Home > Uncategorized > Brain size and correlates with IQ

Brain size and correlates with IQ

When it comes to racial differences in intelligence, the average differences in cranial capacity are an important piece to the puzzle. Whatever their ultimate cause (i.e. whether due to environmental differences or genetic differences that resulted from environmental differences), they establish the deep roots of general mental ability differences. In their recent discussion of race, genes, and IQ, Hunt and Carlson (2007) maintain that differential brain size is a reasonable line of investigation:

Rushton (1995) maintained that one of the reasons for the White–African American disparity in IQ scores is that Whites have larger brain sizes than African Americans. Leaving aside the issue of whether or not one accepts this particular argument, the argument itself illustrates a useful principle. Differences in brain size are associated with intelligence (McDaniel, 2005). Rushton has stated a hypothesis about a biological mechanism, known to influence intelligence, that might explain the difference. Rushton’s claim for a racial disparity in brain sizes was based on exterior skull measures. Further studies, using modern imaging techniques, may provide a more sensitive test of the hypothesis. It would not be appropriate to enter into a detailed discussion here. Our point is simply that discussing this sort of claim is far more likely to increase our understanding of the disparity than is arguing about the percentage of variance associated with biological or environmental variables.

What is the status of this line of investigation?

Obviously, for the brain size explanation to be plausible brain size, cranial capacity, and head circumference need to correlate with differences in intelligence within populations; moreover, brain size, cranial capacity, and head circumference need to be partially heritable. Rushton and Ankney (2009) summarize the findings to date with regards to brain size and intelligence: based on 28 non-clinical published brain imaging samples (N= 1,389) a .40 correlation between IQ and brain size measured by MRI was found; based on 59 published samples (N= 63,405) a .20 correlation between IQ and head circumference was found. These findings are consistent with others. In a meta-analysis McDaniel (2004) found an in vivo brain volume/IQ correlation of 0.33 based on 37 published studies (N= 1535); Reviewing all the data to date (N = 935), Miller and Penke (2007) found a in vivo brain volume/general intelligence (GQ) of .41; the heritability of adult brain volume (N =2494) was found to be .89. Pietschnig, Zeiler, and Voracek, (submitted), found an in vivo brain volume/IQ correlation of .24 based on a meta-analysis of 94 studies published and unpublished.

With regards to the heritability of brain size, based on a review of 14 twin studies using CT to measure brain size, Peper et al. (2007) found a .81 heritablity of global brain measures, .66-97 heritability of brain volume, and .82/.88 heritability of grey/white matter. Moreover, based on a twin study, Posthuma et al. (2002) found a .30 genetic correlation between general intelligence and brain volume, replicating the findings of Pennington et al. (2000), who found a genetic correlation of 0.48 (as cited in Gignac et al, 2003). So, within populations, one can infer a genetic basis to intelligence differences, partially mediated by differences in brain size.

This conclusion is confirmed by the preponderance of the evidence which shows a within family correlation between brain size and IQ (Gignac et al, 2003; Rushton and Jensen, 2010).

In addition to the above two prerequisites, there must be consistent findings of between population differences.

Cranial capacity differences between continental populations

As for cranial capacity differences between regional populations, Beals, et al. (1984) found an average Asian, European, and African cranial capacity of, respectively, 1380, 1362 (sd = 35), and 1276 (sd =85) (N= 20,000); they attributed the selection to cold weather adaptation. (See: Smith and Beals (1990) for population means and standard deviations.) Rushton (2005) summarizes previous global findings: East Asians 1,364, Europeans 1,347, S.Africans 1,267. (See also: Rushton, 1990). Based on a recent study of 699 Nigerians of different ethnicities, Odokuma, et al. (2010) found a mean cranial volume of 1271.

Odokuma, et al. (2010) found a mean cranial volume of 1271; they conclude the following:

The findings in this study are similar to previous studies (Morton, 1839) where the mean cranial volume of the skulls of whites was 1,425 cm³, while that of the Blacks was 1,278 cm³. Based on the measurement of 144 skulls of Native Americans, Morton (1839) reported a figure of 1,344 cm³. Gould (1981) and Rushton (1995) have also showed very similar figures. Tribe had a significant effect on cranial volume at 0.05 levels of significance. Intercultural comparisons demonstrated significant variation as reported by Howells (1989), Froment (1992) and Lahr (1996). While the Ibo’s had an average cranial capacity of 1273.39 cm3 , that of the Urhobo’s was 1255.89 cm3. The Edo’s was 1310.08 cm3. This may be attributable to a . common ancestral origin of the Ibo and Urhobo people or inter marriages which are very common between these cultures with interchange of physical characteristics over the years since these people have been cordial neighbours

There is general acceptance of these findings in the physical anthropology literature. For example, in Chapter 5, Regional Variation and Evolution, of their textbook, Human Lineage, Cartmill and Smith (2009), acknowledge these differences, stating:

They conclude*, as do some others, that the differences are unrelated to IQ, but agree that the differences are genetic, in part. As others argue that the population differences are wholly environmental (e.g., Brody, 2003), it’s important to look at populations that share a common environment.

Cranial capacity differences between racial groups in the US

In the US, it seems that there is indeed a small average difference in cranial circumference between racial groups but more data is needed. (see: Rushton and Ankney, 1999).

Brain mass differences between racial groups in the US

Jensen (1998) summarizes the brain mass findings from the Case-Western Reserve (1980) study (N= 811 W, 450 B). An age matched and height adjusted B-W differences of ~100g (~.78SD) was found, which is commensurate with the findings of Bean (1906), Mall (1909), Pearl (1934), and Vint (1934) as described in Rushton and Ankney (2009). Holloway (2002) found a B-W difference of 63 grams (N = 1,391 W; 615 Black). Similar findings have been found based in imaging studies (see 5). In their study, Isamah, et al. (2010) found that African Americans have 1 SD less total cerebrum volume than European Americans.

Overall, it looks like there’s some consistency among the findings. The results have been numerously replicated, as summarized by Lynn (2006).

Next there must be plausible accounts for selective pressure, relating population differences, brain size, and intelligence. Bailey and Geary (2010) found the following correlations based on the location of 175 archeological sites dating from 1.9 million to 10 thousand years ago.

Cranial capacity positively correlates with latitude and population density both which are positively intercorrelated. It negatively correlates with parasite load and mean temperature.

This conforms with the major evolutionary-hereditarian explanations: Population Density (1), Paleoclimate (2), Geographic Novelty (3), and Disease Burden (4). Ash and Gordon, in Ash and Gordon (2007), give a nice discussion of this perspective:

More recent studies on ASPM and MCPH1, as summarized by Montgomery (2010), bolster this case.

From an environmental standpoint, one would either have to argue that the between population differences in brain size don’t cause between difference in intelligence (a la Cartmill and Smith, 2009) or that the between differences in brain size, while related to differences in cognition, ultimately have an environmental origin. The former requires one to argue that different populations are wired differently, say in the manner of males and females, so that genetic between population differences in size don’t entail between population differences in intelligence; this isn’t supported by within sex studies on brain functioning and structure; and the consistently positive size/IQ correlation within clinal populations (e.g., Sudanese, Guatemalans, African Americans, Asian Indians, Turks, Chileans, etc.) strongly argues against this (see: Rushton and Ankney, 2009). The latter is probably true to some extent; a complete environmental explanation, though, seems very implausible, for several reasons:

1) It requires one to discount the historical evidence which shows a consistent pattern of differences over time and which strongly suggests an adaptive origin to the differences.

2) As the racial population differences in cranial capacity are intercorrelated with numerous other musculoskeletal trait differences, arguing that the cranial capacity differences have a wholly environmental origin require one to maintain that the whole matrix of musculoskeletal trait differences likewise has an environmental origin. Which is implausible. As Rushton and Rushton (2001) note: “[R]ace differences in brain size are correlated with 37 musculoskeletal variables shown in standard evolutionary textbooks to change systematically with increments in brain size. The 37 variables include cranial traits (such as jaw size and shape, tooth size and shape, muscle attachment sites, and orbital bone indentations), and postcranial traits (such as pelvic width, thighbone curvature, and knee joint surface area). Across the three populations, the ‘‘ecological correlations’’ [Jensen, A. R. (1998). The g factor. Westport, CT: Praeger] between brain size and the 37 morphological traits averaged a remarkable r = .94; r = .94. If the races did not differ in brain size, these correlations could not have been found.” See also Rushton and Rushton (2004)

3) Classic studies of geological differences and Cephalic Indexes (CI) contradict a purely environmental hypothesis for differences. For example, Herskovits (1930), found the following association between genealogy an CI:

4) It has been found that population craniometric differences align with genetic differences. As Hubbe et al. (2009) note:

On the other hand, several studies have demonstrated a geographic structure in modern human craniometric diversity on a global level (e.g., Howells, 1973, 1989; Hanihara, 1996). Craniometric data have been found to follow a common geographic pattern with genetic markers, including both classical and microsatellite DNA markers (Relethford, 1994, 2004a, 2009; Manica et al., 2007; Betti et al., 2009). These findings have been interpreted as resulting from an isolation-by-distance model of evolutionary diversification. Furthermore, population relationships inferred from cranial morphology (as reflected both by traditional linear measurements and by 3D geometric morphometric data) have been shown to match those inferred from genetic data (Roseman, 2004; Harvati and Weaver, 2006a,b; Smith, 2009). Taken together, these results suggest that human cranial morphology preserves a relatively strong population history signal, in addition to a climatic and possibly also dietary/masticatory signal (e.g., Relethford, 2004a).

One effectively has to argue that the correlation between craniometric differences and genetic differences is spurious to maintain an environmental case.

Overall, given the reasons above, it seems probable that some of the differences in cranial capacity/brain volume have an evolutionary genetic origin. And, given the genetic correlations between brain volume and IQ, this seems to support a Brain size argument for genotypic IQ differences.


*Cartmill and Smith (2009) cite Deacon (1997). In the cited chapter, “The Size of intelligence: A gross misunderstanding,” 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), “The evolution of primate general and cultural intelligence; Table 4. The relationship between primate general intelligence and brain volume).

In their discussion, Cartmill and Smith (2009( 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).

(1) Cochran, G., & Harpending, H. (2009). The 10,000-year explosion: How civilization
accelerated human evolution.

(2) Kanazawa, 2008. Temperature and evolutionary novelty as forces behind the evolution of general intelligence. Intelligence

Lynn, 1991. The evolution of race differences in intelligence

Templer and Arikawa, 2006. Temperature, skin color, per capita income, and IQ: An international perspective.

(3) Kanazawa, S. (2004b). General intelligence as a domain-specific adaptation.

(4) Eppig, Fincher, and Thornhil, 2010. Parasite prevalence and the worldwide distribution of cognitive ability

We also propose a complementary hypothesis that may explain some of the effects of infectious disease on intelligence. As we mentioned, it is possible that a conditional developmental pathway exists that invests more energy into the immune system at the expense of brain development. In an environment where there has consistently been a high metabolic cost associated with parasitic infection, selection would not favour the maintenance of a phenotypically plastic trait. That is, the conditional strategy of allocating more energy into brain development during periods of health would be lost, evolutionarily, if periods of health were rare. Peoples living in areas of consistently high prevalence of infectious disease over evolutionary time thus may possess adaptations that favour high obligatory investment in immune function at the expense of other metabolically expensive traits such as intelligence. Data do not currently exist on temporal variation of the severity of infectious disease across the world over human history. For genetically distinct adaptations in intelligence to exist based on this principle, parasite levels must be quite consistent over evolutionary time.

(5) Holloway (2008: The Human Brain Evolving: A Personal Retrospective) has an interesting discussion on the brain controversy:

In the late 1970s and early 1980s, I collected autopsy data from the Pathology department at Columbia’s College of Physicians and Surgeons (now CUMS). I was interested in age, sex, and ethnic effects on brain size changes through time as might be found in cross-sectional data. Roughly 2000 cases were collected, without personal identifications, and all cases of brain pathology were culled out of the data set. The results, unpublished, were roughly the same as found in the Ho et al. (1980, 1981) work on a sample from Milwaukee, which indicated that African American brains were statistically significantly lower in weight than were European American brains, that is, of course referring to the mean values. Ho et al. (1980) concluded that cultural effects were the reason behind the difference. Interestingly, Lieberman (2005) in his review of Rushton’s (2000, 2002) claims regarding ethnic (racial) differences in brain sizes and behaviors ignored this work by Ho et al. Needless to say, Tobias’s oft-cited paper on brain weight collecting methods (Tobias 1970) was cited to claim that autopsy data on brain weights are useless. Unfortunately, however problematic such data are, one tends to forget that autopsies are not done discriminately. Once the body is on the morgue slab, the autopsy is conducted in exactly the same fashion irrespective of the cadaver’s race, and thus comparisons of such data collected by the same anatomist or medical examiner are surely valid, depending on which variables are being compared. Comparing data collected by different examiners may of course be difficult, and perhaps statistical metaanalyses would be in order. To my knowledge, none exists.

Simply put, this research area remains an intensely political and near-suicidal enterprise. (Indeed, one colleague suggested I should incinerate the data; another suggested this kind of study had led to his relatives perishing in the Nazi concentration camps.) The continuing gap in African American and European-descent test scores on various cognitive tests (particularly IQ) throughout the United States and the world (Lynn & Vanhanen 2006) is a source of tremendous concern and acrimonious debate. Indeed, Jon Marks claimed he “outed” me as a “racist” (Marks 2000; see Holloway 2000 for reply) in his biological section of the American Anthropologist Newsletter because I had the temerity to defend Arthur Jensen against Loring Brace’s assertion that Jensen was a bigot. I had read much of this literature (e.g., Jensen 1998) including Jensen’s infamous 1969 piece in the Harvard Law Review and did not find him a racist. I remain appalled at our discipline, which regards him as such and which invented the appellation “Jensenism” to tar and feather him. I remain interested in the possibility that different populations have variation both in their brains and their behavior, but the issue is so politically incorrect that one cannot even approach such a study with anything but trepidation. (For example, the Annual Review article by Freedman & DeBoer 1979 was declared by sociocultural students at Columbia as racist and therefore not to be read!) If one disbelieves there are populational differences in the weight and/or structure of the brain, one should examine the papers by Klekamp and his colleagues, particularly regarding the finding that the primary visual striate cortex of Australian aborigines is significantly larger than in brains from people of European descent (Klekamp et al. 1994). This paper is, to my knowledge, the only paper published since the 1930s that demonstrates a real difference in brain morphology between modern populations (the last compilation of some of these earlier studies on brain morphology differences between different populations can be found in C.J. Connolly’s 1950 book, External Morphology of the Primate Brain, which is a sort of bible for most people working in paleoneurology. See also Kochetkova 1978.) Of course, there is Gould’s Mismeasure of Man, another bible of sorts, which should be read along with Michael’s (1988) Current Anthropology paper, which found that Morton’s rankings were correct and which Gould ignored in his later editions of the same book. There is certainly no evidence that Paul Broca used his elbow on the scales when measuring brains of peoples of European descent! Additional autopsy data sets await my attention, including some 5000 cases from Hong Kong, collected by my colleague Philip Beh, and ∼7500 cases from Singapore, the latter of multiple ethnicities. I hope to get to these data sets when I retire.


Ash and Gordon, 2007. Paleoclimatic Variation and Brain Expansion during Human Evolution

Bailey and Geary, 2010. Hominid Brain Evolution: Testing Climatic, Ecological, and Social Competition Models

Beals, et al., 1984. Brain Size, Cranial Morphology, Climate, and Time Machines

Brody, 2003. Jensen’s genetic interpretation of racial differences in intelligence: Critical evaluation

Gignac et al., 2003. Factors influencing the relationship between brain size and intelligence. In: Nyborg H, Ed. The scientific study of general intelligence: Tribute to Arthur R. Jensen.

Herskovits, 1930. The anthropometry of the American Negro

Hubbe et al., 2009. Climate signatures in the morphological differentiation of worldwide modern human populations

Hunt and Carlson, 2007. Considerations relating to the study of group differences in intelligence

Hwang, et al. 1995. Study on the adult Korean cranial capacity

Isamah, et al., 2010. Variability in Frontotemporal Brain Structure: The Importance of Recruitment of African Americans in Neuroscience Research

Jensen, 1998. The G-Factor

Kanazawa, 2008. Temperature and evolutionary novelty as forces behind the evolution of general intelligence.

Lynn, 2006. Race differences in intelligence.

McDaniels, 2003. Big-brained people are smarter: A meta-analysis of the relationship
between in vivo brain volume and intelligence.

Mekel-Bobrov, 2007. The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence and Microcephalin is not explained by increased intelligence

Montgomery, 2010. Brain Evolution: Microcephaly Genes Weigh In.

Peper et al., 2007. Genetic influences on human brain structure: A review of brain imaging studies in twins

Pietschnig, Zeiler, and Voracek, Unpublished. Of valid concerns and invalid effects: Meta-analyzing associations of in-vivo brain volume and IQ.

Posthuma et al., 2002. The association between brain volume and intelligence is of genetic origin.

Odokuma, Igbigbi, Akpuaka, and Esigbenu, 2010. Craniometric patterns of three Nigerian ethnic groups

Rushton, 1990. Race, brain size and intelligence: A rejoinder to Cain and Vanderwolf

Rushton and Ankney, 1999. Size matters: a review and new analyses of racial di􏰀erences in cranial capacity and intelligence that refute Kamin and Omari

Rushton and Rusthon, 2001. Brain size, IQ, and racial-group differences: Evidence from musculoskeletal traits

Rushton and Rusthon, 2004. Progressive changes in brain size and musculo-skeletal traits in seven hominoid populations

Rushton and Ankney, 2009. Whole Brain Size and General Mental Ability: A Review

Rusthon and Jensen, 2010. Race and IQ: A Theory-Based Review of the Research in Richard Nisbett’s Intelligence and How to Get It

Shockely, 1972. Dysgenics, Geneticity, Raceology: A Chalenge to the Intelectual Responsibility of Educators

Smith and Beals, 1990. Cultural correlates with cranial capacity

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  1. skarphedin
    October 6, 2011 at 8:09 pm

    Rushton has referenced Broca on Caucasian brains being more convoluted than Africans. I haven’t been able to find anybody who’s done anything to confirm or disconfirm that. Any information? Seems strange that there’s so little on that. What about differences in the lobes associated with various mental abilities?

  2. skarphedin
    October 6, 2011 at 8:19 pm

    Also, you have probably already seen this, but John Hawks recent paper “Selection for Smaller Brains in Holocone Human Evolution” is interesting in that it establishes that skulls have actually shrunk by over 100 cm3 pretty much around the world over the Holocene (and starting earlier to some degree). This seems at least moderately embarrassing to the brain size-iq argument. Although it seems to indicate that the latitudinal cline to skull size has been in place for some time.

    • Chuck
      October 30, 2011 at 8:07 am

      I saw it, and I find it to be an interesting phenomenon. I don’t understand your comment about it being an embarrassment to the brain size IQ argument. It’s an established fact that brain size does genetically correlate with IQ. Refer above.

  3. June 14, 2013 at 5:33 pm

    I am not a scientist. I am hopeless at maths. Most of the above, therefore, I see only dimly!
    However, I am puzzled mostly by one thing . I accept that brain size and general intelligence must be presumed to be correlated, as in muscle size and strength, eye size and visual acuity etc. in the absence of any reasonable hypothesis which would show how such a presumption was false. I accept that measurement has apparently correlated brain size and intelligence, and that there exists a political movement which attempts to not just refute, but to suppress this quite reasonable hypothesis,
    What I am really unclear on, however, is just what constitutes a race. Surely intelligence is the most defining human characteristic of all, and the one which makes the most difference to an individual’s capacity to prosper within a community, so why is there such an effort to apportion it between groups which have been postulated without previous reference to it? Ought not the enquiry to begin from the other direction, ie what other defining characteristics do people with an IQ of, say, over 160 possess? What skin colour, facial characteristics, blood group, or genetic markers are found in association with it. And what of Iqs from 120 to 160, say? Or below 75? Surely IQ should be a major definer of a group, rather than a characteristic apportioned between pre existing groups?
    Also, should not the constitution of a group – and perhaps, whether any group so distilled from individual creatures is a real thing, or a construction of the mind, a mental shorthand – be at least part of the question?
    This is not a suggestion or an attempt to denigrate this research, as I find it enormously interesting, if difficult to understand! It is a question from an uninformed and interested individual.

    • Chuck
      June 15, 2013 at 2:24 am

      You said: “This is not a suggestion or an attempt to denigrate this research, as I find it enormously interesting, if difficult to understand! It is a question from an uninformed and interested individual.”

      I reply: You are missing the prerequisite background information necessary to make the discussion intelligible. For that information, start here. And then proceed to here.

      Briefly, within populations there is a substantial amount of variance in phenotypic and genotypic general mental ability. Between geographically defined populations, there are measurable phenotypic differences. The question arises as to whether these phenotypic differences between populations are partially genetically conditioned as are the differences within populations. Differences are conceptualized as average differences, for example, as in the well established average differences, in this same trait, between college grads and high school drop outs. As background, it is known that selection pressures related to general mental ability were not identical across the globe. It is also known that different geographic populations faced different selection pressures for many neurologically related traits. For example:

      Natural selection acts to maintain diversity between Out of Africa and sub-Saharan African populations in genes related to neurological processes and brain development.

      The Yemeni and Mozabite are closely related Out of Africa (OOA) populations from the Arabian Peninsula and North Africa respectively, while the Maasai are a sub-Saharan African (SSA) population. Using genome-wide SNP data (publicly available for the Mozabite and Maasai, and collected here for the Yemeni) we show the Yemeni to have ~7% and the Mozabite to have ~26% recent sub-Saharan admixture, while the Maasai have ~27% Middle Eastern admixture. We use an adaptation of the locus specific branch length method to look for the effects of natural selection on alleles introduced to the three populations through admixture. We specifically look for 1) the adaptive introgression of alleles from SSA into the Yemeni and Mozabite, 2) the adaptive introgression of alleles from OOA into the Maasai, 3) purifying selection of SSA alleles out of the Yemeni and Mozabite, and 4) purifying selection of OOA alleles out of the Maasai. We found correspondence in patterns of adaptive introgression and purifying selection between the populations for 18 genomic loci, all of which contain protein-coding genes. The correspondence in signatures of selection between three independent populations is strong evidence for natural selection, rather than the false positive signals common in genome-wide scans of selection. Strikingly, of the regions where purifying selection is acting to maintain diversity between the Out of Africa and sub-Saharan African populations, eight out of twelve genes with known ontologies are involved in neurological processes or brain development. A binomial test found this enrichment to be significant.

      Modern human phenotypic variation: Exploring patterns of differentiation within and between continents.

      Current consensus is that modern human cranial phenotypic variation is a result of isolation-by-distance and neutral evolutionary processes, with natural selection acting most notably in some anatomical regions of populations living in extreme environmental conditions. Under this model, there is an underlying assumption that the rate of morphological differentiation in the past was uniform across the planet. The goal of this study is to test this assumption by comparing the morphological differentiation of human groups between and within geographic regions. We analyzed a large sample of male individuals from 135 human series. Craniometric variation was assessed through Fst estimates calculated from 33 linear measurements for each pair of series. Series were grouped into 15 geographic regions and the average Fst values within regions were then compared with the average linear geographic distances between series. Our results show a very strong linear correlation between average Fst values and geographic distances within each geographic region. However, between-regions analyses generally show lower correlations with distances, suggesting that geographic distance is not a good predictor for the Fst values between continents. These preliminary results indicate that the processes associated with the morphological differentiation within and between continents may have differed considerably, and probably a more significant influence of bottlenecks and natural selection. With the exception of the differences between North America and the Old World, isolation by distance may not be a good predictor of morphological differentiation between groups located in different continents. Alternative models should be taken into account to explain this pattern.

      Given that geographic populations differ neurologically — see also here and here — given that selections pressures related to general intelligence were not globally identical, given that there is measured phenotypic variance between these geographically defined populations, given that within these populations there is substantial variance in congenital intelligence, it is hypothesized that some of the phenotypic differences between populations are genetically conditioned.

      You say: “Surely intelligence is the most defining human characteristic of all, and the one which makes the most difference to an individual’s capacity to prosper within a community.”

      I reply: I, for one, don’t define one’s human worth in terms of one’s level of general intelligence — mostly, because I’m an intellectually honest egotists of mediocre intelligence. And I’m not going to admit that I am less human than others on account of this trait. But also because I am not going to degrade the feeble minded relative to those not, etc. So I reject, out of hand, this whole line of meditation.

      You say: “so why is there such an effort to apportion it between groups which have been postulated without previous reference to it?”

      That there are average psychometric intelligence differences between the groups discussed is established. The cause is the question. Again the context is that the overwhelming portion of the variance in this traits is within populations. The issue of between population variance only comes up because there is great concern about outcome differences between these same populations.

      You said: “Ought not the enquiry to begin from the other direction, ie what other defining characteristics do people with an IQ of, say, over 160 possess?”

      This is a different query. You can read the answer to it in a standard book on psychometric intelligence.

      You said: “What skin colour, facial characteristics, blood group, or genetic markers are found in association with it. And what of Iqs from 120 to 160, say? Or below 75? Surely IQ should be a major definer of a group, rather than a characteristic apportioned between pre existing groups?”

      The exploration of the genetic architecture of psychometric intelligence is well under way. What polymorphisms are associated with higher and lower intelligence is an interesting question which will largely be answered in the next 10-15 years. But that is not the concern of this piece. Here is discussed the possibility that the phenotypic variance between geographically defined populations in the trait (or in measures of it) is conditioned just as the variance within populations is, that is, by both genes and environment.

      You say: “Also, should not the constitution of a group – and perhaps, whether any group so distilled from individual creatures is a real thing, or a construction of the mind, a mental shorthand – be at least part of the question?”

      No. Because, as noted variously, we are discussing mean differences. For mean differences e.g., in skin color, height, personality, etc. you obviously don’t need a special kind of group.

      • Roy Irons
        June 15, 2013 at 11:48 am

        Thank you for taking the time and trouble to reply to my questions. I shall read it all up with interest.I hope that I can understand it when I do!
        I must just comment quickly, however, and endorse your comment re intelligence and worth. High intelligence is a defining human characteristic, because that has raised humans above all other animals in power. High intelligence, however, does not prevent a person being nasty or unpleasant or downright wicked, and it doesn’t prevent them from being completely wrong!. But worth is an entirely separate characteristic, and is by no means confined to humans anyway. I would certainly condemn myself to worthlessness if I equated it with intelligence!
        Thanks again – it was very kind of you to reply.

      • Chuck
        June 15, 2013 at 6:01 pm

        Generally, I distinguish between intrinsic and extrinsic superiority. Intrinsic superiority/inferiority describes one’s rank in the hierarchy of being e.g., mortals are intrinsically inferior to gods. Extrinsic superiority/inferiority describes the rank of one’s traits e.g., the ill have inferior health relative to the hale. I reject the identification of the intrinsic with the innate. The phrase “innate worth” makes no sense to me insofar as “innate” refers to genes and “worth” refers to being-status. With regards to value, having a congenital disease is no different from having an environmentally conditioned one (e.g., Lyme disease versus chronic fatigue syndrome). I class both as extrinsic inferiorities (to the extent that I use the term “inferiority” to describe the class).

        So I don’t reject the idea that some people (and I guess potentially peoples on average) are “inferior” or “worth less” in an extrinsic sense of inferior in traits x, y, and x. Rather, I reject the lack of distinction between the intrinsic and extrinsic and I reject the identification of genes with markers of the intrinsic. This is consistent with natural law theory. I’m not sure why people moved away from this thinking. But I do believe that statistician David Bartholomew was onto something when he wrote:

        The problem actually goes deeper still. It is not variation as such which causes the trouble. After all, we vary in our tastes in food and music. Some of us are tall and others short. Some are athletic and others are not. Except in rather restricted circumstances, such as among school children of a certain age, such things are not equated with feelings of inferiority or superiority. What is it about intelligence that links it so intimately with human worth?

        It is obviously something to do with our brains, the seat of our consciousness, but it is what we do with those brains that counts. Intelligence is held to influence our ability to acquire power, influence and wealth. In short, those who have it in good measure are seen as being better endowed and able to lead a fuller life. Perhaps the link derives from the ultimate source of our values and beliefs; it may also have to do with the fact that they have changed in much of western society as a whole. The Judeo-Christian belief in ‘Man made in the image of God’ provided a sufficient basis for equality. With that fact securely in place, it was easy to be relaxed about other inequalities which spoke of the diversity of the creation rather than differences in worth. Without that foundation, one has to cast around for some other basis for ascribing worth to individuals. Science is seen, by some, as providing the only objective basis of knowledge and hence as the obvious route to a rational assessment of the human person. What is more natural then than to turn to a measure of our most distinctive and impressive attribute – our intelligence? For many therefore, intelligence and intelligence testing impinge on their understanding of themselves at the deepest level. It is hardly surprising that they find it hard to be rational about it. (Measuring Intelligence Facts and Fallacies).

        The issue is complex because it was the Hellenic-Christian perspective which raised the capacity for human reason to the status human being marker i.e., Man as rational animal. So that perspective set the foundation for the equation of inter-human intelligence differences with inter-human being-worth differences. Once the Aristotelian-Thomist perspective on nature and substance was rejected, once genes were equated with nature and essence was equated with accident, it became easy to translate individual and average differences in the capacity for reasoning (as understood as general mental ability) with differences in human-ness. I appreciate this.

        But, given the vast individual differences in the trait, it makes no sense to oppose the idea of group differences on the grounds that these differences imply differences between groups in human worth and that such an implication is morally problematic. Regardless of group differences, the same logic would imply massive within group differences in intrinsic value. If one opposes the idea of inter-human intrinsic inferiority/superiority, one must reject the equation of intelligence qua general intelligence differences with intrinsic worth differences. If not, one must accept that some groups are intrinsically inferior to others just as nearly all individuals are to some others.

        I see two strategies for avoiding the immediately above conclusion: distinguish between intrinsic and extrinsic as I do and treat mental functioning as just another extrinsic trait — or identify reason with human-ness and return to the idea of lex naturalis, where human nature is conceived as an essential (or substantial) property in which all humans equally participate, regardless of imperfect realization.

        Whatever the case, the context of this discussion is that there are vast inter-group congenital differences in the trait, differences of a magnitude which dwarf any between geographic population deviations. In light of this, the question should be: Why make much ado of between geographic group differences? But the context is that much ado is made of these differences. For example, see number 4 on the social science’s top ten list of fundamental concerns. This disproportionate concern stems from the view that racial differences (national and international) are the product of social injustice (legacy of x, y, and z). My point is that there is a different perspective.

      • Chuck
        June 15, 2013 at 6:32 pm

        “I must just comment quickly, however, and endorse your comment re intelligence and worth. High intelligence is a defining human characteristic, because that has raised humans above all other animals in power.”

        The type of “intelligence” that varies between species is qualitatively different from the type of intelligence (i.e., general intelligence) that varies within the human species. Jensen (1998) makes this point. I mean, when we are talking about sub/inter-species differences we are talking about the types of differences that exist between members of your family — not the type of difference between you and a zebra. Again, this is more background understanding. Regardless, I would agree that certain types of mental capacity distinguish humans as a species from other species on the earth. I don’t imagine that high intelligence of the inter-species type is unique, in the whole universe, to humans, though. And, moreover, I would note that this is a species level difference. I imagine that by most standards some non-human animals are more intelligent than some human animals e.g., an adult Orca versus a human fetus — no?

  4. March 11, 2014 at 4:57 am

    1st we must know all the facts & properly analyze all the data &
    In here lies the rub. European and Asians also have bigger eyeballs. In fact all people living in and or native to northern latitudes have bigger brains and eyeballs. http://www.sciencedaily.com/releases/2011/08/110804214410.htm
    You can see this dynamic with in the populations living at varying latitudes in Europe. As you travel north in latitude studies show Northern Europeans have bigger eyeballs and brains than Southern Europeans
    This being the case the people with the biggest brains and eyeballs are Eskimos or the Inuit people who live near the North Pole. European and Asian brains are bigger in the areas dealing with sight. During the ice age it was dark and difficult to see. Also today due to sunlight refraction it is more difficult to see in Northern latitudes.
    Neanderthal man who was an early cousin to homo sapiens (us) evolved during the ice age and had a bigger brain than any human. http://www.sciencedaily.com/releases/2013/03/130319093639.htm

    We can tell from early Neanderthal artifacts they were less intelligent than homo sapiens archaic or homo sapiens sapiens(us/modern man) so their bigger brains did not equate to a higher intelligence. In fact it appears to have been substantially lower Europeans and Asians like Neanderthal developed in the northern hemisphere during ice age climatic changes. Both Asians and Europeans evolved bigger eyes and optic nerve areas to see better and developed bigger brains to deal with the extra sight sensory input
    So now we understand “why” Europeans and Asians have bigger brains. The next question is does this correlate with their inherently having a higher IQ than Africans?

    First lets look at two areas of the brain Asians and Europeans have increased size and have more grey matter dedicated to:

    The visual cortex . Again note this extra brain matter is dedicated to analyzing increased visual input. Another area larger in Asians and Europeans are the lateral ventricles.These are filled with cerebrospinal fluid (CSF) which bathes and cushions the brain and spinal cord . The lateral ventricles do not consist of any grey or white matter.

    Cognitive functions such as -executive functions, planning, abstract reasoning, impulse control, sustained attention and insight are not carried out in these areas. These functions are carried out primarily in the frontal lobe were the Orbitofrontal Cortex is located.

     African Americans in fact have a largerorbitofrontal cortex  than Caucasians.

    So knowing this we have to look into other reasons for higher intelligence. You have to analyze the psychological, economic & social effects on populations with traditionally lower IQ. Yes bigger brains due seemed to be tied to higher Intelligence but I theorize that an African with a bigger brain with in the African range would be just as intelligent as a European with a bigger brain with the European range but three average sized brained African, Asian & Europeans would all have equalling average IQ as long as all three were raised with equal health, economic & educational access. Today we are comparing socially disadvantaged people’s some who come from pastrolist & hunter gatherings cultures. I theorize this is why Native Africans & East Indians score low in their native lands but those that immigrate to America score in the 100’s

    • Chuck
      March 11, 2014 at 3:08 pm

      Hi hucipher,

      If you wish to discuss this topic I will oblige you, though, I may request that you do some background reading. Regarding your comments:

      (a) “I theorize this is why Native Africans & East Indians score low in their native lands but those that immigrate to America score in the 100’s”

      This is true regarding Asian Indian migrants, but not so regarding African migrants.

      (b) This being the case the people with the biggest brains and eyeballs are Eskimos or the Inuit people who live near the North Pole.

      Regarding the study cited, Anthropologist Peter Frost has noted:

      “Pearce and Dunbar (2011) argue that bigger brains are an adaptation to lower levels of ambient light. Specifically, dimmer light requires larger eyes, which in turn require larger visual cortices in the brain. Using 73 adult crania from populations located at different latitudes, the two authors found that both eyeball size and brain size correlate positively with latitude. The correlation was stronger with eyeball size, an indication that this factor was driving the increase in brain size…How credible is this explanation? First of all, visual cortex size was not directly measured. The authors inferred that this brain area was responsible for the increase in total cranial capacity. Obviously, they couldn’t have done otherwise. They were measuring skulls, not intact brains…But there’s another problem—one in the realm of logic. A lot of things correlate with latitude: pigmentation, mating systems, rules of descent, degree of paternal investment, and so on. If one of them correlates more strongly with latitude than the others, does it therefore cause the others? Not at all. It may be closer than the others to this shared cause, but it doesn’t necessarily lie on the same causal chain as the others…In other words, the level of ambient light does not produce a single cascade of consequences, with eyeball size being the first consequence. There are probably many different cascades.”

      Whatever the case, a visual cortex hypothesis is plausible. If so, global Southerners might be expected to do worse than Northerners on spatially loaded IQ subtests. Whether this is the case or not, I know not. There are models of intelligence (e.g., dynamic mutualism) by which the general factor emerges out of the interaction between lower factors; if these models are correct, a spatial cognitive advantage could translate into a general intelligence one. The issues then is complicated.

      (c) “We can tell from early Neanderthal artifacts they were less intelligent than homo sapiens archaic or homo sapiens sapiens(us/modern man) so their bigger brains did not equate to a higher intelligence.”

      I haven’t investigated the Neanderthal-IQ issue. Whatever the case, among humans, the correlation between brain size and general intelligence is about 0.4, which means that only 20% of variance in IQ is explained by brain size; there is much room for other factors.

      • Nicholas
        August 19, 2014 at 10:23 pm

        Well this is strange because on the contrary in my college Africans are outscoring everyone in Houston. Doesn’t matter if you’re White, African-American, Asian, or German. The African immigrants are overflowing all the bio-chemistry and math classes. I truly believe there is a lack of research regarding this. I understand environments have a great ordeal in regards to intelligence but in order to put race in the matter there needs to be a more global effort with all different nationalities in one room to provide as much of an unbiased research as possible. The thing about research is that 80% of it is always biased whether it’s intentional or un-intentional…and “WE” did a research on that in Psychology class!

      • Chuck
        August 19, 2014 at 11:34 pm

        “Well this is strange because on the contrary in my college Africans are outscoring everyone in Houston… The African immigrants are overflowing all the bio-chemistry and math classes. I truly believe there is a lack of research regarding this.”

        I find that unlikely. See the relevant discussion here: http://openpsych.net/ODP/2014/07/ethnicrace-differences-in-aptitude-by-generation-in-the-united-states-an-exploratory-meta-analysis/ Also, individuals who are citizens of Black African counties do awful on the GRE math. See: “Snapshot of the Individuals Who Took the GRE® revised General Test 2011-2012″. One would only expect that those who come to the U.S. would do equally poorly, barring rather strong migrant selection. Whatever the case, if you could find and send me test score data broken down by region of origin I would appreciate it. The data overall indicates that Black West Indian and African immigrants perform about as poorly on measure of general aptitude as do native African Americans. Though, disentangling West Indian and African origin Black immigrants is not without difficulty.

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