INTELLIGENCE 15, 223-228 (1991)

In Vivo Brain Size and Intelligence LEE WILLERMAN

ROBERT SCHULTZ

University of Texas at Austin

J. NEAL RUTLEDGE

University of Texas at Austin Austin Radiological Association

ER1N D. BIGLER University of Texas at Austin Austin Neurological Clinic

It is widely believed that human brain size and intelligence are only weakly related to each other. Using magnetic resonance imaging, we show that larger brain size (corrected for body size) is associated with higher IQ in 40 college students equally divided by high versus average IQ, and by sex. These results suggest that differences in human brain size are relevant to explaining differences in intelligence test performance.

Studies have shown positive but modest relations between external head size and psychometric intelligence (e.g., Van Valen, 1974, for a review of earlier re- search; Susanne, 1979). An assumption in much of that work was that head and brain size were fungible, and if head size were not powerfully predictive of intelligence, brain size would not do much better (Gould, 1981). Large between- person variation in average skull thickness compromises extrapolations of exter- nal cranial measurements to predict cranial capacity, however (Rogers, 1984).

The evolution of brain size, sex differences, and intelligence have been classic questions (Jerison, 1973). Postmortem evidence brought to bear in the 19th and early 20th centuries was deficient in sampling and methodology: Sex, age, body size, brain-fixation procedures, weighing before or after fixation, inclusion or exclusion of meninges, and cause of death were often inadequately controlled. Elderly personages with small brains were said to illustrate the irrelevance of brain size in high achievement, and large-brained personages were thought to

Erin D. Bigler is now at the Department of Psychology, Brigham Young University. We thank N. Gopal and P. Watson of the University of Texas Advanced Graphics Laboratory for

computer programing, and J.C. Loehlin, T. Schallert, and W. Wilczynski for advice and encouragement.

Correspondence and requests for reprints should be sent to Lee Willerman, Department of Psychology, University of Texas at Austin, Austin, TX 78712.

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224 WILLERMAN, SCHULTZ, RUTLEDGE, AND BIGLER

champion the opposing view. However, brain size at the height of intellectual power would have been a better index than aged brain size postmortem. Relating intelligence to intracranial volume rather than to postmortem brain size might have finessed some of these problems, but no such study has been done. The advent of in vivo brain imaging would now seem to have rendered studies of head size and intelligence obsolete. Brain size in living younger subjects can be measured reliably, and statistical controls for body size and socioeconomic back- ground can be applied.

Given the limitations of previous work, this study was designed specifically to examine brain morphometric correlates of intelligence in an ostensibly healthy college-age sample using magnetic resonance imaging (MRI).

METHOD

Subjects The final sample consisted of 40 right-handed Anglo introductory psychology students (M = 18.9 years, SD = 0.6) who had indicated no history of alco- holism, unconsciousness, brain damage, epilepsy, or heart disease. These sub- jects were drawn from a larger pool of introductory psychology students with total Scholastic Aptitude Test scores of --> 1350 or

BRAIN SIZE AND INTELLIGENCE 225

within a slice was r = .99 (n = 10). The computer then counted all pixels with nonzero gray scale values for brain size in each slice, their summed value serving as the index of overall brain size. We report here only on total brain size; a future report will focus on cortex versus subcortex volumes.

RESULTS

Based on scores on a iaterality questionnaire (Oldfield, 1971), the high-lQ group was more right-lateralized (p < .05), but this difference did not interact with any other results. Average-IQ men were taller than high-IQ men (p < .05). Mid- parent years of completed education for the two IQ groups did not differ signifi- cantly: high IQ = 16.95 years, SD = 1.79 and average IQ = 16.0 years, SD = 1.95, p >. 10. Controlling for midparent education had no effect on the results to be reported.

Brain tissue was present on only 17 of 18 slices for 12 of the subjects (9 women). After controlling for sex, correlations of height and weight with brain size were only r = .09 and r = . 10. Body size, nevertheless, was partialled out because studies using larger samples have shown moderate brain-size-body-size correlations (Dekaban & Sadowsky, 1978; Ho, Roessman, Straumfjord, & Munroe, 1980; Holloway, 1980), and brain size adjusted for body size is believed to approximate more closely the proportion of brain devoted to intellectual func- tion (Jerison, 1973).

Analysis of covariance (ANCOVA) contrasted the two dichotomous variables: high versus average 1Q and male versus female, controlling for body size (height and weight). Results showed that the high-lQ group had greater brain size, F(1, 34) = 6.6, rph = .40, p > .05, as did the men, F(1, 34) = 4.7, r~, h = .35, p < .05, with no interaction between IQ and sex. An analysis adjusting for height and weight separately by sex yielded essentially the same results. An analysis that excluded a single outlier with larger body size did not affect the brain-size-IQ relation, but reduced the sex difference to marginal significance (p = .08); thus, the sex difference in adjusted brain size must be considered tentative.

A stepwise multiple regression with the dichotomous IQ classification entered first and the actual IQ scores entered second, tested whether IQ scores contrib- uted to the prediction of adjusted brain size within the two IQ subgroups. Results showed a significant contribution of residualized IQ scores (partial r = .41, p < .05) after controlling for IQ classification, suggesting generalizability to a con- tinuous IQ distribution. Correlations of brain size with IQ scores were higher among men than women, although not significantly so. Among men, 1Q scores as a continuous variable correlated with brain size, before and after adjusting for body size, r = .51 (p < .05) and r = .65 (p < .01). Corresponding correlations for women were r = .33 and .35, both n.s. With sexes pooled, the IQ-adjusted brain-size correlation was r = .51 (p < .01). This correlation is higher than

226 WILLERMAN, SCHULTZ, RUTLEDGE, AND BIGLER

expected for the general population because of selection of extreme IQ groups. Applying a statistical correction (Guilford & Fruchter, 1973) predicted a correla- tion of r = .35 for a more representative sample.

Not all brain levels contributed equally to the brain-size-IQ correlation (Fig- ure 1); size differences were greatest for ventricular-level slices in men. These levels include language circuits, association fibers, and association cortex (Dea- con, 1988), but more direct anatomic comparison in specific regions of interest is required for substantiation.

The axial image approximating the glabella-opisthocranion plane was used to estimate head perimeter. Head-perimeter-IQ correlations in men (r = . 17) and women (r = .31) were not significant, nor was the unadjusted brain-size-head- perimeter correlation in men (r = .24). However, the corresponding correlation was significant in women (r = .68, p < .01). The perimeter-lQ correlations are consistent with previous reports of such relationships (e.g., Van Valen, 1974), and more variability in skull thickness in men could account for the sex dif- ference in the head-perimeter-brain-size correlation (Rogers, 1984).

1.0 n~

0 .) 01

0.5

Z

o.o n

N

-0.5 Z

m

-I.0 - I0

I I I I I I I I

-8 -6 -4 -2 0 2 4 6

SLICE LEVEL

FIG. 1. Brain area in standard scores (M = 0 +- SD = l ) by high versus average IQ and sex, adjusted for height and weight. Squares refer to men and circles to women; empty symbols refer to high IQ, filled symbols to average IQ. Slice 0 is at the midventricle level as shown in the insert; 5 mm thick MR1 images are separated by 2.5 ram. Large brain-area-IQ differences in men near ventricles (slices -3 below to + 1 above 0) include neural substrates of language and association. A repeated-measures ANOVA on ventricle size across slices -3 to + 1 reveals no effect of sex, IQ, or their interaction (all ps > .20) so ventricles are included in area measurements. Tissue above or below graphed levels are excluded because all subjects could not be represented.

BRAIN SIZE AND INTELLIGENCE 227

DISCUSSION

A previous study of brain size and IQ (Yeo, Turkheimer, Raz, & Bigler, 1987) obtained a correlation of r = .07 with sexes pooled, using eight or nine con- tiguous computerized tomography (CT) slices maximally encompassing about 53% of the brain. The patients had medically unconfirmable neurologic symp- toms and many had elevated psychopathology scores. A correlation of r = .57 (p < .01) obtained for relative hemisphere size and superiority of verbal or nonver- bal IQ within subjects is difficult to reconcile with the low between-subjects correlation for brain size and IQ. A recent CT study found an eta correlation of .35 (p < .05) between area of a single ventricular-level slice and occupational class in normal adults, but the sample's racial heterogeneity makes interpretation ambiguous (Pearlson et al., 1989). Corresponding results were obtained for Frontal Lobe Size Educational Achievement (r = .31) in an MR1 study (Andreason et al., 1990). Neither of the last two studies controlled for sex, and it is, thus, not feasible to examine possible sex differences in the correlations.

Although the sex difference in the adjusted brain-size-IQ correlation was not significant here, its absolute magnitude (r = .35 vs. r = .65) warrants mention of possible sex differences in brain organization. Aphasia in women is com- paratively less frequent following focal left hemisphere damage, except in one region where it is more likely (Kimura, 1987). There is also some indication that men and women have the same number of cortical neurons despite differences in overall brain size (Haug, 1987). That the sexes may package similar processing capacities differently implies alternative routes to achieving similar adaptive goals.

It is possible that unknown environmental factors common to both brain size and IQ could be responsible for their correlation. One candidate is nutritional variation, but that seems improbable because the middle-class background of our subjects makes prenatal or postnatal undernutrition unlikely. Future research, however, should be directed to establishing whether the brain-size-IQ effect occurs within, as well as between, families (Jensen & Sinha, in press).

Brain size is correlated with cortical surface area (Haug, 1987) so that larger size might reflect more cortical columns available for analyzing high-noise or low-redundancy signals, thus enabling more efficient information processing pertinent to IQ test performance (Raz, Willerman, & Yama, 1987). Embryologic factors may be important because the panoply of stem cells giving rise to cortical neurons is fully present by the 40th day after fertilization, and the full comple- ment of cortical neurons is present before birth (Rakic, 1988; Williams & Her- rup, 1988). Glial cells contribute to postnatal brain growth, but their influence on intelligence is unknown. Attractive hypotheses to explain larger cortices are a greater number of stem cells, an increased number of mitotic divisions producing more descendant neurons, or different rates of neuronal death.

228 WILLERMAN, SCHULTZ, RUTLEDGE, AND BIGLER

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