Human variation in cranial capacity. Black, 1,450 cc and over; checkerboard, 1,400-49 cc; crosshatching, 1,350-99 cc; horizontal striping, 1,300-49 cc; diagonal striping, 1,250-99 cc; dots, 1,200-49 cc; white areas, under 1,200 cc (Beals et al., 1984)
So Stephen Jay Gould was wrong, and Samuel George Morton was right. Brain size does vary among human populations. But what does this variation mean? Why does it exist?
This question is briefly addressed by Lewis et al. (2011):
[…] cranial capacity variation in human populations appears to be largely a function of climate, so, for example, the full range of average capacities is seen in Native American groups, as they historically occupied the full range of latitudes
For more information, the reader is referred to an earlier study: Beals, Smith, and Dodd (1984). The latter plotted cranial capacity from 122 human populations. The resulting map is shown above. A few caveats: there is a LOT of interpolation and extrapolation in the above map. There are no data points from Cambodia or southern Vietnam, and hence nothing to justify the very low values assigned to that region. Southern India is assigned a very low cranial capacity on the basis of a small sample that includes Veddas—a relic group of hunter-gatherers from Sri Lanka.
All in all, and keeping these caveats in mind, cranial capacity does seem to correlate with latitude. Why? According to the authors, heads are larger at higher latitudes to reduce heat loss. An object will lose less heat if its ratio of volume to surface area is high. There has thus been natural selection to make heads broader and more globular at higher latitudes. The increase in brain size is incidental.
This explanation was challenged in the comments section following the Beals, Smith, and Dodd (1984) article. A Japanese commenter, Iwatoro Morimoto, pointed out that “in recent centuries, brachycranic skulls show a considerable increase in frequency in Eurasian populations, including the Japanese.” Since mean temperatures have changed little in recent centuries, there must have been another factor at work. Unfortunately, Morimoto provided no references to back up this counter-argument.
Another commenter, Erik Trinkaus, similarly pointed out that Neanderthal cranial capacity was no bigger during glacial periods than during interglacials. The same was true for early modern humans. For populations already established at northern latitudes, cranial capacity shows no evidence of rising and falling with mean temperature.
A recent analysis has nonetheless found a significant correlation between cranial capacity and latitude among ancestral hominids in general, ranging from A. Afarensis to H. sapiens (Henneberg and Miguel, 2004). The correlation remained even when the authors controlled for each skull’s time period and, thus, was not due to the overall rise in cranial capacity over time and the parallel expansion of ancestral hominids into higher latitudes.
In sum, cranial capacity does correlate with latitude. It is less clear, however, whether this correlation is mediated by mean temperature and the need to reduce heat loss.
Higher cognitive demands at higher latitudes?
Could it be that cognitive demands increased as ancestral humans entered higher latitudes? Not because mean temperatures were lower but because the yearly cycle presented a greater diversity of environments and required much more foresight. Between ‘summer’ and ‘winter,’ the differences are much greater in the temperate and arctic zones than in the tropics.
This point is elaborated upon by Hoffecker (2002, p. 135). Among early modern humans, tools and weapons were more complex at arctic latitudes than at tropical latitudes. “Technological complexity in colder environments seems to reflect the need for greater foraging efficiency in settings where many resources are available only for limited periods of time.” Arctic humans coped with resource fluctuations and high mobility requirements by planning ahead and by developing untended devices (e.g., traps and snares) and means of food storage.
In addition, these increased cognitive demands fell on both men and women. Paternal and maternal investment were much more equal than in the tropics, where women provided for their families year-round with less male assistance (Kelly, 1995, pp. 268-269; Martin, 1974, pp. 16-18). Indeed, because men were the main food providers beyond the tropical zone, women could care for their families by developing a new range of tasks: food processing (e.g., butchery and carcass transport); shelter building; garment making; leather working; transport of material goods; etc. (Waguespack, 2005). This technological revolution would ultimately lead to what we now call ‘civilization’ (Frost, 2008).
Curiously, Beals, Smith, and Dodd (1984) cite Gould’s 1978 Science article—the one claiming that Morton had unconsciously fudged his data to make brains look bigger among Europeans than among sub-Saharan Africans. Yet these authors declined to mention the inconsistencies between Gould’s findings and their own. Their reference to Gould is studiously neutral: “Critiques of the use of brain size in typology have been offered by Gould.”
There has not been much comment on the Beals, Smith, and Dodd (1984) article. The most substantive one seems to be a blog post by Robert Lindsay (2010) who calls their map a “train wreck” for claims that cranial capacity correlates with IQ:
White racists like to make a big deal about the supposed correlation between head size and intelligence and race. A nice little chart showing the basically dishonest portrayal they attempt based on cherry-picking data is below.
Methinks that Lindsay takes the fine details on that map a bit too seriously. Many of the details are simply creative extrapolation and infilling; otherwise, the map roughly corresponds with world distribution of mean IQ. Furthermore, no one is claiming that cranial capacity is the only determinant of IQ. There are undoubtedly many others: cortical surface area, myelinization of nerve fibers, relative importance of domain-general thinking, etc.
But he does make a good point about the Amerindian data.
As you can see, in the Americas, there is no good evidence whatsoever for head size and IQ. I am not aware that Amerindian IQ varies in the Americas. The average is apparently 87 across the continent. If anyone can show me that it varies by latitude, please do.
Agreed. No one can, for now. But a hypothesis is not false because no one has bothered to test it.
Beals, K.L., C.L. Smith, and S.M. Dodd (1984). Brain size, cranial morphology, climate, and time machines, Current Anthropology, 25, 301–330.
Frost, P. (2008). The path to civilization? Evo and Proud, March 10, 2008.
Henneberg, M. and C. de Miguel. (2004). Hominins are a single lineage: brain and body size variability does not reflect postulated taxonomic diversity of hominins, Journal of Comparative Human Biology, 55, 21–37
Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe. New Brunswick: Rutgers University Press.
Kelly, R.L. (1955). The Foraging Spectrum. Diversity in Hunter-Gatherer Lifeways. Washington: Smithsonian Institution Press.
Lewis, J.E., D. DeGusta, M.R. Meyer, J.M. Monge, A.E. Mann, R.L. Holloway. (2011). The Mismeasure of Science: Stephen Jay Gould versus Samuel George Morton on Skulls and Bias, PLoS Biology, 9(6) e1001071
Lindsay, R. (2010). The Head Size/IQ/Race Trainwreck, March 11
Martin, M.K. (1974). The Foraging Adaptation — Uniformity or Diversity? Addison?Wesley Module in Anthropology 56.
Waguespack, N.M. (2005). The organization of male and female labor in foraging societies: Implications for early Paleoindian archaeology. American Anthropologist, 107, 666-676.