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Does Brain Size Matter?

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Does Brain Size Matter? CONSCIOUSNESS REDUX NEUROANATOMY Does Brain Size Matter? A recent discovery proves embarrassing to any notion of humanity’s innate superiority While “size does not matter” is a uni- versally preached dictum among the politically correct, everyday experience tells us that this can’t be the whole sto- ry—under many conditions, it clearly does. Consider the size of Woody Allen’s second favorite organ, the brain. Adjectives such as “highbrow” and “lowbrow” have their origin in the belief, much expounded by 19th-century phre- nologists, of a close correspondence be- tween a high forehead—that is, a big brain—and intelligence. Is this true? Does a bigger brain make you necessari- ly smarter or wiser? And is there any sim- to fourth decade of life. An MRI study whereas writer Anatole France’s brain ple connection between the size of a ner- of 46 adults of mainly European descent could barely bring half of that weight on vous system, however measured, and the found that the average male had a brain the scale at 1,017 grams. (Note that post- mental powers of the owner of this ner- volume of 1,274 cubic centimeters (cm3) mortem measures are not directly com- vous system? While the answer to the for- and that the average female brain mea- parable to data obtained from living mer question is a conditional “yes, some- sured 1,131 cm3. Given that a quart of brains.) In other words, gross brain size what,” the lack of any accepted answer milk equals 946 cm3, you could pour a varies considerably across healthy adults. to the second one reveals our ignorance bit more than that into a skull without What about smarts? We all know of how intelligent behavior comes about. any of it spilling out. Of course, there from our day-to-day interactions that is considerable variability in brain vol- some people just don’t get it and take a Bigger Is Slightly Better ume, ranging from 1,053 to 1,499 cm3 in long time to understand a new concept; The human brain continues to grow men and between 975 and 1,398 cm3 in others have great mental powers, al- ) until it reaches its peak size in the third women. As the density of brain matter is though it is impolite to dwell on such dif- Koch just a little bit above that of water plus ferences too much. Think of Bertie CABE ( some salts, the average male brain Wooster, an idle but clueless rich man, C BY CHRISTOF KOCH weighs about 1,325 grams, close to the and Jeeves, his genius valet, in a series of proverbial three pounds often cited in novels by P. G. Wodehouse and their ); SEAN M Christof Koch is president U.S. texts. successful British adaptation to the and chief scientific officer of Removing brains after their owners small screen. the Allen Institute for Brain died revealed that Russian novelist Ivan Individuals differ in their ability to illustration Science in Seattle. He serves on Scientific American Turgenev’s brain broke the two-kilo- understand new ideas, to adapt to new Mind’s board of advisers. gram barrier, coming in at 2,021 grams, environments, to learn from experience, ALAMY ( 22 SCIENTIFIC AMERICAN MIND JANUARY/FEBRUARY 2016 Exploring the riddle of our existence to think abstractly, to plan and to rea- In healthy volunteers, total brain volume is there a difference in the basic number son. Psychologists have sought to cap- weakly correlates with intelligence, with of switching elements? ture these differences in mental capaci- a correlation value between 0.3 and 0.4 It is also well established that the cra- ties via a number of closely related con- out of a possible 1.0. In other words, nial capacity of Homo neanderthalen- cepts such as general intelligence ( g, or brain size accounts for between 9 and sis, the proverbial caveman, was 150 to general cognitive ability) and fluid and 16 percent of the overall variability in 200 cm3 bigger than that of modern crystalline intelligence. These differenc- general intelligence. Functional scans, humans. Yet despite their larger brain, es in people’s ability to figure things out used to look for brain areas linked to par- Neandertals became extinct between on the spot and to retain and apply in- ticular mental activities, reveal that the 35,000 and 40,000 years ago, when sights that they learned in the past to parietal, temporal and frontal regions of Homo sapiens shared their European current circumstances are assessed by the cortex, along with the thickness of environment. What’s the point of having psychometric intelligence tests. These these regions, correlate with intelligence big brains if your small-brained cousins observations are reliable, in that differ- but, again, only modestly so. Thus, on outcompete you? ent tests strongly correlate with one an- average, a bigger brain is associated with other. They are also stable across de- somewhat higher intelligence. Whether a Brain Size across Species cades. That is, measures such as the in- big brain causes high intelligence or, Our lack of understanding of the mul- telligence quotient (IQ) can be repeatedly more likely, whether both are caused by tiplicity of causes that contribute to intel- and reliably obtained from the same other factors remains unknown. ligence becomes even more apparent when subjects nearly 70 years later. Recent experiments take into account looking outside the genus Homo. Many Differences in general intelligence, the particular connections among neu- animals are capable of sophisticated be- assessed in this way, correlate with suc- rons in certain regions of an individual’s haviors, including sensory discrimina- cess in life, with social mobility and job brain, much like a neural fingerprint. tion, learning, decision making, planning performance, with health and with life They do better at predicting fluid intelli- and highly adaptive social behaviors. span. In a study of one million Swedish gence (the capacity to solve problems in Consider honeybees. They can recog- men, an increase in IQ by one standard novel situations, to find and match pat- nize faces, communicate the location and quality of food sources to their sisters via the waggle dance, and navigate complex mazes with the help of cues they store in RUSSIAN NOVELIST IVAN TURGENEV’S BRAIN short-term memory. And a scent blown BROKE THE TWO-KILOGRAM BARRIER. into a hive can trigger a return to the site where the bees previously encountered WRITER ANATOLE FRANCE’S this odor, a type of associative memory BRAIN WAS BARELY HALF THAT. that guides them back and that was made famous by Marcel Proust in his Remem- brance of Things Past ( À la Recherche du deviation, a measure of variability, was terns, to reason independently of specif- Temps Perdu). The insect does all of this associated with an amazing 32 percent ic domains of knowledge), explaining with fewer than one million neurons that reduction in mortality. Smarter people about 25 percent of the variance in this weigh around one thousandth of a gram, do better in life. Whereas a high IQ may measure from one person to the next. less than one millionth the size of the hu- not predispose people to be happy or to Our ignorance when it comes to how man brain. Yet are we really a million understand the finer points of dating, the intelligence arises from the brain is ac- times smarter? Certainly not if I look at highly intelligent are more likely to be centuated by several further observa- how well we govern ourselves. found among hedge-fund managers than tions. As alluded to earlier, the adult The prevailing rule of thumb holds among supermarket checkout clerks. male’s brain is 150 grams heavier than that the bigger the animal, the bigger its What about any numerical relation the female’s organ. In the neocortex, the brain. After all, a bigger creature has between brain size and intelligence? Such part of the forebrain responsible for per- more skin that has to be innervated and correlations were difficult to establish in ception, memory, language and reason- more muscles to control and requires a the past when only pathologists had ac- ing, this disparity translates to 23 billion larger brain to service its body. Thus, it cess to skulls and their content. With neurons for men versus 19 billion for makes sense to control for overall size structural MRI imaging of brain anato- women. As no difference exists in the av- when studying brain magnitude. By this my, such measurements are now routine. erage IQ between the two genders, why measure, humans have a relative brain- MIND.SCIENTIFICAMERICAN.COM SCIENTIFIC AMERICAN MIND 23 CONSCIOUSNESS REDUX Human Bottlenose dolphin Manatee Chimpanzee Polar bear African lion Cat Mouse From man to mouse: Frontal slices of eight mammalian brains reveal the immense variety in the size of the organ and the patterning of surface convolutions that have evolved over the course of tens of millions of years. The smoothness of the manatee’s brain surface contrasts with the cauliflowerlike branching in the bottlenose dolphin. Structural differences extend down to the microscopic scale. Scientists are still laboring to understand what the varying anatomy means for animal intelligence. to-body mass of about 2 percent. What group. Thus, if we consider all mammals Yet it is not quite clear what all this about the big mammals—elephants, dol- and compare them against the cat as a means in terms of the cellular constitu- phins and whales? Their brains far out- reference animal (which therefore has an ents of brains. Neuroscientists always weigh those of puny humans, up to 10 ki- EQ of 1), people come out on top with an assumed that humans have more nerve lograms for some whales.
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