on brain size because some people have larger-than-average heads without their mothers being dead.
Presumably head size is partly determined by environmental factors outside genetic control, and presumably having your mother die in childbirth is a really big disadvantage, much worse than being slightly less intelligent. If that’s so, then what should it look like if we, as a species, are hard against that wall? (Which I take to mean that any overall increase in head size would be bad even if being cleverer is a big advantage.) I suggest we’d see head sizes that are far enough away from outright disaster that, even given that random environmental variation, death in childbirth is still pretty rare, but not completely unknown. And, of course, that’s just what we see; death in childbirth is very rare now, in prosperous advanced countries, but if you go back 100 years or look in less fortunate parts of the world it’s not so rare at all.
This could be quantified, at least kinda. We could look at how the frequency of death in childbirth, in places without modern medical care, varies with head size (though controlling this adequately would be hard); we could look at the (rather weak and confounded with other things) relationship between head size and intelligence; and we could say that it looks as if d(IQ)/d(head size) points of IQ are about as valuable, evolutionarily, as a probability -d(maternal mortality)/d(head size) of having your mother die when you’re born.
Without doing that calculation or something like it, I don’t think Eliezer is justified in saying that it doesn’t look as if there’s been strong selective pressure for bigger brains; it seems to me like the pressure could be pretty strong without the picture being qualitatively different from what we actually see.
Alternately, the selection could in reality be applied more strongly to helplessness of the infant or shape of the pelvis rather than head size at birth.
I get the impression that deaths from childbirth don’t, usually, come about because the head is too large, but because there are other complications: Breech births, especially.