First, let’s presume I’m correct and the correlation between visuospatial IQ and brain size is approximately linear in humans, and that late neanderthals and early modern humans were similar enough to modern humans to be comparable. White people have an average brain size of 1350 cc and a standard deviation of approximately 130 cc, and a visuospatial IQ of 100 by definition. Neanderthals and EMH had an average brain size of 1600 cc. This is +1.9 SD above the modern human mean, predicting a nonverbal IQ of 128.5.
If we correct for encephalization by multiplying by the ratio of neanderthal EQ to modern human EQ (4.8/5.4 = 0.889), then 1600 cc times 0.889 is 1422 cc. This is +0.55 SD above the white mean, predicting an average visuospatial IQ of 108.3.
The same analysis for Amud 1 (1740 cc) yields an expected nonverbal IQ of +3 SD. If I understand correctly, you have to multiply that by the strength of the correlation when estimating individual IQs. This would be a conservative estimate because Amud was an ectomorphic neanderthal.
In addition, Pumpkin Person reports (reluctantly?):
Based on the fact that they left behind no drawings, I now estimate Neanderthals, like Homo erectus, had an artistic IQ of about 26. However in the documentary Apocalypse Neanderthal, a scientist mentions that it took him a year and a half to learn to make the stone tools Neanderthals made all the time. Since scientists probably average about 125 IQ, that might suggest Neanderthals had a spatial IQ as high as 125!
If something like the Levallois technique is going to survive tens of thousands of years by cultural exchange, the average individual in the culture has to be able to learn it.
Let me know if I’m fucking anything up here.