Now published in the Canadian Journal of Experimental Psychology (Vol. 49, No. 4)

Does Brain size matter?

A Reply to Rushton and Ankney

MICHAEL PETERS

Abstract

 Rushton and Ankney (1995) show that new Magnetic Resonance Imaging mri studies allow the statement that brain size and IQ are correlated. However, the evidence does not justify statements about the relation of brain size and IQ across race and sex. Rushto n & Ankney's comparisons of brain size across races are based on brain weights corrected for body weights, but the corrections are not justifiable for within-species comparisons. Large variations of brain size within race groupings (as defined by Rushton & Ankney), and large secular changes in brain size add to doubts about the validity of Rushton & Ankney's arguments about race/brain size/IQ relations. Rushton & Ankney suggest that sex differences in brain size relate to sex differences in IQ and spatial abilities; however, the sex differences in IQ do not stand in proportion to differences in brain size, and spatial performance in women does not correlate with brain size. Scaling brain size across sexes remains an unsolved problem.

"nutritional and environmental conditions which foster good development of intelligence can be expected to foster good physical brain development as well. "

"the relation between brain size and IQ is confounded by nutritional and environmental conditions"
 

Rushton and Ankney (1995) suggest that I was in error when rejecting the claim (Lynn, 1993) that a relation between brain size and intelligence is firmly established. Since my note went to press (Peters, 1993), several papers have appeared which indicate a sizeable relation between brain size, as determined by mri scan, and IQ (Andreasen, Flaum, Swayze, O'Leary, Alliger, Cohen, Ehrhardtm, & Yuhet, 1993; Egan, Chiswick, Santosh, Naidu, Rimmington, & Bestet, 1994; Raz, Torres, Spencer, Millman, Baertschi, & Sarpel, 1993; Wickett, Vernon, & Lee, 1994). Each of the above studies has some problems posed by method and interpretations or by the findings themselves. For example, Egan et al. found the highest correlation not between brain matter and IQ, but between cerebrospinal fluid volume and IQ (r = .8), an inexplicable result for those who argue that it is brain matter which is correlated with IQ. Nevertheless, as a group, these studies justify the conclusion that there is a positive and sizable correlation between brain size and IQ. This lends credibility to earlier claims by Willerman, Schultz, Rutledge, & Bigler, 1991; 1992) and supports Rushton & Ankney's critIQue. The new mri data are important because studies thatrelate IQ to brain size as estimated through cranial parameters remain contradictory. For example, Reed and Jensen (1993) report an equivalent cranial capacity of 1550 cm3 for a high IQ group (124-136) and 1549 cm3 for a low IQ group (87-111), whereas studies listed by Wickett et al. (1994) report positive correlations between cranial capacity and IQ.


Accepting the fact that MRI data document a significant correlation between brain size and intelligence within demographically homogeneous groups, the question of causality arises. Neural systems, by definition, have evolved to interact with the environment, and the very significant expansion of brain size after birth, driven by a growth of synapses and cortical interconnections, is interactive with environmental input (Bedi, Massey, & Smart, 1989; Jacobsen, 1991, pp. 266-270; Walsh, 1981). Thus, nutritional and environmental conditions which foster good development of intelligence can be expected to foster good physical brain development as well. My seemingly nonsensical position, that under some conditions it is useful to control for body size when look ing at brain/IQ relations, is based on the possibility that the relation between brain size and IQ is confounded by nutritional and environmental conditions (cf. Passingham, 1979; Rodriguez, Donnadien, Martinez, & Chavez, 1979). This does not change the basic observations, but it does change arguments about causation.
 
 

Rushton & Ankney reiterate the conclusion, already reached by Gould (1981, p. 106), that women have absolutely much smaller, and relatively somewhat smaller brains than men. I continue to disagree with Rushton & Ankney on the issue of scaling. The statement that allometric techniques standard in comparative biology have been used does not assure that these are appropriately used for between sex comparisons. In the available brain size/IQ studies, the differences in IQ are small or non-existent (Passingham , 1979; Raz et al., 1993; Willerman et al., 1991), in spite of very large differences in brain size. Three interpretations offer themselves: (a) women do have lower IQ's than men after all (Lynn, 1994), (b) the gross brain size in women is not a meaningful index for comparison because of difficulties in scaling body/brain parameters across sexes and possible differences in fine structure (Peters, 1991; Willerman et al., 1991), and (c) women and men differ especially on specific spatial tasks (Rushton & An kney, 1995). Because most of the mri evidence is cast in terms of standard IQ test/brain size relations, it is interesting to note that the sex differences in Wechsler IQ which are summarized by Lynn are very small relative to the sex differences in brain size. This supports interpretation (b). Rushton & Ankney's point (c) is difficult to evaluate at this point. Rushton & Ankney emphasize spatial ability, and point out that there are significant sex differences in 3-d spatial abilities, especially mental rotation performance. However, Wickett et al. find no significant correlation between brain size and mental rotation performance in their sample of women, and considerably lower correlations between Wechsler performance IQ and brain size than for Wechsler verbal IQ and brain size. Caution is advised in attributing spatial performance to brain size or neuron counts because such performance is very sensitive to practice; women can improve their mental rotation performance by 30% to 50 % (Peters, Chisholm, & Laeng, 1995) after only a single exposure to the test. Such rapid improvement is difficult to reconcile with the idea that brain size, within normal limits, is the limiting factor in 3-d spatial performance.
 
 

Rushton and Ankney (1995) classify races into three groups, and this has been a source of contention (e.g., Weizmann et al., 1990). The issue of race classification is a major problem in anthropology (Harrison, Tanner, Pilbeam, & Baker, 1988; p. 326) and cannot be addressed here. Rushton & Ankney have adopted an essentially operational definition that shares the strengths and weaknesses of such definitions. To simplify things, I shall adopt the terms Caucasoid, Mongoloid, and Negroid when referring to the data in Rushton's work.

Much of Rushton and Ankney's (1995) case on racial differences in brain size is based on estimates derived from cranial measures. Such estimates are not without problems (Hoadley & Pearson, 1929; Wickett et al., 1994, Willerman et al., 1991), and may have different validity for men and women. As an illustration, Willerman et al. (1992), found a significant correlation between brain size and head perimeter for women, but not for men. Nevertheless, let us assume that cranial measures do provide an imperfect but acceptable estimate of brain size. Rushton & Ankney encounter several problems in the attempt to generalize the mri/brain size/IQ data to the general context of racial comparisons. One of the problems lies in the simplified grouping of races, because this tends to give insufficient weight to within- group variations.

Rushton's (1991) own data illustrate the point. Within the Caucasoid grouping, a 1969 sample of Iranian soldiers is given an average estimated cranial capacity of 1356 cm3, whereas a 1966 sample of American Army soldiers has an average of 1470 cm3. Similarly, a 1967 us Air Force sample has 1539 cm3, whereas a 1975 German Air Force sample has 1455 cm3. Within the Mongoloid grouping, a 1963 sample of Thai soldiers has an estimated average of 1340 cm3, whereas a 1965 sample of South Vietnamese has 1299 cm3. All of these differences are much larger than the differences obtained between Negroid (1449 cm3), Caucasoid (1468 cm3), and Mongol- oid (1464 cm3) enlisted men from a 1988 us Army cohort using Rushton's (1992) terms and data. There are also secular changes within a culture and racial group (Haug, 1984; Miller & Corsellis, 1977). For example, us Air Force personnel measured in 1967 had estimated brain sizes that exceeded by 68 cm3 values from a us Air Force sample drawn in 1950 (Rushton, 1991). In face o f such variation within groupings, between cultures for similar groupings, and between different cohorts drawn within a culture, generally valid statements about race differences are difficult to make.

Two additional points need to be made. Rushton and Ankney (1995) suggest that cranial capacity estimates for Mongoloid-, Caucasoid-, and Negroid-Americans are 1416, 1380, and 1359 cm3, respectively, indicating larger differences than Rushton's (1992) values given above for these groups (1464, 1468, 1449cm3). The former values for the three groups represent cranial capacity estimates which are based on values corrected for body parameters (Rushton, 1992). To perform this correction, Rushton used slopes for the log/log plot of brain against body weight which are not appropriate for within- species comparisons (Harvey, 1988). For comparison of individuals drawn from the same species, a slope which is almost horizontal is appropriate, and should be close to the .08 determined empirically by Reed and Jensen (1993). This is borne out by other available evidence. Wickett et al. (1994) state that for their sample of white women, it would appear that the size of the brain is largely independent of body size (p. 836). Similarly, Jerison (1979) found no significant association between body weight or height and brain weight for men within the age range of 29 to 41 years of age. A conservative conclusion is that there is no legitimate reason for using steep slopes in comparing brain/body size relations across races. As a result, statements about brain size differences between races should not rely on adjusted values, and it is not appropriate to conclude that higher IQ's in Asians are linked to larger brain size.

The issue of race/brain size/IQ invites a return to the sex/brain size/IQ issue. Rushton's (1992) data show that the estimated cranial capacity of Negroid-American men is some 13-14% higher than that of Caucasoid-American women, even though the average I Q for the former is presumably lower. How can this be integrated into a model of larger brain => higher IQ without qualifying the meaning of brain size comparisons across sexes, or revisiting the issue of what factors other than brain size have a bearing on IQ? This question once again emphasizes the unresolved issues of how brain weight/ body parameters can be compared across sexes, races, and age cohorts.

Finally, the small absolute differences in brain size between Mongoloids, Caucasoids, and Negroids in Rushton's (1992) data base should be evaluated relative to cohort data. We have seen that values from two Air Force samples drawn 17 years apart showed estimated brain size differences that are larger than the differences reported between races in the 1988 common age cohort. It is legitimate to ask whether the contemporary Negroid- and Caucasoid-American samples described in the common 1988 age cohort could not differ as much from each other in undefined demographic and nutritional variables as the cohort samples from 1950 and 1967 differed from each other.

I am not going to address the issue of racial differences in IQ relative to brain size, because this cannot be resolved here. Clearly, Rushton & Ankney feel that sufficient evidence is available to make their point on racial differences in IQ. The issue is not whether such differences can be observed; they are observed and they are marked and important for a number of reasons. What to make of them is another matter. If cohort differences across time and culture complicate interpretation of brain size dif ferences across races, the additional uncontrolled effects of community variables in the determination of IQ (Church & Katigbak, 1991; Coon, Carey & Fulker, 1992; Wachs, Moussa, Bishry, Yunis, Sobhy, McCabe, Jerome, Galal, Harrison, & Kirksley, 1993) across races render conclusive statements about racial IQ differences even more difficult, if not impossible at present.

Send correspondence to:
M. Peters, Ph.D
Deptartment of Psychology,
University of Guelph,
Guelph, Ontario
N1G 2W1
CANADA

 E-mail: mpeters@uoguelph.ca.

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Canadian Journal of Experimental Psychology
Copyright Canadian Psychological Association

Date of acceptance: December 30, 1994
 
 


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