Human Intelligence

How do you measure brain size?
Answered by Charles W. Bryant and Discovery Channel
  • Charles W. Bryant

    Charles W. Bryant

  • Discovery Channel

    Discovery Channel

  1. Charles W. Bryant Co-Host, Stuff You Should Know
    When it comes to the brain, there are really two ways that researchers measure its physical size -- weight and volume. Weight can give you an indication of the density of the brain's neurons. Scientists turn to volume to measure the size of fossilized brains. Brain imaging techniques can show a neurologist how "big" a brain is in a living human, but it's tough to get a truly accurate measurement unless the brain is outside of the skull.

    If you're looking to measure the level of smarts, or computational power of the brain, then you need to examine the cortical surface area. The cerebral cortex, which is key to things like memory, thought, language and attention, is folded in humans and other larger mammals. The total surface area of the cerebral cortex, folds included, can tell us a lot about the ultimate intelligence of the animal.

    You may also hear scientists mention something called the encephalization quotient when they're talking about brain size. This measurement looks at how big the brain is in proportion to an animal's entire body.

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  2. Measuring the skull cavity gives anthropologists an idea of the brain size of our ancestors. Advanced brain imaging allows scientists and doctors to look at how big brains really are, but it's still hard to measure a brain. Weight, circumference and number of neurons are all factors of brain size, and there remains the question of whether body size should be considered when comparing brains. In addition, different parts of the brain may be of a variety of sizes.

    Scientists have debated for years whether brain size really makes a difference in intelligence. It may matter some between species but size carries little weight, if you will, within a species. The Queensland Institute of Medical Research specifically compared people's genes, intelligence and brain size to determine how reportedly "smart" versions of some genes affect brain size or intelligence. The mutated versions of these genes cause a condition called microcephaly, or an abnormally small brain, and can cause delayed development and mental retardation. In short, the researchers found that people with normal variations in these genes (not the mutation that causes microcephaly) do not have significant differences in brain size or intelligence. In other words, a person with a highly evolved version of the same gene that can cause a small brain if mutated did not have a large brain and higher intelligence than a person with a normal version of the gene [source: ABC News].

    Scientists also recently recorded changes in human brains and bodies. Brain growth was gradual long ago during human evolution and then rapid during a climate change period between 800,000 and 200,000 years ago [source: Smithsonian]. Marta Lahr from the Cambridge University Leverhulme Centre for Human Evolutionary Studies has researched humans evolution and found that about 10,000 years ago, Homo sapiens suddenly became about 10 percent shorter and lost 10 percent of our brain size. A male human brain now has a volume of about 1,350 cubic centimeters (about 82 cubic inches) [source: Leake]. The reason may have to do with humans becoming more farmers than hunters and gatherers. But we are no less intelligent today.

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