NYT: Brain Size & Cognitive Advancement

In the NYT Science section is an article on a recent thesis correlating human brain size with intelligence:

At three pounds, it is gigantic relative to our body size. Our closest living relatives, chimpanzees, have brains that are only a third as big.

Scientists have long suspected that our big brain and powerful mind are intimately connected. Starting about three million years ago, fossils of our ancient relatives record a huge increase in brain size. Once that cranial growth was underway, our forerunners started leaving behind signs of increasingly sophisticated minds, like stone tools and cave paintings.

But scientists have long struggled to understand how a simple increase in size could lead to the evolution of those faculties. Now, two Harvard neuroscientists, Randy L. Buckner and Fenna M. Krienen, have offered a powerful yet simple explanation.

In our smaller-brained ancestors, the researchers argue, neurons were tightly tethered in a relatively simple pattern of connections. When our ancestors’ brains expanded, those tethers ripped apart, enabling our neurons to form new circuits.

Dr. Buckner and Dr. Krienen call their idea the tether hypothesis, and present it in a paper in the December issue of the journal Trends in Cognitive Sciences.

Nowhere in the article is there any mention of longstanding scientific facts surrounding human brain size and… drumroll… race.

Someday, though, there will be such an article in the NYT.

Just as the reality of brain size differences as a subject is (now) no longer taboo in the Times, the Truth on brain size & race can only be suppressed for so long.

Below is the entirety of “Section 6: Race, Brain Size, and Cognitive Ability” from the 2005 paper “Thirty Years Of Research On Race Differences In Cognitive Ability” by J. Philippe Rushton & Arthur R. Jensen:

Studies on over 700 participants show that individuals with larger brain volumes have higher IQ scores. About two dozen  studies using magnetic resonance imaging (MRI) to measure the volume of the human brain have found an overall correlation with IQ of greater than .40 (Rushton & Ankney, 1996; P. A. Vernon, Wickett, Bazana, & Stelmack, 2000). The correlation of .40 using MRI is much higher than the .20 correlation found in earlier research using simple head size measures, although the .20 correlation is also reliable and significant. Rushton and Ankney (1996) reviewed 32 studies correlating measures of external head size with IQ scores or with measures of educational and occupational achievement, and they found a mean r = .20 for people of all ages, both sexes, and various ethnic backgrounds, including African Americans.

The most likely reason why larger brains are, on average, more intelligent than smaller brains is that they contain more neurons and synapses, which make them more efficient. Haier et al. (1995) tested the brain efficiency hypothesis by using MRI to measure brain volume and glucose metabolic rate to measure glucose uptake (an indicator of energy use). They found a correlation of –.58 between glucose metabolic rate and IQ, suggesting that more intelligent individuals have more efficient brains because they use less energy in performing a given cognitive task. Several other studies supporting the brain-size/efficiency modelwere reviewed in Gignac, Vernon, and Wickett (2003). In any individual, however, energy use increases with the increasing complexity of the cognitive task.

Estimates from twin studies indicate that genes contribute from 50% to 90% of the variance to both cranial capacities based on external head size measures and to brain volume measured by MRI (Bartley, Jones, & Weinberger, 1997; Pennington et al., 2000; Posthuma et al., 2002; Rushton & Osborne, 1995; Thompson et al., 2001). Common genetic effects mediate from 50% to 100% of the brain-size/IQ correlation (Pennington et al., 2000; Posthuma et al., 2002). Studies have also shown that correlations between brain size and IQ also hold true within families as well as between families (Gignac et al., 2003; Jensen, 1994; Jensen & Johnson, 1994), which also implies shared genetic effects. However, one study that examined only sisters failed to find the within-family relation (Schoenemann, Budinger, Sarich, & Wang, 2000). Families with larger brains overall tend to have higher IQs and, within a family, the siblings with the larger brains tend to have higher IQ scores. 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, that differ between families. Race differences in average brain size are observable at birth. A study by Rushton (1997) analyzed recorded  head circumference measurements and IQ scores from 50,000 children in the Collaborative Perinatal Project followed from birth to age 7 (Broman, Nichols, Shaugnessy, & Kennedy, 1987). Using the head circumference measures to calculate cranial capacity at birth, 4 months, 1 year, and 7 years, at each of these ages, the Asian American children averaged larger cranial volumes than did the White children, who averaged larger cranial volumes than did the Black children. Within each race, cranial capacity correlated with IQ scores. By age 7, the Asian American children averaged an IQ of 110; the White children, 102; and the Black children 90. Because the Asian American children were the shortest in stature and the lightest in weight while the Black children were the tallest in stature and the heaviest in weight, these average race differences in brain-size/IQ relations were not due to body size.

External head size measurements (length, width, height) also have been used to estimate cranial capacities in adults. Rushton carried out five studies of large archival data sets. The first (Rushton, 1991) examined head size measures in 24 international military samples collated by the U.S. National Aeronautics and Space Administration. After adjusting for the effects of body height, weight, and surface area, the mean cranial capacity for East Asians was 1,460 cm3 and for Europeans 1,446 cm3. The second (Rushton, 1992) demonstrated that even after adjusting for the effects of body size, sex, and military rank in a stratified random sample of over 6,000 U.S. Army personnel, the average cranial capacity of East Asians, Whites, and Blacks were 1,416, 1,380, and 1,359 cm3, respectively. The third study (Rushton, 1993) reanalyzed a set of anthropometric data originally published by Melville Herskovits (who concluded there were not race differences in cranial capacity) and found Whites averaged a cranial capacity of 1,421 and Blacks, 1,295 cm3. The fourth study (Rushton, 1994) analyzed data obtained on tens of thousands of people from around the world collated by the International Labor Office in Geneva, Switzerland. It found that after adjusting for the effects of body size and sex, samples from the Pacific Rim, Europe, and Africa had average cranial capacities, of 1,308, 1,297, and 1,241 cm3  respectively. Finally, Rushton and Osborne (1995) analyzed the Georgia Twin Study of adolescents and found that after correcting for body size and sex, Whites had an average cranial capacity of 1,269 cm3, Blacks 1,251 cm3.

Rushton’s results, based on calculating average cranial capacity from external head size measures, join those from dozens of other studies from the 1840s to the present on different samples using three different methods (endocranial volume from empty skulls, wet brain weight at autopsy, and high-tech MRI). All show the same strong pattern of East Asians averaging larger and heavier brains than Whites who average larger and heavier brains than Blacks. For example, using MRI technology, Harvey, Persaud, Ron, Baker, and Murray (1994) found that 41 Blacks in Britain averaged a smaller brain volume than did 67 British Whites. The American anthropologist Samuel George Morton (1849) filled over 1,000 skulls with packing material to measure endocranial volume and found that Blacks averaged about 5 cubic inches less cranial capacity than Whites. His results were confirmed by Todd (1923), H. L. Gordon (1934), and Simmons (1942). The most extensive study of race differences in endocranial volume to date measured 20,000 skulls from around the world and reported East Asians, Europeans, and Africans had average cranial volumes of 1,415, 1,362, and 1,268 cm3, respectively (Beals, Smith, & Dodd, 1984).

Using the method of weighing brains at autopsy, Paul Broca (1873) reported that Whites averaged heavier brains than did Blacks, with larger frontal lobes and more complex convolutions. (Broca also used endocranial volume and found East Asians averaged larger cranial capacities than Europeans, who averaged larger than Blacks.) Other early autopsy studies found a mean Black–White group difference in brain weight of about 100 g (Bean, 1906; Mall, 1909; Pearl, 1934; Vint, 1934). A more recent autopsy study of 1,261 American adults found that the brains of 811 White Americans in their sample averaged 1,323 g and the brains of 450 Black Americans averaged 1,223 g—a difference of 100 g (Ho,  Roessmann, Straumfjord, & Monroe, 1980). Because the Blacks and Whites in the study were similar in body size, this was not responsible for the differences in brain weight.

Rushton (2000; Rushton & Ankney, 1996) summarized the world database using the three methods on which there are a sufficient number of studies (autopsies, endocranial volume, and head measurements), as well as head measurements corrected for body size (see Rushton, 2000, pp. 126–132, Table 6.6). The results in cm3 or equivalents were as follows: East Asians = 1,351, 1,415, 1,335, and 1,356 (M = 1,364); Whites = 1,356, 1,362, 1,341, and 1,329 (M = 1,347); and Blacks = 1,223, 1,268, 1,284, and 1,294 (M = 1,267). The overall mean for East Asians is 17 cm3 more than that for Whites and 97 cm3 more than that for Blacks. Within-race differences due to differences in method of estimation averaged 31 cm3. Because 1 cubic inch of brain matter contains millions of brain cells and hundreds of millions of synapses or neural connections, these group differences in average brain size may explain group differences in average IQ.

Jensen and Johnson (1994) showed that for both Blacks and Whites, the head size by IQ correlation is true within families as well as between families, indicating the intrinsic or functional relationship mentioned earlier. Further, within each sex, Blacks and Whites fit the same regression line of head size on IQ. When Blacks and Whites are perfectly matched for true-score IQ (i.e., IQ corrected for measurement error) at either the Black mean or the White mean, the overall average Black–White group difference in head circumference is virtually nil. (Matching Blacks and Whites for IQ eliminates the average difference in head size, but matching the groups on head size does not equalize their IQs. This is what one would expect if brain size is only one of a number of brain factors involved in IQ.)

In another analysis of the Georgia Twin Study, Jensen (1994) showed that the mean Black–White group difference in head/brain size is also related to the magnitude of the mean Black–White group difference in g. The correlation coefficient of each test with the head measurements was correlated with the magnitude of the Black–White group difference on that test, thus forming two vectors. The column vector of IQ test and head size correlations indicated a correlation of .51 (p _ .05) with the vector of standardized Black–White group differences on each of the tests.

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