Note from: P. Tom Schoenemann Department of Anthropology University of California, Berkeley (email@example.com) Jerison (1973) lists the following "encephalization quotients" (which take account of the relationship between body size and brain size across vertebrates) for chimps, gorillas, and orangutans: Chimp (male): 2.48 (56.7 kg body weight, 440 g brain weight) Chimp (female): 2.17 (44.0 kg body weight, 325 g brain weight) Gorilla (male): 1.53 (172.4 kg body weight, 570 g brain weight) Gorilla (female): 1.76 (90.7 kg body weight, 426 g brain weight) Human (male): 7.79 (55.5 kg body weight, 1361 g brain weight) Human (female): 7.39 (51.5 kg body weight, 1228 g brain weight) These are individual specimens, but they are representative of what I have seen in other studies. Clearly, body size does not come close to explaining the huge increase in brain size in hominids, and it isn't even necessary to calculate EQ's to see this. With respect to the question of brain size and intelligence, the most recent review I know of (there have been others) concerning the correlation between IQ and head size looked at 25 separate studies (going back to the turn of the century), comprising 39 independent normal samples (total N = 51,931; Wickett, et al. in press). They report that most correlations range between r = .10 to r = .30, with an n-weighted mean of r = .194. This is highly statistically significant, though head dimensions clearly do not explain very much of the variation in IQ. More interestingly, 4 recent studies of this question for the first time derived estimates of brain size from high quality magnetic resonance imaging (MRI), instead of using external cranial dimensions. All 4 studies show much higher correlations: Willerman et al. (1991) report an estimated correlation of r = .35 (N = 40); Andreasen et al. (1993) found a correlation of r= .38 (N = 67); Raz et al (in press) found a correlation of r = .43 (N = 29); and Wickett et al. (in press) report a correlation of r = .395 (N = 40, all females). These are all statistically significant. It is quite simply a myth that brain size and IQ are empirically unrelated in modern populations. It is also a myth that brain size has never been shown to relate to behavioral differences in any non-human species. Hamilton showed way back in 1935 that rats selected for 12 generations to be either "maze-bright" or "maze-dull" differed by about 2.5 standard deviations in brain weight. Within unselected control rats there was a correlation of r = .25 between maze ability and brain weight. Recently Anderson (1993) reported data on rats in which several cognitive tasks were given and a general factor extracted, and brain weights were obtained. The correlation between this general factor and brain weights in these rats was r = .48. All the way back in 1974 Van Valen pointed out that if brain size and intelligence were truely functionally related, then the correlations between IQ (an imperfect measure of "intelligence") and head dimensions (an imperfect measure of brain size) would necessarily be attenuated. The fact that the MRI derived correlations are approximately double the average derived from external cranial dimensions is strong support for this view. The next logical step will be to see if other dimensions of cognitive ability besides IQ show higher correlations with MRI derived brain size measures. However, Van Valen (1974) also showed that a correlation of r = .30 would have been big enough to account for the rapid evolution of brain size assuming selection for intelligence (or whatever behavioral variable correlated this highly with brain size). REFERENCES: Anderson, Britt,1993, "Evidence from the rat for a general factor that underlies cognitive performance and that relates to brain size: Intelligence?" NEUROSCIENCE LETTERS v.153: 98-102. Andreason, Nancy C., Michael Flaum, Victor II Swayze, Daniel s. O'Leary, Randall Alliger, Gregg Cohen, James Ehrhardt, and William T.C. Yuh,1993, "Intelligence and brain structure in normal individuals," AMERICAN JOURNAL OF PSYCHIATRY v.150: 130-134. Hamilton, J. A. 1935, THE ASSOCIATION BETWEEN BRAIN SIZE AND MAZE ABILITY IN THE WHITE RAT, Dissertation, University of California at Berkeley. Jerison, H. J., 1973, EVOLUTION OF THE BRAIN AND INTELLIGENCE. New York: Academic Press. Raz, N., Torres, I. J., Spencer, W. D., Millman, D., Bertschi, J. C., Sarpel, G., in press, "Neuroanatomical corelates of age-sensitive and age-invariant cognitive abilities: An _in vivo_ MRI investigation," INTELLIGENCE. Van Valen, Leigh.,1974, "Brain size and intelligence in man," AM. J. PHYS. ANTHROP. 40: 417-424. Wickett, John C., Philip A. Vernon, and Donald H. Lee., in press, "_In vivo_ brain size, head perimeter, and intelligence in a sample of healthy adult females," PERSONALITY AND INDIVIDUAL DIFFERENCES. Willerman, Lee, Robert Schultz, J. Neal Rutledge, and Erin D. Bigler, 1991, "_In vivo_ brain size and intelligence," INTELLIGENCE v.15: 223-228.