Reinhard_Heini
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A review of the world literature on brain size and IQ by Rushton [Rushton, J. P. (1995). Race, evolution, and behavior: a life history perspective. New Brunswick, NJ: Transaction] found that African-descended people (Blacks) average cranial capacities of 1267 cm3, European-descended people (Whites) 1347 cm3, and East Asian-descended people (East Asians) 1364 cm3. These brain size differences, containing millions of brain cells and hundreds of millions of synapses, were hypothesized to underlie the race differences on IQ tests, in which Blacks average an IQ of 85, Whites 100, and East Asians 106. The validity of the race differences in brain size, however, continues to be disputed. In the present study, the race 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; ρ = .94. If the races did not differ in brain size, these correlations could not have been found. It must be concluded that the race differences in average brain size are securely established. As such, brain size-related variables provide the most likely biological mediators of the race differences in intelligence.
Among individuals, intelligence is related to brain size. This has been documented in about two dozen studies using magnetic resonance imaging (MRI) to measure brain volume. The overall correlation between IQ and brain size measured by MRI is >.40. Gignac, Vernon, and Wickett (in press), Rushton and Ankney (1996), and Vernon, Wickett, Bazana, and Stelmack (2000) have reviewed the evidence. Altogether there are now about 15 studies on over 700 subjects showing that individuals with larger brain volumes have higher IQ scores. This is much higher than the .20 correlation found in earlier research using simple head size measures, although even simple head size measures also show the relationship. Rushton and Ankney reviewed 32 studies correlating measures of external head size with IQ scores, or with measures of educational and occupational achievement, and found a mean r = .20 (P < 10−10) with people of all ages, both sexes, and various ethnic backgrounds.
The races differ in average brain size and this shows up at birth. Rushton (1997) analyzed the enormous US data set known as the Collaborative Perinatal Project. It recorded head circumference measurements and IQ scores from 50,000 children followed from birth to age 7 (Broman, Nichols, Shaugnessy, & Kennedy, 1987). The results showed that at birth, 4 months, 1 year, and 7 years, the East Asian children in the study averaged larger cranial volumes than did the White children, who averaged larger cranial volumes than did the Black children. Within each race, the children with the larger head sizes had the higher IQ scores and by age 7, the East Asian children averaged an IQ of 110, White children an IQ of 102, and Black children an IQ of 90. Moreover, the East Asian children, who averaged the largest craniums, were the shortest in stature and the lightest in weight, whereas the Black children, who averaged the smallest craniums, were the tallest in stature and the heaviest in weight. Therefore, the race differences in brain size were not due to body size.
Studies have also shown that the correlation between brain size and IQ holds true withinfamilies as well as between families Gignac et al., in press, Jensen, 1994, Jensen & Johnson, 1994; although one study that examined only sisters failed to find the within-family relation; Schoenemann, Budinger, Sarich, & Wang, 2000). The within-family finding is of special interest because it controls for most of the sources of variance that distinguish families such as social class, styles of child rearing, and general nutrition, which differ between families. Jensen and Johnson's study, based on head size measurements of over 7000 sibling pairs, showed the within-family relation for Blacks, as for Whites. When Blacks and Whites were matched for IQ, the Black–White difference in head size disappeared.
Many are surprised to learn that the races differ in brain size (e.g., Brody, in press, Graves, 2002, Kamin & Omari, 1998, Lieberman, 2001) and they question how reliable the evidence is. In fact dozens of studies from the 1840s to the 1990s, using different methods on different samples, reveal the same strong pattern. Four different methods of measuring brain size—MRI, endocranial volume measured from empty skulls, wet brain weight at autopsy, and external head size measurements—all produce the same results. Using MRI, for example, Harvey, Persaud, Ron, Baker, and Murray (1994) found that 41 Africans and West Indians had a smaller average brain volume than did 67 Caucasians.
Using the method of measuring endocranial volume, the American anthropologist Samuel George Morton (1849) filled over 1000 skulls with packing material and found that Blacks averaged about 5 in.3 less cranial capacity than Whites. These results have stood the test of time Gordon, 1934, Simmons, 1942, Todd, 1923. More recently, Beals, Smith, and Dodd (1984) carried out the largest study of race differences in endocranial volume to date, with measurements of up to 20,000 skulls from around the world. Their study found that East Asians, Europeans, and Africans averaged cranial volumes of 1415, 1362, and 1268 cm3, respectively. The skulls from East Asia were 3 in.3 larger than those from Europe, which in turn were 5 in.3 larger than those from Africa.
Using the method of weighing brains at autopsy, the famous French neurologist Paul Broca (1873) found that Whites averaged heavier brains than Blacks with more complex convolutions and larger frontal lobes. (He corroborated the Black–White difference using endocranial volume and also found that East Asians averaged larger cranial capacities than did Whites.) The autopsy results too have stood the test of time. Subsequent studies have found an average Black–White difference of about 100 g Bean, 1906, Mall, 1909, Pearl, 1934, Vint, 1934. Some studies have found that the more White admixture (judged independently from skin color), the greater the average brain weight in Blacks Bean, 1906, Pearl, 1934. More recently, Ho, Roessmann, Straumfjord, and Monroe (1980) found in an autopsy study of 1261 American adults, that 811 White Americans averaged 1323 g and 450 Black Americans averaged 1223 g—a difference of 100 g. Since the Blacks and Whites in the study were similar in body size, differences in body size cannot explain away the differences in brain weight.
A final way of estimating brain size is by cranial volume calculated from external head size measurements (length, width, height). The results again confirm the racial differences. Rushton, 1991, Rushton, 1992, Rushton, 1993, Rushton, 1994 and Rushton and Osborne (1995) carried out a series of studies estimating brain size this way from five large archival data sets. In the first of these studies, Rushton (1991) examined head size measures in 24 international military samples collated by the U.S. National Aeronautics and Space Administration and after adjusting for the effects of body height, weight, and surface area, found that the cranial capacity for East Asians was 1460 and for Europeans it was 1446 cm3. In the most comprehensive of these studies, Rushton (1992) calculated average cranial capacities for East Asians, Whites, and Blacks from a stratified random sample of over 6000 U.S. Army personnel. The East Asians, Whites, and Blacks averaged 1416, 1380, and 1359 cm3, respectively. The East Asians averaged 36 cm3more capacity than did the Whites, and the Whites averaged 21 cm3 more capacity than did the Blacks. This study allowed precise adjustments for all kinds of body size measures. Yet adjusting for these, or other variables, did not erase the average racial differences in cranial capacity.
Are these findings attributable simply to race differences in body size? The world database from: (a) autopsies, (b) endocranial volume, (c) head measurements, and (d) head measurements corrected for body size was summarized by Rushton (1995, pp. 126–132, Table 6.6). The results in cubic centimeters or equivalents were: East Asians = 1351, 1415, 1335, 1356 (mean = 1364); Whites = 1356, 1362, 1341, 1329 (mean = 1347); and Blacks = 1223, 1268, 1284, and 1294 (mean = 1267). The review found the overall mean for Asians to be 17 cm3 more than that for Europeans and 97 cm3 more than that for Africans. Within-race differences, due to the method of estimation, averaged 31 cm3. Since 1 in.3 of brain matter contains millions of brain cells and hundreds of millions of synapses or neural connections, Rushton hypothesized that these brain size differences help to explain why the races differ in average IQ. Similarly, Jensen (1998, p. 443) calculated an “ecological correlation” of .998 between brain size and IQ across the three races.
It is important to note, for the historical record, that by 1871, Charles Darwin considered the race differences in brain size so well established that he relied on them as evidence in favor of his then controversial theory of human origins. Even Franz Boas (1894), who is sometimes described as the “true” founder of American anthropology for being the first to challenge “Eurocentric racism,” initially accepted that the races differed in brain size and drew important conclusions about relative intellectual performance based on the amount of overlap in the distributions. Boas analyzed data from Topinard (1890) and found that only 27% of Blacks exceeded the White brain size average, rather than the 50% that should have done had the races been equal. Arguing that “the greater the central nervous system, the higher the faculty of the race and the greater its aptitude to mental development,” Boas concluded: “We might, therefore, anticipate a lack of men of high genius (among Blacks).”
Such conclusions were widely shared among scientists up until the 1930s. Darwin's cousin, Sir Francis Galton (1869), began a scientific research program on intelligence that continues to this day. Galton (1888) examined the relation between brain size and cognitive ability by multiplying head length by breadth by height in 1095 university students, plotting the results against class rank, and calculating that those who obtained high honors had a brain size 2–5% greater than those who did not. As described, modern studies have corroborated Galton's results Rushton & Ankney, 1996, Vernon et al., 2000.
Despite 150 years of evidence that the races differ in brain size, and that brain size is related to intelligence, this research is often claimed to be inconclusive or to reflect little more than personal bias Brody, in press, Gould, 1996, Graves, 2002, Kamin & Omari, 1998, Lieberman, 2001. The change in view from Darwin's time to today did not occur because of more and better data or methods of analysis, but because of changes in the political climate. This began when Franz Boas (1938) and his students chipped away at traditional “hierarchical” thinking throughout the 1920s and 1930s, rejecting an evolutionary explanation of IQ and instead championing the omnipotence of culture. Subsequent political events such as World War II and the reactions to the Holocaust, the U.S. Civil Rights Movement, and the struggle for the hearts and minds of the Third World during the Cold War, resulted in a success for Boas' mission to decouple biology from culture Degler, 1991, Miele, 2001. For many years, empirical research on race differences in brain size and intelligence virtually ceased, with the literature dominated by vigorous critiques, notably from Gould, 1981, Gould, 1996, Kamin (1974), and Tobias (1970).
Critiques of the race/brain size/IQ relationship continue to be forthcoming, including of the most recent literature (e.g., Brody, in press, Graves, 2002, Kamin & Omari, 1998, Lieberman, 2001). The main analytical arguments offered against accepting race differences in brain size at face value include alleged inadequacies in the control of variables such as body size, social class, and nutritional status; the unrepresentativeness of the samples; artifacts of data preparation, including personal bias; computational errors; weak effect sizes and their possible interactions with other variables; and the overgeneralization of conclusions. Although many of these criticisms have been tested, refuted, and replied to, and serious errors of omission and commission found in the critiques (e.g., Rushton, 2001, Rushton & Ankney, 2000), the belief remains widespread that the races do not differ reliably in brain size.
The present study aims to reduce substantially the uncertainty about race differences in brain size by analyzing data used by forensic anthropologists to identify race, age, and sex from the bones of skeletons (Byers, 2002). A broad-based extension and conceptual replication, such as the one provided here, will extend the parameters of the debate which has to take the totality of the evidence into account. The data and analyses presented here are not as susceptible to such ad hoc explanations as “unrepresentative samples” and “lack of controls for social class and nutrition,” for these are not known to affect the variables under discussion in any major way. Indeed, some of the musculoskeletal traits considered here (e.g., jaw size) are more widely identified with racial characteristics than is brain size, although their association with brain size has been much less widely known.
Source from the article/ https://www.sciencedirect.com/science/article/abs/pii/S016028960200137X
Introduction
In the US and around the world, East Asians and their descendants average an IQ of about 106, Europeans and their descendants about 100, and Africans and their descendants about 85. The lowest average IQ scores are reported for sub-Saharan Africa, about 70 Jensen, 1998, Lynn & Vanhanen, 2002, Rushton, 2000. Average IQ differences between individuals and groups, including mean racial-group differences, show up before age 5, and they last a lifetime (Jensen, 1998).Among individuals, intelligence is related to brain size. This has been documented in about two dozen studies using magnetic resonance imaging (MRI) to measure brain volume. The overall correlation between IQ and brain size measured by MRI is >.40. Gignac, Vernon, and Wickett (in press), Rushton and Ankney (1996), and Vernon, Wickett, Bazana, and Stelmack (2000) have reviewed the evidence. Altogether there are now about 15 studies on over 700 subjects showing that individuals with larger brain volumes have higher IQ scores. This is much higher than the .20 correlation found in earlier research using simple head size measures, although even simple head size measures also show the relationship. Rushton and Ankney reviewed 32 studies correlating measures of external head size with IQ scores, or with measures of educational and occupational achievement, and found a mean r = .20 (P < 10−10) with people of all ages, both sexes, and various ethnic backgrounds.
The races differ in average brain size and this shows up at birth. Rushton (1997) analyzed the enormous US data set known as the Collaborative Perinatal Project. It recorded head circumference measurements and IQ scores from 50,000 children followed from birth to age 7 (Broman, Nichols, Shaugnessy, & Kennedy, 1987). The results showed that at birth, 4 months, 1 year, and 7 years, the East Asian children in the study averaged larger cranial volumes than did the White children, who averaged larger cranial volumes than did the Black children. Within each race, the children with the larger head sizes had the higher IQ scores and by age 7, the East Asian children averaged an IQ of 110, White children an IQ of 102, and Black children an IQ of 90. Moreover, the East Asian children, who averaged the largest craniums, were the shortest in stature and the lightest in weight, whereas the Black children, who averaged the smallest craniums, were the tallest in stature and the heaviest in weight. Therefore, the race differences in brain size were not due to body size.
Studies have also shown that the correlation between brain size and IQ holds true withinfamilies as well as between families Gignac et al., in press, Jensen, 1994, Jensen & Johnson, 1994; although one study that examined only sisters failed to find the within-family relation; Schoenemann, Budinger, Sarich, & Wang, 2000). The within-family finding is of special interest because it controls for most of the sources of variance that distinguish families such as social class, styles of child rearing, and general nutrition, which differ between families. Jensen and Johnson's study, based on head size measurements of over 7000 sibling pairs, showed the within-family relation for Blacks, as for Whites. When Blacks and Whites were matched for IQ, the Black–White difference in head size disappeared.
Many are surprised to learn that the races differ in brain size (e.g., Brody, in press, Graves, 2002, Kamin & Omari, 1998, Lieberman, 2001) and they question how reliable the evidence is. In fact dozens of studies from the 1840s to the 1990s, using different methods on different samples, reveal the same strong pattern. Four different methods of measuring brain size—MRI, endocranial volume measured from empty skulls, wet brain weight at autopsy, and external head size measurements—all produce the same results. Using MRI, for example, Harvey, Persaud, Ron, Baker, and Murray (1994) found that 41 Africans and West Indians had a smaller average brain volume than did 67 Caucasians.
Using the method of measuring endocranial volume, the American anthropologist Samuel George Morton (1849) filled over 1000 skulls with packing material and found that Blacks averaged about 5 in.3 less cranial capacity than Whites. These results have stood the test of time Gordon, 1934, Simmons, 1942, Todd, 1923. More recently, Beals, Smith, and Dodd (1984) carried out the largest study of race differences in endocranial volume to date, with measurements of up to 20,000 skulls from around the world. Their study found that East Asians, Europeans, and Africans averaged cranial volumes of 1415, 1362, and 1268 cm3, respectively. The skulls from East Asia were 3 in.3 larger than those from Europe, which in turn were 5 in.3 larger than those from Africa.
Using the method of weighing brains at autopsy, the famous French neurologist Paul Broca (1873) found that Whites averaged heavier brains than Blacks with more complex convolutions and larger frontal lobes. (He corroborated the Black–White difference using endocranial volume and also found that East Asians averaged larger cranial capacities than did Whites.) The autopsy results too have stood the test of time. Subsequent studies have found an average Black–White difference of about 100 g Bean, 1906, Mall, 1909, Pearl, 1934, Vint, 1934. Some studies have found that the more White admixture (judged independently from skin color), the greater the average brain weight in Blacks Bean, 1906, Pearl, 1934. More recently, Ho, Roessmann, Straumfjord, and Monroe (1980) found in an autopsy study of 1261 American adults, that 811 White Americans averaged 1323 g and 450 Black Americans averaged 1223 g—a difference of 100 g. Since the Blacks and Whites in the study were similar in body size, differences in body size cannot explain away the differences in brain weight.
A final way of estimating brain size is by cranial volume calculated from external head size measurements (length, width, height). The results again confirm the racial differences. Rushton, 1991, Rushton, 1992, Rushton, 1993, Rushton, 1994 and Rushton and Osborne (1995) carried out a series of studies estimating brain size this way from five large archival data sets. In the first of these studies, Rushton (1991) examined head size measures in 24 international military samples collated by the U.S. National Aeronautics and Space Administration and after adjusting for the effects of body height, weight, and surface area, found that the cranial capacity for East Asians was 1460 and for Europeans it was 1446 cm3. In the most comprehensive of these studies, Rushton (1992) calculated average cranial capacities for East Asians, Whites, and Blacks from a stratified random sample of over 6000 U.S. Army personnel. The East Asians, Whites, and Blacks averaged 1416, 1380, and 1359 cm3, respectively. The East Asians averaged 36 cm3more capacity than did the Whites, and the Whites averaged 21 cm3 more capacity than did the Blacks. This study allowed precise adjustments for all kinds of body size measures. Yet adjusting for these, or other variables, did not erase the average racial differences in cranial capacity.
Are these findings attributable simply to race differences in body size? The world database from: (a) autopsies, (b) endocranial volume, (c) head measurements, and (d) head measurements corrected for body size was summarized by Rushton (1995, pp. 126–132, Table 6.6). The results in cubic centimeters or equivalents were: East Asians = 1351, 1415, 1335, 1356 (mean = 1364); Whites = 1356, 1362, 1341, 1329 (mean = 1347); and Blacks = 1223, 1268, 1284, and 1294 (mean = 1267). The review found the overall mean for Asians to be 17 cm3 more than that for Europeans and 97 cm3 more than that for Africans. Within-race differences, due to the method of estimation, averaged 31 cm3. Since 1 in.3 of brain matter contains millions of brain cells and hundreds of millions of synapses or neural connections, Rushton hypothesized that these brain size differences help to explain why the races differ in average IQ. Similarly, Jensen (1998, p. 443) calculated an “ecological correlation” of .998 between brain size and IQ across the three races.
It is important to note, for the historical record, that by 1871, Charles Darwin considered the race differences in brain size so well established that he relied on them as evidence in favor of his then controversial theory of human origins. Even Franz Boas (1894), who is sometimes described as the “true” founder of American anthropology for being the first to challenge “Eurocentric racism,” initially accepted that the races differed in brain size and drew important conclusions about relative intellectual performance based on the amount of overlap in the distributions. Boas analyzed data from Topinard (1890) and found that only 27% of Blacks exceeded the White brain size average, rather than the 50% that should have done had the races been equal. Arguing that “the greater the central nervous system, the higher the faculty of the race and the greater its aptitude to mental development,” Boas concluded: “We might, therefore, anticipate a lack of men of high genius (among Blacks).”
Such conclusions were widely shared among scientists up until the 1930s. Darwin's cousin, Sir Francis Galton (1869), began a scientific research program on intelligence that continues to this day. Galton (1888) examined the relation between brain size and cognitive ability by multiplying head length by breadth by height in 1095 university students, plotting the results against class rank, and calculating that those who obtained high honors had a brain size 2–5% greater than those who did not. As described, modern studies have corroborated Galton's results Rushton & Ankney, 1996, Vernon et al., 2000.
Despite 150 years of evidence that the races differ in brain size, and that brain size is related to intelligence, this research is often claimed to be inconclusive or to reflect little more than personal bias Brody, in press, Gould, 1996, Graves, 2002, Kamin & Omari, 1998, Lieberman, 2001. The change in view from Darwin's time to today did not occur because of more and better data or methods of analysis, but because of changes in the political climate. This began when Franz Boas (1938) and his students chipped away at traditional “hierarchical” thinking throughout the 1920s and 1930s, rejecting an evolutionary explanation of IQ and instead championing the omnipotence of culture. Subsequent political events such as World War II and the reactions to the Holocaust, the U.S. Civil Rights Movement, and the struggle for the hearts and minds of the Third World during the Cold War, resulted in a success for Boas' mission to decouple biology from culture Degler, 1991, Miele, 2001. For many years, empirical research on race differences in brain size and intelligence virtually ceased, with the literature dominated by vigorous critiques, notably from Gould, 1981, Gould, 1996, Kamin (1974), and Tobias (1970).
Critiques of the race/brain size/IQ relationship continue to be forthcoming, including of the most recent literature (e.g., Brody, in press, Graves, 2002, Kamin & Omari, 1998, Lieberman, 2001). The main analytical arguments offered against accepting race differences in brain size at face value include alleged inadequacies in the control of variables such as body size, social class, and nutritional status; the unrepresentativeness of the samples; artifacts of data preparation, including personal bias; computational errors; weak effect sizes and their possible interactions with other variables; and the overgeneralization of conclusions. Although many of these criticisms have been tested, refuted, and replied to, and serious errors of omission and commission found in the critiques (e.g., Rushton, 2001, Rushton & Ankney, 2000), the belief remains widespread that the races do not differ reliably in brain size.
The present study aims to reduce substantially the uncertainty about race differences in brain size by analyzing data used by forensic anthropologists to identify race, age, and sex from the bones of skeletons (Byers, 2002). A broad-based extension and conceptual replication, such as the one provided here, will extend the parameters of the debate which has to take the totality of the evidence into account. The data and analyses presented here are not as susceptible to such ad hoc explanations as “unrepresentative samples” and “lack of controls for social class and nutrition,” for these are not known to affect the variables under discussion in any major way. Indeed, some of the musculoskeletal traits considered here (e.g., jaw size) are more widely identified with racial characteristics than is brain size, although their association with brain size has been much less widely known.
Method
In the present study, the race differences in brain size are correlated with race differences in 37 musculoskeletal traits identified in standard evolutionary anatomy textbooks as being systematically related to brain size increments over the hominoid line Aiello & Dean, 1990, Conroy, 1997, Fleagle, 1999. The data on the racial attributes are taken from standard forensic anthropology textbooks Binkley, 1989, Brues, 1990, Byers, 2002, Krogman & Iscan, 1986, Reichs, 1998. Whenever a differentResults
There were missing data for 6/111 (5%) of the categories, all involving East Asians, and ties on 4 of the 105 remaining ones (4%). The first set of correlations was calculated using a pairwise deletion method to handle missing data, thereby retaining as many traits as possible for analysis (all 37 traits). Absolute and relative brain size (Traits 1 and 4) intercorrelated 1.00, and both correlated with a mean r of .94 (median=1.00) across the 37 traits. They showed similar high correlationsDiscussion
As brain size increases from 1356 to 1383 cm3across the three racial groups, 37 musculoskeletal traits vary accordingly with a remarkable mean r = .94, ρ = .94. These changes occurred on both cranial traits (temporalis fossae, postorbital constrictions, mandibles, dentition, neck muscle attachments), and postcranial traits (pelvic widths, femoral heads, tibial plateaus). Because the physical traits are not completely independent of each other, with some being logically as well as adaptivelySource from the article/ https://www.sciencedirect.com/science/article/abs/pii/S016028960200137X
