Although brain size roughly tripled as humans evolved from apes, one mystery is the little known brain shrinkage that occurred in the last 10,000 years or so.   The bulk of the evidence for this comes from Maciej Henneberg who published the following table:

One problem with this table is that cranial capacity is not measured directly by actually filling the skull with mustard seeds, water or beads etc, but rather by using regression equations which may or may not apply to all samples.

However there’s also evidence from directly measured cranial capacity suggesting brains have shrunk since the upper paleolithic.

So it seems that brain size had been shrinking since the stone age until at least the industrial revolution;  the question is what’s happened since?

According to Richard Lynn, brain size has since been increasing because of better health and nutrition,  and this is matched by a similar increase in IQ test performance, known as the Flynn effect.

Unfortunately I don’t know of any directly measured cranial capacities from 21st century young adults.  The closest we have are MRI scans of intracranial volume, but one scientist working in this field recently told me that these can give different results depending on the scanning (MRI) machine and the algorithms used.  For more details, see here and here.

Thus, I turn once again to the Lee and Pearson regression formulas, for estimating cranial capacity from external head measures which Lee and Pearson found agreed with the direct packing method far better than the packing method applied by two different packers!

Using 2012 anthropometric data from the U.S. army,  I found that men have a head length, head breadth and head height of 199.5 mm, 154.3 mm and 131.1 mm respectively.  Comparable figures for women are 189.8 mm, 147.8 mm, and 126.5 mm.

Assuming these forumulas are still valid today,  21st century U.S. army men average 1499 cc and their female counterparts average 1337 cc, for a sex-combined average of 1418 cc.

The simplest explanation for the fall and recent rise of brain size is the one advanced by Richard Lynn.  As humans switched from hunting to farming, malnutriton and disease caused brain size (and height) to plummet,  but with 20th century advances in health and nutrition, First World countries have (largely) recovered our former brain size.

But why is the 2012 U.S. army sample still 80 cc lower than people 50 to 30 kya?  I see four reasons:

• Most of the upper paleolithic samples are probably from Europe, while the U.S. army is more diverse.
• The U.S. army is slightly shorter than their civilian  counterparts, and so their heads might be slightly smaller.
• Even in 2012, much of the U.S. still suffered from sub-optimum nutrition and inadequate health care during their prenatal and perinatal development.
• Upper paleolithic people may have been genetically (if not phenotypically) more robust than people today. For example Richard Klein once claimed that if a paleolithic (but anatomically modern) human walked into the room, the only thing we’d notice about him is that he was extraordinarily well built.  Among 1989 army personnel of the same rank, race, and sex, weight correlates 0.41 with cranial capacity (at least as estimated by the Lee & Pearson formula).  Among the entire army, it correlated 0.66.  Thus, there may have been a slight genetic decrease in brain size related to body size, but the lion’s share of the decrease and all of the increase was likely health and nutrition.