One of the great mysteries of human evolution is why after four million years of brains getting bigger in the hominin line leading to humans, did they suddenly start to shrink sometime after modern humans colonized the World and replaced all rival homo species. While Lynn (1990) attributes this to malnutrition and disease caused by agriculture, others have argued it was a genetic change.
However Peter Frost argues that it might (at least in part) be a statistical artifact:
Perhaps smaller skulls are more likely to decompose faster. The skulls we unearth would therefore be a biased sample, and this bias toward preservation of larger skulls would gradually increase for skulls that have been in the ground longer.
The problem of “preservation bias” has already been noted with respect to female and infant remains
This makes perfect sense but how big is the effect and is there anyway to adjust for it? Frost points us to a paper by Walker et al. (1988) that describes a California cemetery for the mission’s Indian neophytes who died circa 1831. Although the mission records showed that 53% of the buried people were female, the skeletal record showed only 50% were. The inference to be drawn from this is that because females are smaller, they decompose faster.
Now imagine if we tried to estimate the average cranial capacity of the mission Indians from the skeletal record. Considering men have brains about 12% bigger than women do, such a decrease in the percentage of women alone would increase the mean cranial capacity of the entire sample by 0.34% in just the 157 years after death that the data was published. Assuming the process is linear, that leads to a 87% increase in 40,000 years. So instead of Cro-Maagnons having brains of 1498 cm3(80 cm3 bigger than the average American today), they’d have brains that averaged 1498 cm3/1.86 = 805 cm3
A more reasonable hypothesis is that instead of preservation bias inflating brain size estimates by 0.34% per 157 years, it inflates it by 0.34% per the natural logarithm of 157 years, so 5.06 log years. And if Cro-Magnons lived around 40,000 years ago, or 10.6 log years ago, this would make the 1498 cm3 brain size estimate for Cro Magnon 0.71% inflated, and thus the true value 1498 cm3/1.0071 = 1487 cm3. One should note that weighting the samples by sex will not entirely solve this problem because sex is simply being used here as a proxy for big vs small brains and for most skulls, the sex is probably not known, they just assume the bigger skulls are male.
Pumpkin Person 
hey Pumpkin person, off topic, would you do a post on assigning an ACT cutoff for mensa and triple nine? What would be your approach to this problem? What arguments would you make about the validity of the modern ACT?
I did do a post converting ACT to IQ (from which Mensa & TNS cut-offs can be derived):
nice, thanks. I’ve read that post before, but bringing it to point helps. Now, would you do anything with restriction of range on that one for the purposes of converting to iq, a la your suggestion about mensa and it’s sat cutoff?
I wonder if you could infer the relation between the average act score and its corresponding IQ score, if that score is above 100 due to range restriction effects, from the average of black and white races taking the ACT. I believe those statistics exist. Would the difference between the racial averages on act differ from the well-documented 1.1 standard deviation in IQ if you were dealing with a skewed distribution taking the ACT? In other words, could you figure out where on each bell curve of the racial populations was the average ACT taker falling? Am I making sense?
Racial IQ differences would probably be a bit smaller among the ACT takers than among random Americans, because the former are self-selected.
Ever seen the movie “get out”? Since you’re a horror and brain guy thought you might appreciate that one. It’s about old powerful white people transplanting their brains into the bodies of black victims to attain their “physical advantages”. Thoughts?
I really enjoyed it, but didn’t quite get it. The altered blacks acted like they were lobotomized, not like they had the brains of whites.
The frequency at which skulls are preserved is very low. Many more people are born and live till reproduction than get preserved. So how do we really know what size brain is required to survive in certain environments? If brains shrank in cities this does not automatically mean they shrank in the wild. We would need to know all the distributions just about as to the environments and time periods. Especially since the sample is mostly in Europe and in England I think?
One could look at different skulls at the same time. For example, while mesolithic Eastern Europeans were not cro magnons (upper paleolithic and mesolithic Europeans are different populations), they had larger skulls than their contemporary yet culturally neolithic farmer neighbors of closeby regions. These farmers were mostly descended from neolithic Anatolian migrants. And not just larger skulls but also larger brains. There was this thing called the burned house horizon at the time and one explanation is that it was a result of conflict between the hunter gatherers and the farmers. Personally I think that might be a partial but not complete explanation of the phenomenon, but I do suspect that it contributed to the existence of the burned house horizon.