Number cortical neurons != brain size. Orcas have ~2x the number of cortical neurons, but much larger brains. Assuming brain weight is proportional to volume, with human brains being typically 1.2-1.4kg, and orca brains being typically 5.4-6.8kg, orca brains are actually like 6.1/1.3=4.7 times larger than human brains.
I think cortical neurons is a better proxy than brain size and I expect that the relation between cortical neurons and brain size differs substantially between species. (I expect more similarity within a species.)
My guess is that neuron density is actually somewhat anti-correlated with brain size
This might be true in mammals (and/or birds) overall, but I’m kinda skeptical this is a big effect within humans. Like I’d guess that regression slope between brain size and cortical neurons is ~1 in humans rather than substantially less than 1.
that number of cortical neurons would be correlated with IQ rather at ~0.4-0.55 in humans
I agree you’ll probably see a bigger correlation with cortical neurons (if you can measure this precisely enough!). I wouldn’t guess much more though?
Overall, I’m somewhat sympathetic to your arguments that we should expect that multiplying cortical neurons by X is a bigger effect than multiplying brain size by X. Maybe this moves my estimate of SDs / doubling of cortical neurons up by 1.5x to more like 1.8 SD / doubling. I don’t think this makes a huge difference to the bottom line.
Yeah I think I came to agree with you. I’m still a bit confused though because intuitively I’d guess chimps are dumber than −4.4SD (in the interpretation for “-4.4SD” I described in my other new comment).
I think cortical neurons is a better proxy than brain size and I expect that the relation between cortical neurons and brain size differs substantially between species. (I expect more similarity within a species.)
This might be true in mammals (and/or birds) overall, but I’m kinda skeptical this is a big effect within humans. Like I’d guess that regression slope between brain size and cortical neurons is ~1 in humans rather than substantially less than 1.
I agree you’ll probably see a bigger correlation with cortical neurons (if you can measure this precisely enough!). I wouldn’t guess much more though?
Overall, I’m somewhat sympathetic to your arguments that we should expect that multiplying cortical neurons by X is a bigger effect than multiplying brain size by X. Maybe this moves my estimate of SDs / doubling of cortical neurons up by 1.5x to more like 1.8 SD / doubling. I don’t think this makes a huge difference to the bottom line.
Yeah I think I came to agree with you. I’m still a bit confused though because intuitively I’d guess chimps are dumber than −4.4SD (in the interpretation for “-4.4SD” I described in my other new comment).