Note from:  

P. Tom Schoenemann
Department of Anthropology
University of California, Berkeley
(schoenem@qal.berkeley.edu)

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.