In The Descent of Man, Charles Darwin detailed the following paradox:

No one, I presume, doubts the large proportion which the size of man’s brain bears to his body, compared to the same proportion in the gorilla or orang, is closely connected with his higher mental powers. . . . On the other hand, no one supposes that the intellect of any two animals or of any two men can be accurately gauged by the cubic contents of their skulls. (Darwin, 1871, p. 37)

Ignoring Darwin, many have tried to tie individual intelligence to gross brain size. If the functional unit of the brain is the brain cell and if larger human brains have more brain cells, does it not follow that brain size and intelligence are related? It depends.

The evolutionary approach that we have been using to explain how the large human brain evolved is based on comparisons between species. Special care attends the extension of evolutionary principles to physical comparisons within species, especially biological comparisons within or among groups of modern humans. We now illustrate the difficulty of within-species comparisons by considering the complexity of correlating human brain size with intelligence (Deary, 2000). Then we turn to another aspect of studying the brain and behavior in modern humans—the fact that unlike that of other animals, so much modern human behavior is culturally learned.

Over a century ago some investigators promoted a simple conclusion: people with the largest brains display the most intelligent behavior. The late Stephen Jay Gould, in his 1981 book The Mismeasure of Man, reviews much of this early literature and criticizes the research on three counts: brain measurement, correlating brain size and intelligence, and what intelligence is.

First, measuring a person’s brain is difficult. If a tape measure is simply placed around a person’s head, factoring out skull thickness is impossible. There is also no agreement about whether volume or weight is a better measure. And no matter which indicator we use, we must consider body size. The human brain varies in weight from about 1000 grams to more than 2000 grams, but people also vary in body mass. To what extent should we factor in body mass in deciding whether a particular brain is large or small? And how should we measure body mass, given that a person’s total weight can fluctuate widely over time?

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Large differences between the brains of individual people do exist, but the reasons for these differences are numerous and complex. Consider some examples. People may have larger or smaller brain cells. Larger people are likely to have a larger brain than smaller people. Men have a somewhat larger brain than women, but they are proportionately physically larger. Nevertheless, girls mature more quickly than boys, so in adolescence the brain and body size differences may be absent. As people age, they generally lose brain cells, so their brain shrinks.

Neurological diseases associated with aging accelerate the age-related decrease in brain size. Brain injury near the time of birth often results in a dramatic reduction in brain size, even in regions distant from the damage. Stress associated with physical or behavioral deprivation in infancy also reduces brain size (Herringa et al, 2013). Neurological disorders associated with a parent’s abuse of alcohol or other drugs are associated with conditions such as fetal alcohol spectrum disorder (FASD), in which the brain can be greatly reduced in size. Autism spectrum disorder (ASD), a largely genetic condition affecting development, produces a wide variety of brain abnormalities, including either increases or decreases in brain size in different individuals.

Brain size may also increase in individuals. For example, just as good nutrition in the early years of life can be associated with larger body size, good nutrition can also be associated with an increase in brain size. The brain’s plasticity—its ability to change—in response to an enriched environment is associated with growth of existing brain cells and thus an increase in brain size. Furthermore, one way in which the brain stores new skills and memories is to form new connections among brain cells, and these connections in turn contribute to increased brain size.

Finally, we must also consider what is meant by intelligence. When we compare behavior across species, we are comparing species-typical behavior—behavior displayed by all members of a species. For example, lamprey eels do not have limbs and cannot walk, whereas salamanders do have limbs and can walk: the difference in brain size between the two species can be correlated with this trait. When we compare behavior within a species, however, we are usually comparing how well one individual performs a certain task in relation to others—how well one salamander walks relative to how well another salamander walks, for example.

We can make intraspecies comparisons for humans, but this likewise presents problems. For one thing, individual performance on a task is influenced by many factors unrelated to inherent ability, among them opportunity, interest level, training, motivation, and health. For another, people vary enormously in their individual abilities, depending on the particular task. One person may have superior verbal skills but mediocre spatial abilities; another person may be adept at solving spatial puzzles but struggle with written work; still another may excel at mathematical reasoning and be average in everything else. Which of these people should we consider the most intelligent? Should certain skills carry greater weight as measures of intelligence? Clearly, it is difficult to say.

Early in the twentieth century, Charles Spearman carried out the first formal performance analysis among various tests used to rate intelligence. He found a positive correlation among tests and suggested that a single common factor explained them. Spearman named it g for general intelligence factor, but it turns out that g also varies. Many factors unrelated to inherent ability—among them opportunity, interest level, training, motivation, and health—influence individual performance on a task.

For example, when IQ tests that were given to young adults of one generation are given to the next generation, scores increase by as much as 25 points, a phenomenon called the Flynn effect (Flynn, 2012). Taken at face value—though it shouldn’t be—the increase suggests that human g has risen to such a degree in two generations that most young adults fall in the superior category relative to their grandparents. Obviously, the score change has not been accompanied by a similar increase in brain size. It is more likely that education and other life experiences explain the Flynn effect.

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Howard Gardner (2006), furthermore, proposes that humans have a number of intelligences—verbal, musical, mathematical, social, and so on. Each type of intelligence is dependent on the function of a particular brain region or regions. Hampshire and colleagues (2012), who presented participants with a battery of typical intelligence assessment tests, support Gardner’s idea. As participants took the tests, their brain activity was imaged and recorded. The study identified three separate abilities—reasoning, short-term memory, and verbal ability—each associated with a different brain network. The experimenters argue that this finding provides little support for Spearman’s g. They further suggest that a wider array of assessments would reveal additional intelligence networks.

Given the difficulty in measuring brain size and in defining intelligence, it is not surprising that scant research appears in the contemporary literature on the problem of gross brain size and intelligence. If you are wondering whether having a larger brain might mean you could study a little less, consider this. The brains of people who are widely considered highly intelligent have been found to vary in size from the low end to the high end of the range for our species. The brain of the brilliant physicist Albert Einstein was average in size.

In evolutionary terms, the modern human brain developed rapidly. Many behavioral changes differentiate us from our primate ancestors, and these adaptations took place more rapidly still, long after the modern brain had evolved. The most remarkable thing that our brains have made possible is ever more complex culture—learned behaviors passed from generation to generation through teaching and experience.

Cultural growth and adaptation render many contemporary human behaviors distinctly different from those of Homo sapiens living 200,000 years ago. Only 30,000 years ago, modern humans made the first artistic relics: elaborate paintings on cave walls and carved ivory and stone figurines. Agriculture appears still more recently, about 15,000 years ago, and reading and writing were invented only about 7000 years ago.

Most forms of mathematics and many of our skills in using mechanical and digital devices have still more recent origins. Early H. sapiens brains certainly did not evolve to select smart phone apps or imagine traveling to distant planets. Apparently, the things that the human brain did evolve to do contained the elements necessary for adapting to more sophisticated skills.

Alex Mesoudi and his colleagues (2006) suggest that cultural elements, ideas, behaviors, or styles that spread from person to person—called memes (after genes, the elements of physical evolution)—can also be studied within an evolutionary framework. They propose that individual differences in brain structure may favor the development of certain memes. Once developed, memes would in turn exert selective pressure on further brain development. For example, chance variations in individuals’ brain structure may have favored tool use in some individuals. Tool use proved so beneficial that toolmaking itself exerted selective pressure on a population to favor individuals well skilled in tool fabrication.

Similar arguments can be made with respect to other memes, from language to music, from mathematics to art. Mesoudi’s reasoning supports neuroscience’s ongoing expansion into seemingly disparate disciplines, including linguistics, the arts, business, and economics. Studying the human brain, far from examining a body organ’s structure, means investigating how it acquires culture and fosters adaptation as the world changes and as the brain changes the world.

Modern Human Brain Size and Intelligence

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Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Answers appear in the Self Test section of the book.