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!

Assuming about a 0.35 correlation between artistic IQ and technological IQ, we might very crudely estimate an overall IQ of 71 for Neanderthals.  I realize this is all very speculative, but scientist Steve Hsu arrived at a similar figure, stating:

It’s very likely these Neanderthals, although able to interbreed with humans, and probably capable of speech, will be on average considerably less intelligent than humans. If I had to guess I would suppose their average adult IQ to be about 70, or -2 SD relative to modern humans. You might wonder how they could have survived for 300k+ years with such modest intelligence, but based on my experiences with 5-10 year old kids I don’t think that a sub-adult level of maximum intelligence precludes the ability to form societies and function as hunter-gatherers. (Apes survive with even less cognitive ability.) I just don’t think that higher developments (e.g., invention of writing) are likely for such a population. What Homo Sapiens accomplished in 50-100k years far outstrips Neanderthal accomplishments over a much longer period of time.

Modern humans differ from each other at about 1 in 1000 places in the genome, whereas a Neanderthal and a human differ at a few per 1000 places. Some subset of these additional differences cause them to be broader, more powerfully muscled, and, most likely, less intelligent.

Hsu doesn’t explain why he estimates an IQ of 70, but notice how he cites the fact that genetically  Neanderthals were twice as different from modern humans as modern humans are from one another, so perhaps he’s simply doubling the IQ standard deviation of 15 to guess that Neanderthals were 30 points below IQ 100 (defined as average on IQ tests).

An average IQ of 70 appears to be the threshold for discovering agriculture, something Neanderthals failed to do even during the Eemian, when they had 15,000 years of warm climate to do so.  Anatomically modern humans also failed to make the leap to agriculture during those same 15,000 years, suggesting no modern human population had an average IQ above 70 before the upper Paleolithic, around the time scholar Richard G Klein believes a massive brain mutation occurred causing behavioral modernity.

What kind of genetic change might have occurred?  Hsu once mentioned that a high quality Neanderthal genome had a genetic variant associated with developmental delay and autism.  Perhaps prior to behavioral modernity, modern humans also had this variant in large numbers?