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JAMA 100 Years Ago
August 18, 1999

BRAIN WEIGHT AS INDEX OF INTELLIGENCE.

Author Affiliations
 

Edited by Jennifer Reiling, Editorial Assistant.

JAMA. 1999;282(7):616F. doi:10.1001/jama.282.7.616

Opinions regarding the weight of the brain being an index of intelligence have fluctuated within wide limits. Before the days of cerebral localization there were many physiologists who believed that an individual's intelligence was directly proportionate to the total weight of the encephalon. A large brain meant large mentality. As the encephalon began to be more closely investigated and the post-mortem records were more carefully compared, it was discovered that large brains were not always possessed by those who exhibited the highest intelligence. In fact some of the greatest thinkers in the world were the owners of brains much below the average in absolute weight and size. This apparently upset the earlier physiologic dicta in regard to mind activity being dependent on the physical brain. The discussion at once assumed two aspects. Some physiologists frankly declared that mind was not solely a product of brain function, but that it was an entity dependent for its manifestation on the brain, but nevertheless quite separate and distinct from the latter. Scalpel and alembic might analyze the brain, but they could do nothing to explain the mind. These views characterized the metaphysical physiologists. The materialistic physiologists, on the other hand, took the ground that the mind was solely and in toto a product of brain activity, and that accordingly it must follow that a large mind necessarily presupposes a large brain. In attempting to harmonize their views with the apparently contradictory observation that many of the most intelligent men were the possessors of undersized brains, these physiologists were again divided into two camps, as it were; some taking the ground that the cellular richness of the cortex, including its convolutional development, was the real indicator of intelligence irrespective of the total brain weight, while others maintained that the total brain weight was the potential indicator though not necessarily the actual one. The latter insisted that a large brain represented large capacity for mentalization, and that the brain, like the muscles, was capable of development, and the greater that development so much greater will be the resulting mentalization. Its potentiality, according to these physiologists, rather than its manifested activity is what is shown in its total weight.

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