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DanArmak comments on When does heritable low fitness need to be explained? - Less Wrong Discussion
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Part of the reason is probably because of a conflict between the mother, father, and baby. The baby gets all of the benefit of, say, having a bigger head at birth but some of the cost is born by the mother and little by the father so from the baby and father's viewpoint the head size will be bigger than it would be for the mom.
That seems orthogonal to NancyLebovitz's point: some women have lower risk than other women when birthing infants with a given head size.
Also, if a woman dies in childbirth, or even if she's injured enough to not lactate well in the days immediately following birth, that strongly impacts the baby. Evolution may not care about how painful birth is, or even how long it takes (now that we don't need to fear predators), but it certainly cares about risk of injury or death to the mother.
Some gene increases the size of the baby's head at birth raising the baby's eventual IQ by X, but also increases the chance that the mom will die during childbirth by Y. There will exist (X,Y) such that the baby having the gene will increase the expected number of great grandchildren that the dad and baby will have, but decrease the expected number of great grandchildren that the mom will have.
How big a selection pressure does higher IQ command? Wikipedia on Fertility and Intelligence:
Many local times & places have had a negative correlation between IQ and fertility.
Of course, fertility isn't precisely the same as inclusive fitness, but it's strongly correlated.
On the other hand, many women die in childbirth or suffer significant complications in many parts of the world even today.
Enough that Homo Sapiens has a brain volume nearly twice as high as Homo Habilis. (~600 cc to ~1200 cc).
I'm asking about selection today, or in the last few millenia, not two million years ago in a different species.
Exactly.
Thanks. I was just going to make that point.
Pelvic structure is in play as well as head size.
Also note that there's more to raising children than lactating. Women typically needed to be in good enough shape to do hunting/gathering/food growing.
See Sarah Hrdy's Mother Nature for a reminder that motherhood takes place in the world, not just between the mother and child.