(re: Nisbett (2007). All brains are the same color)
When it comes to the issue of population differences in intelligence, the average differences in cranial capacity are an important piece of evidence. Whatever their ultimate cause (i.e., whether due to environmental differences or genetic differences that resulted from environmental differences), they establish the deep rootedness and biologicality of (some of the) global GMA differences. There is, naturally, a good deal of resistance to this conclusion, which is reflected in the fraudulent discussions of cranial variation in many anthropological texts.
For example, In Chapter 5, Regional Variation and Evolution, of their textbook, Human Lineage, Cartmill and Smith (2009) authoritatively state:
While correctly noting the population differences in CC/brain size (see: Appendix A in Smith and Beals 1990 and table 2 in Beals et al., 1984), they claim that the differences are unrelated to differences in intelligence. In making their case, they cite chapter 5, The Size of intelligence: A gross misunderstanding, of Deacon (1997), The symbolic species: the co-evolution of language and the brain
In the cited chapter, Deacon, in fact, does not discuss intrahuman variation in intelligence or brain size. Rather, he argues, as the title of the chapters suggests, that bigger brains did not make humans smarter than other primates. (Cf. Reader (2011); Table 4. The relationship between primate general intelligence and brain volume).
In their discussion, Cartmill and Smith not only misrepresent Deacon’s chapter but, having brought up the issue, neglect to inform the readers about the copious amount of evidence that has accumulated internationally since the ’90s which establishes that bigger human brains are, indeed, smarter (McDaniel, 2005; Rushton and Ankney, 2009). Moreover, they cite Beals et al’s thermoregulatory explanation for human variation without making mention that this climatic explanation is complementary with a size-intelligence one (e.g Ash and Gordon 2007; see also Schwartzman and Middendor (2009) for hominid evolution in general).
Of course, Cartmill and Smith might be correct in their key assertion. Perhaps different populations are wired differently such that between differences in size don’t entail differences in intelligence. [The consistently positive size/IQ correlation within clinal populations (e.g. Sudanese, Guatemalans, African Americans, Asian Indians, Turks, Chileans, etc) strongly argues against this.] Moreover, it’s rather plausible that the differences are mostly environmental in origin. Finding out, though, requires an honest appraisal of the evidence.
References not cited above.
Ash and Gordon, 2007. Paleoclimatic Variation and Brain Expansion during Human Evolution
Beals, et al., 1984. Brain Size, Cranial Morphology, Climate, and Time Machines
McDaniel, 2005. Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence
Reader et al., 2011. The evolution of primate general and cultural intelligenc
Rushton and Ankney, 2009. Whole Brain Size and General Mental Ability: A Review
Schwartzman and Middendor, 2009. Was climate the prime releaser for encephalization? An editorial
Smith and Beals, 1990. Cultural correlates with cranial capacity