Autopsies on prominent humans have enabled the investigation of human intelligence relative to brain size. Though some highly intelligent people were found to have possessed significantly larger than average brains, there were many who did not. Gauss, regarded by historians of mathematics as one of the three greatest mathematicians of all time, weighed in at what has been called an “embarrassing” 1492 grams–slightly greater than an average-sized brain.*
Einstein’s brain, part of which resides at McMaster University in Hamilton, Ontario, tipped the scales at 1230 g, which is below average.** However, subsequent studies have revealed that the brains of both Gauss and Einstein are much more convoluted in structure than average. Einstein’s brain was found to have a higher density of neurons than normal. Some cognitive scientists suggest that high cognitive ability may be associated with a density of connections rather than total brain mass.
With the development of brain scan technology, it became possible to measure actual brain size in humans in vivo, i.e., before shrinkage through age or disease. Studies involving the MRI (Magnetic Resonance Imaging) techniques indicate that there is only a moderate correlation between brain size and intelligence (as measured by IQ tests). The 2003 publication The Scientific Study of General Intelligence asserts:***
The large number of MR [magnetic resonance] studies replicated multiple times by independent groups has unequivocally confirmed a relationship between brain volume and higher IQ scores for normal men and women. The value of this correlation hovers near r = 0.35.
This suggests that other factors such as, neuron density or convolution, must account for most of the difference in intelligence among humans. For the correlation between forehead size and intelligence, visit: Does forehead size say anything about intelligence? – Intelligence and IQ
The relationship between intelligence and brain size has meaning only within a particular species. For example, the African elephant brain contains about 260,000,000,000 (a quarter of a trillion) neurons while the human brain, consists of 90,000,000,000 (90 billion) neurons, or about 35% of the number in the average elephant brain. Yet, we humans are believed to have cognitive function that exceeds that of the elephants. Although we do not know the reason for that difference, some scientists have proposed that it’s not the total mass of the neurons in the brain, but the densities and locations of the neurons.
About, 98% of the neurons in the elephant brain are located in the cerebellum where they are involved in functions such as balance, movement and muscle control. The elephant cerebral cortex, which has twice the mass of the human cerebral cortex, holds only 5.6 billion neurons, which is about one third of the number of neurons found in the human cerebral cortex. It has also been conjectured that the ratio of brain size to body size in comparing species may be strongly correlated to intelligence. However, checking the validity of these conjectures will require a clearer definition of intelligence as it relates to different species and greater knowledge of the contribution to intelligence from the different regions of the brain. In order to determine why we humans have such large brains relative to our body size, we will need more information, or larger brains.
*. Reference to Gauss’s brain size found in Gould, Stephen Jay. 1981, 1996. The Mismeasure of Man. New York: W. W. Norton and Company. p. 125.
**. Witelson, Sandra F., Debra L Kigar, and Thomas Harvey. “The Exceptional Brain of Albert Einstein.” Lancet, 1999, Vol. 353, Hamilton, ON: Department of Psychiatry and Behavioural Neurosciences, McMaster University. pp. 2149–53. Accessible at:http://www.bic.mni.mcgill.ca/users/elise/Alberts_brain.pdf
***. Gignac, Gilles, Philip A. Vernon, and John C. Wickett. 2003.
The Scientific Study of General Intelligence: Tribute to Arthur Jensen Chapter 6: Factors Influencing the Relationship Between Brain Size and Intelligence. Amsterdam: Elsevier
