While "size does not thing" is a universally preached dictum amongst the politically correct, everyday feel tells us that this can't be the whole story—under many conditions, information technology clearly does. Consider the size of Woody Allen's 2d favorite organ, the encephalon. Adjectives such equally "highbrow" and "lowbrow" have their origin in the belief, much expounded past 19th-century phrenologists, of a close correspondence between a high brow—that is, a big brain—and intelligence. Is this true? Does a bigger brain brand yous necessarily smarter or wiser? And is there any simple connexion between the size of a nervous organisation, notwithstanding measured, and the mental powers of the owner of this nervous arrangement? While the answer to the showtime question is a conditional "yes, somewhat," the lack of any accustomed answer to the second query reveals our ignorance of how intelligent beliefs comes about.

Bigger is slightly better

The human brain continues to grow until it reaches its peak size in the third to fourth decade of life. An MRI study of 46 adults of mainly European descent plant that the average male had a encephalon volume of ane,274 cubic centimeters (cm3) and that the average female encephalon measured 1,131 cm3. Given that a quart of milk equals 946 cm3, you could cascade a fleck more than that into a skull without any of it spilling out. Of course, in that location is considerable variability in brain volume, ranging from 1,053 to ane,499 cm3 in men and betwixt 975 and one,398 cmiii in women. As the density of brain matter is only a footling bit above that of water plus some salts, the average male brain weighs about one,325 grams, close to the proverbial 3 pounds frequently cited in U.S. texts.

Removing brains later their owners died revealed that Russian novelist Ivan Turgenev'southward brain broke the two-kilogram barrier, coming in at two,021 grams, whereas writer Anatole France's encephalon could barely bring half of that weight to the scale at ane,017 grams. (Note that postmortem measures are not straight comparable to information obtained from living brains.) In other words, gross brain size varies considerably beyond healthy adults.

What nearly smarts? We all know from our day-to-24-hour interval interactions that some people just don't get it and accept a long time to understand a new concept; others have great mental powers, although it is boorish to dwell on such differences too much. Think of Bertie Wooster, an idle only clueless rich human being, and Jeeves, his genius valet, in a series of novels by P. G. Wodehouse and their successful British adaptation to the pocket-sized screen.

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Source: "The Evolution of the Brain, the Human Nature of Cortical Circuits, and Intellectual Inventiveness," by Javier DeFelipe, in Frontiers in Neuroanatomy, Vol. 5, Article No. 29. Published online May 16, 2011

Individuals differ in their ability to understand new ideas, to adapt to new environments, to learn from experience, to recollect abstractly, to plan and to reason. Psychologists take sought to capture these differences in mental capacities via a number of closely related concepts such every bit general intelligence (g, or general cognitive power) and fluid and crystalline intelligence. These differences in people'southward ability to figure things out on the spot and to retain and apply insights that they learned in the past to current circumstances are assessed by psychometric intelligence tests. These observations are reliable, in that different tests strongly correlate with one another. They are as well stable across decades. That is, measures such as the intelligence caliber (IQ) tin can be repeatedly and reliably obtained from the same subjects nearly 70 years afterwards.

Differences in general intelligence, assessed in this way, correlate with success in life, with social mobility and job performance, with wellness and with life span. In a study of 1 meg Swedish men, an increase in IQ by ane standard difference, a measure of variability, was associated with an amazing 32 percent reduction in bloodshed. Smarter people practice meliorate in life. Whereas a loftier IQ may not predispose people to be happy or to understand the finer points of dating, the highly intelligent are more than likely to be found amidst hedge fund managers than among supermarket checkout clerks.

What about any numerical relation betwixt encephalon size and intelligence? Such correlations were difficult to establish in the past when only pathologists had access to skulls and their content. With structural MRI imaging of encephalon anatomy, such measurements are now routine. In salubrious volunteers, full encephalon volume weakly correlates with intelligence, with a correlation value betwixt 0.3 and 0.4 out of a possible ane.0. In other words, brain size accounts for betwixt 9 and 16 percentage of the overall variability in general intelligence. Functional scans, used to expect for encephalon areas linked to particular mental activities, reveal that the parietal, temporal and frontal regions of the cortex, along with the thickness of these regions, correlate with intelligence just, once more, only modestly then. Thus, on average, a bigger brain is associated with somewhat higher intelligence. Whether a large brain causes high intelligence or, more probable, whether both are caused past other factors remains unknown.

Recent experiments take into account the particular connections amidst neurons in certain regions of an private's brain, much like a neural fingerprint. They practice ameliorate at predicting fluid intelligence (the capacity to solve problems in novel situations, to detect and friction match patterns, to reason independently of specific domains of knowledge), explaining about 25 per centum of the variance in this measure from one person to the next.

Our ignorance when it comes to how intelligence arises from the brain is accentuated past several further observations. As alluded to earlier, the adult male'due south encephalon is 150 grams heavier than the female'southward organ. In the neocortex, the function of the forebrain responsible for perception, memory, linguistic communication and reasoning, this disparity translates to 23 billion neurons for men versus nineteen billion for women. Every bit no departure exists in the average IQ betwixt the two genders, why is there a deviation in the basic number of switching elements?

It is besides well established that the cranial capacity of Homo neanderthalensis, the proverbial caveman, was 150 to 200 cmiii bigger than that of modern humans. Notwithstanding despite their larger brain, Neandertals became extinct between 35,000 and 40,000 years ago, when Homo sapiens shared their European surround. What's the point of having big brains if your small-brained cousins outcompete you?

Brain size beyond species

Our lack of understanding of the multiplicity of causes that contribute to intelligence becomes fifty-fifty more apparent when we look outside the genus Homo. We notice that many animals are capable of sophisticated behaviors, including sensory discrimination, learning, decision-making, planning and highly adaptive social behaviors.

Consider honeybees. They can recognize faces, communicate the location and quality of food sources to their sisters via the waggle dance, and navigate circuitous mazes with the aid of cues they store in short-term memory. And a aroma blown into a hive can trigger a return to the site where the bees previously encountered this odor, a type of associative memory that guides them dorsum and that was made famous past Marcel Proust in his Remembrance of Things Past (À la Recherche du Temps Perdu). The insect does all of this with fewer than 1 million neurons that counterbalance around i thousandth of a gram, less than ane millionth the size of the human brain. Withal are we really a meg times smarter? Certainly not if I await at how well we govern ourselves.

The prevailing rule of thumb holds that the bigger the animal, the bigger its encephalon. Later on all, a bigger creature has more peel that has to be innervated and more than muscles to control and requires a larger brain to service its body. Thus, information technology makes sense to control for overall size when studying brain magnitude. By this measure, humans have a relative encephalon-to-body mass of about two percent. What almost the large mammals—elephants, dolphins and whales? Their brains far outweigh those of puny humans, up to 10 kilograms for some whales. Given their body mass, ranging from vii,000 kg (for male African elephants) up to 180,000 kg (for blue whales), their brain-to-body ratio is under a tenth of a percent. Homo brains are far bigger relative to people'due south sizes than those of these creatures. Smugness is non in store, though. We are outclassed by shrews, molelike mammals, whose brain takes upwardly about ten percent of their entire trunk mass. Even some birds beat united states on this mensurate. Hmm.

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Source: "Quantitative Relationships in Delphinid Neocortex," by Heidi S. Mortensen et al., inFrontiers in Neuroanatomy, Vol. 8, Article No. 132. Published online November 26, 2014 (chart); Heidi Southward. Mortensen (encephalon); Ólavur Frederiksen (pilot whale)

I small alleviation is an invention of neuroanatomists chosen the encephalization quotient (EQ). It is the ratio of the mass of the brain of the species under investigation relative to a standard brain belonging to the aforementioned taxonomic group. Thus, if nosotros consider all mammals and compare them against the cat as a reference animal (which therefore has an EQ of 1), people come out on top with an EQ of 7.5. Stated differently, the human encephalon is 7.5 times bigger than the brain of a typical mammal weighing as much as nosotros do. Apes and monkeys come in at or below 5, every bit do dolphins and other cetaceans. We finally fabricated it to the acme, validating our ineradicable belief in humanity's exceptionalism.

Nonetheless it is not quite clear what all this means in terms of the cellular constituents of brains. Neuroscientists always assumed that humans accept more nerve cells where information technology counts, in the neocortex, than any other species on the planet, no matter the size of their brain.

A 2014 report of x long-finned pilot whales from the Faeroe Islands plays havoc with this hypothesis. Defenseless as part of a local hunt in the common cold waters of the North Atlantic between Scotland and Republic of iceland, these graceful mammals—also known every bit blackfish—are actually dolphins. The number of nervus cells making up their highly convolved neocortex was estimated in a few sample slices and so extrapolated to the entire structure. The total came to an amazing 37.2 billion neurons. Astonishing because this implies that the long-finned pilot whale has about twice every bit many neocortical neurons as humans do!

If what matters for cognitive performance is the number of neocortical neurons, these dolphins should be smarter than all other extant creatures, including u.s.. Whereas the highly playful and social dolphins showroom a diversity of skills, including the ability to recognize themselves in a mirror, they do non possess linguistic communication or any readily discernible powers of abstraction that stand out from those of other nonhuman animals. So what gives? Is the complexity of the nerve cells themselves essentially less than cells establish in people, or is the way these neurons communicate or learn less sophisticated? We don't know.

People forever ask for the single thing that distinguishes humans from all other animals, on the supposition that this 1 magical property would explain our evolutionary success—the reason nosotros tin build vast cities, put people on the moon, write Anna Karenina and compose Eroica. For a while it was causeless that the undercover ingredient in the human brain could be a particular blazon of neuron, so-called spindle or von Economo neurons, named for Baron Constantin von Economo (1876–1931).

But we at present know that not only great apes but as well whales, dolphins and elephants take these neurons in their frontal cortex. So information technology is not encephalon size, relative brain size or absolute number of neurons that distinguishes us. Peradventure our wiring has become more streamlined, our metabolism more efficient, our synapses more sophisticated.

As Charles Darwin surmised, it is very likely a combination of a great many unlike factors that jointly, over the gradual course of development, made us singled-out from other species. We are unique, simply and so is every other species, each in its own way.