Ever since being presented with the opportunity to cheat on the StarCon Aptitude Test by copying the answers of a fellow cadet with a remarkably large “brain pan”, I have been curious about the relationship of brain size and intelligence. The traditionally assumed positive correlation has apparently stood the test of time: a major recent study finds that there is a definite, albeit weak, correlation between brain size and intelligence, although one of the authors recommends against “measuring job candidates’ head sizes during the hiring process” (or, presumably, when determining whose answers to copy on aptitude tests):
Summary: Using a large dataset and controlling for a variety of factors, including sex, age, height, socioeconomic status, and genetic ancestry, scientists found that people with larger brains rated higher on measures of intelligence and educational attainment. Size was far from everything, however, explaining only about two percent of the variation in smarts.
The English idiom “highbrow,” derived from a physical description of a skull barely able to contain the brain inside of it, comes from a long-held belief in the existence of a link between brain size and intelligence. …
A new study, the largest of its kind, led by Gideon Nave of the University of Pennsylvania’s Wharton School and Philipp Koellinger of Vrije Universiteit Amsterdam, has clarified the connection. Using MRI-derived information about brain size in connection with cognitive performance test results and educational-attainment measures obtained from more than 13,600 people, the researchers found that, as previous studies have suggested, a positive relationship does exist between brain volume and performance on cognitive tests. But that finding comes with important caveats.
“The effect is there,” says Nave, an assistant professor of marketing at Wharton. “On average, a person with a larger brain will tend to perform better on tests of cognition than one with a smaller brain. But size is only a small part of the picture, explaining about 2 percent of the variability in test performance. For educational attainment the effect was even smaller: an additional ‘cup’ (100 square centimeters) of brain would increase an average person’s years of schooling by less than five months.” Koellinger says “this implies that factors other than this one single factor that has received so much attention across the years account for 98 percent of the other variation in cognitive test performance.”
“Yet, the effect is strong enough that all future studies that will try to unravel the relationships between more fine-grained measures of brain anatomy and cognitive health should control for total brain volume. Thus, we see our study as a small, but important, contribution to better understanding differences in cognitive health.” …
One of the notable findings of the analysis related to differences between male and females. “Just like with height, there is a pretty substantial difference between males and females in brain volume, but this doesn’t translate into a difference in cognitive performance,” Nave says.
A more nuanced look at the brain scans may explain this result. Other studies have reported that in females, the cerebral cortex, the outer layer of the front part of the brain, tends to be thicker than in males.
“This might account for the fact that, despite having relatively smaller brains on average, there is no effective difference in cognitive performance between males and females,” Nave says. “And of course, many other things could be going on.”
The authors underscore that the overarching correlation between brain volume and “braininess” was a weak one; no one should be measuring job candidates’ head sizes during the hiring process, Nave jokes. Indeed, what stands out from the analysis is how little brain volume seems to explain. Factors such as parenting style, education, nutrition, stress, and others are likely major contributors that were not specifically tested in the study.
Here’s a slightly older comprehensive review of the scientific literature on the topic, whose introduction surveys nineteenth and early twentieth century attitudes:
We review the literature on the relation between whole brain size and general mental ability (GMA) both within and between species. Among humans, in 28 samples using brain imaging techniques, the mean brain size/GMA correlation is 0.40 (N = 1,389; p < 10−10); in 59 samples using external head size measures it is 0.20 (N = 63,405; p < 10−10). In 6 samples using the method of correlated vectors to distill g, the general factor of mental ability, the mean r is 0.63. We also describe the brain size/GMA correlations with age, socioeconomic position, sex, and ancestral population groups, which also provide information about brain–behavior relationships. Finally, we examine brain size and mental ability from an evolutionary and behavior genetic perspective. …
Throughout the nineteenth and early twentieth centuries, the relation between whole brain size and GMA was almost universally accepted (Broca, 1861; Darwin, 1871; Morton, 1849; Topinard, 1878). The renowned French neurologist Paul Broca (1824–1880) measured external and internal skull dimensions and weighed wet brains at autopsy and observed that mature adults averaged a larger brain than either children or the very elderly, skilled workers averaged a larger brain than the unskilled, and eminent individuals averaged a larger brain than the less eminent. Charles Darwin (1871) cited Broca’s studies in The Descent of Man to support his theory of evolution:
No one, I presume, doubts that the large size of the brain in man, relatively to his body, in comparison with that of the gorilla or orang, is closely connected with his higher mental powers. We meet the closely analogous facts with insects, in which the cerebral ganglia are of extraordinary dimensions in ants; these ganglia in all the Hymenoptera being many times larger than in the less intelligent orders, such as beetles…
The belief that there exists in man some close relation between the size of the brain and the development of the intellectual faculties is supported by the comparison of the skulls of savage and civilized races, of ancient and modern people, and by analogy of the whole vertebrate series.
I recently encountered a nineteenth / early twentieth century source not cited by the paper, the always fascinating Rav Eliyahu Kalatzkin’s endorsement of the correlation between brain size and intelligence. Due to a curious intersection of the timeless principle that correlation does not imply causation and some classic nineteenth century scientific confusion, however, he rejects the common assumption that brain size affects intelligence, arguing that this is actually an example of reverse causation: the self-evident (to him) nature of an extreme form of Cartesian dualism – “it is inconceivable that the brain, combined of phosphorus and albumen and the like, can engender intelligence and knowledge” – prompts his suggestion that it is actually greater intelligence that causes greater brain size, rather than the reverse:
וההכרח לומר דהא דקאמר שמעת מימיך אומר חמור זה יוצא צינה עליו, הכוונה שאינה מסובבת לו יסורין וחולי, כאשר הרגש הכאב הוא בעצבי ההרגשה המובילין להמוח, ואין שוטה נפגע, וביחוד החמור המצויין בסכלותו,1
ובהבעלי חיים הפקחים כמו הקוף ירבה כמות המוח,
[ויש שאמרו כי גם באנשים ירבה כמות המוח לפי ערך תבונתם, ומזה ישפטו כי המוח יראה פעולתו על כח השכל, ופליאה דעת איך יחולל המוח המורכב מפאספאר ואלבומין וכדומה, את כח השכל והמדע, וקרוב הדבר אשר יחליפו הסיבה להמסובב וכי יגדל וירבה כמות המוח על ידי עבודת העיון והתבונה, והעבודים בעמל ידים ירבה מרוצת הדם בשרירי ידיהם, ומזה יגדלו ויתחזקו שרירי ידיהם, ואנשי מדע אשר יעמלו ברעיוני חכמה והרגשות עדינות יעוררו רגשות עדינות וקלות בהעצבים ומרוצת הדם, וכאשר הקל יצוף למעלה, מקומן בראש ומוח, ורגש תאוה מוחשית מקומה באברים תחתונים, ורגשות זכות המחוללים תנועה זכה בדם ועצבים, מקומן הגבה מעלה, ובעבדות המוח אשר יתעצם שמה מרוצת ותנועת הדם, מזה יגדל וירבה כמות המוח, אם אך לא יחלישו ברוב יגיעה ועבודה שאינו לפי כחו, ובקלקול ופצע במוח יאבד האדם כח תבונתו, אף כי התבונה והשכל הוא ענין רוחני, כאשר יסתמא בקלקול העין, כאשר מוחש ההבטה הרוחנית, תראה פעולתה על ידי העין]
ובהם יוסיף גם הרגש ומכאוב2
R. Kalatzkin acknowledges that damage to the brain will impair the intellect; I do not understand his reconciliation of this fact with his stance of substance dualism.
[Further reading: another review of the scientific literature; some popular articles; Wikipedia discussions.]
- It is unclear where the donkey gets its reputation for foolishness. Livestock.com claims (also without any sourcing): “In general donkeys are quite intelligent, cautious, friendly, playful, and eager to learn. Donkeys have a notorious reputation for stubbornness, but this has been attributed to a very strong sense of self preservation.” [↩]
- אמרי שפר סימן ל”ד אות י”ב [↩]