Brain size - IQ correlations
Originally posted by zoobyshoe
I'd like to see some studies that correlate brain size to intelligence. By your reckoning a person's I.Q. ought to be reflected in their brain size. Got any studies showing this?
--
The physical characteristics correlated with g that are empirically best established are stature, head size, brain size, frequency of alpha brain waves, latency and amplitude of evoked brain potentials, rate of brain glucose metabolism, and general health. [/color]
--
The g Factor.
Chapter 6: Biological Correlates of g. p137.
http://www.questia.com/PM.qst?a=o&d=24373874
--
...the technology of magnetic resonance imaging (MRI) now makes it possible to obtain a three-dimensional picture of the brain of a living person. A highly accurate measure of total brain volume (or the volume of any particular structure in the brain) can be obtained from the MRI pictures. Such quantitative data are now usually extracted from the MRI pictures by computer.
To date there are eight MRI studies ^{[10]} of the correlation between total brain volume and IQ in healthy children and young adults. In every study the correlations are significant and close to +.40 after removing variance due to differences in body size. (The correlation between body size and brain size in adult humans is between +.20 and +.25.) Large parts of the brain do not subserve cognitive processes, but govern sensory and motor functions, emotions, and autonomic regulation of physiological activity. Controlling body size removes to some extent the sensorimotor aspects of brain size from the correlation of overall brain size with IQ. But controlling body size in the brain × IQ correlation is somewhat problematic, because there may be some truly functional relationship between brain size and body size that includes the brain's cognitive functions. Therefore, controlling body size in the IQ × brain size correlation may be too conservative; it could result in overcorrecting the correlation. Moreover, the height and weight of the head constitute an appreciable proportion of the total body height and weight, so that controlling total body size could also contribute to overcorrection by removing some part of the variance in head and brain size along with variance in general body size. Two of the MRI studies used a battery of diverse cognitive tests, which permitted the use of correlated vectors to determine the relationship between the column vector of the various tests' g factor loadings and the column vector of the tests' correlations with total brain volume. In one study, ^{[10f]} based on twenty cognitive tests given to forty adult males sibling pairs, these vectors were correlated +.65. In the other study, ^{[10g]} based on eleven diverse cognitive tests, the vector of the tests' g loadings were correlated +.51 with the vector of the tests' correlations with total brain volume and +.66 with the vector of the tests' correlations with the volume of the brain's cortical gray matter...
Metabolically, the human brain is by far the most "expensive" organ in the whole body, and the body may have evolved to serve in part like a "power pack" for the brain, with a genetically larger brain being accommodated by a larger body. It has been determined experimentally, for example, that strains of rats that were selectively bred from a common stock exclusively to be either good or poor at maze learning were found to differ not only in brain size but also in body size. ^{[11]} Body size increased only about one-third as much as brain size as a result of the rats being selectively bred exclusively for good or poor maze-learning ability. There was, of course, no explicit selection for either brain size or body size, but only for maze-learning ability. Obviously, there is some intrinsic functional and genetic relationship between learning ability, brain size, and body size, at least in laboratory rats. Although it would be unwarranted to generalize this finding to humans, it does suggest the hypothesis that a similar relationship may exist in humans. It is known that body size has increased along with brain size in the course of human evolution. The observed correlations between brain size, body size, and mental ability in humans are consistent with these facts, but the nature and direction of the causal connections between these variables cannot be inferred without other kinds of evidence that is not yet available.
The IQ × head-size correlation is clearly intrinsic, as shown by significant correlations both between-families (r = +.20, p < .001) and within-families (r = +.11, p < .05) in a large sample of seven-year-old children, with head size measured only by circumference and IQ measured by the Wechsler Intelligence Scale for Children. ^{[12]} (Age, height, and weight were statistically controlled.) The same children at four years of age showed no significant correlation of head size with Stanford-Binet IQ, and in fact the WF correlation was even negative (-.04). This suggests that the correlation of IQ with head size (and, by inference, brain size) is a developmental phenomenon, increasing with age during childhood.
One of the unsolved mysteries regarding the relation of brain size to IQ is the seeming paradox that there is a considerable sex difference in brain size (the adult female brain being about 100 cm^3 smaller than the male) without there being a corresponding sex difference in IQ. ^{[13]} It has been argued that some IQ tests have purposely eliminated items that discriminate between the sexes or have balanced-out sex differences in items or subtests. This is not true, however, for many tests such as Raven's matrices, which is almost a pure measure of g, yet shows no consistent or significant sex difference. Also, the differing g loadings of the subscales of the Wechsler Intelligence Test are not correlated with the size of the sex difference on the various subtests. ^{[14]} The correlation between brain size and IQ is virtually the same for both sexes.The explanation for the well-established mean sex difference in brain size is still somewhat uncertain, although one hypothesis has been empirically tested, with positive results. Properly controlling (by regression) the sex difference in body size diminishes, but by no means eliminates, the sex difference in brain size. Three plausible hypotheses have been proposed to explain the sex difference (of about 8 percent) in average brain size between the sexes despite there being no sex difference in g:
- Possible sexual dimorphism in neural circuitry or in overall neural conduction velocity could cause the female brain to process information more efficiently.
- The brain size difference could be due to the one ability factor, independent of g, that unequivocally shows a large sex difference, namely, spatial visualization ability, in which only 25 percent of females exceed the male median. Spatial ability could well depend upon a large number of neurons, and males may have more of these "spatial ability" neurons than females, thereby increasing the volume of the male brain.
- Females have the same amount of functional neural tissue as males but there is greater "packing density" of the neurons in the female brain. While the two previous hypotheses remain purely speculative at present, there is recent direct evidence for a sex difference in the "packing density" of neurons. ^{[15]} In the cortical regions most directly related to cognitive ability, the autopsied brains of adult women possessed, on average, about 11 percent more neurons per unit volume than were found in the brain of adult men. The males and females were virtually equated on Wechsler Full Scale IQ (112.3 and 110.6, respectively). The male brains were about 12.5 percent heavier than the female brains. Hence the greater neuronal packing density in the female brain nearly balances the larger size of the male brain. Of course, further studies based on histological, MRI, and PET techniques will be needed to establish the packing density hypothesis as the definitive explanation for the seeming paradox of the two sexes differing in brain size but not differing in IQ despite a correlation of about +.40 between these variables within each sex group.
[/color]--
The g Factor. pp147-149.
http://www.questia.com/PM.qst?a=o&d=24373874
--
- (a) Andreasen et al., 1993; (b) Egan et al., 1994; (c) Raz Torres, et al., 1993; (d) Wickett at al., 1994; (e) Willerman et al., 1991; (f) Wickett et al., 1996; (g) Schoenemann , 1997.
- Hamilton, 1935.
- Jensen & Johnson, 1994; also see Johnson, 1991, for additional evidence of a within-families correlation between head size and IQ.
- Ankney, 1992.
- Jensen, 1980a, pp. 622-627. This chapter affords a fairly comprehensive review of sex differences in mental abilities and references to much of the literature prior to 1980.
- Wittelson et al., 1995.
[/color]--
The g Factor. p167.
http://www.questia.com/PM.qst?a=o&d=24373874
(References in next message[/color])
[Edit: format adjusted][/color]
-Chris