Is there a correlation between total neurons and intelligence?

Is there a correlation between total neurons and intelligence?

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First off, I am not a biologist, just a curious layman, so I apologize in advance if this isn't a "good" question. Please don't downvote me into oblivion.

I read today that the human brain has about 100 billion neurons and that got me wondering: is that number pretty standard for everyone or does someone like say, Einstein, have many more neurons than this daft poster.


Is there a correlation between total neuron count and intelligence, or does intelligence depend more on the way neurons are used--or some other factor like previous experience--rather than their total count?

Can this question be answered?

There are two very valid points-of-view posted as answers below, but it sounds like we simply don't have a solid answer to this question at this point in time, so I hesitate to select a "correct" answer.

There are an estimated 100 billion neurons within the human brain. In general a minor variation in the number of neurons should not effect individuals too much, however when there is a more significant loss, such as brain injury or in some forms of dementia cognitive abilities do decline. So in this sense yes the number of neurons does relate to intelligence. However this variation is not what accounts for general variation of intelligence in the population.

I know of no correlation between number of neurons in cortex and intelligence. This question is fraught with controversy because there has been very little work on it but much speculation. Some have suggested that the connectivity between neurons is what is important rather than the number which is logically possible but remains to be supported by definitive evidence.

Unlike a computer, the speed at which any brain can perform a computation is related to the number of synapses it goes through. This means fewer synapses in series correlate with decreased reaction time.

An example of fewer neurons correlating with a decrease in reaction time is exemplified in sensory neurons. All sensory nerve cell bodies are all located in the dorsal ganglia of the spine. Only projections from single nerve cells here reach our tissues. Furthermore, it's only in ganglia, nuclei, and the cortex that we see additional nerve cells. This is because each nerve cell in series introduces latency.

Latency in our sensory and motor circuits is very important to survival. Our intelligence is limited by a pressure to have a desirable reaction time. Reaction time does correlate intelligence. This is why tests of the sort are timed.

There is another factor in intelligence relating to number of neurons: An increase in grey matter means an increase in white matter. There has been some speculation that humans have nearly reached the limit of our cognitive ability, as our ability to store white matter has been reached.

We seem to be occupying the intelligence Goldilocks zone where reaction time, energy consumption, white matter are all in near optimal balance. It's the balance of these things, the sophistication of the circuits themselves that determine intelligence, not neuron count.

Here's a news link about running out of room for white matter

Here's a study which correlates perception time with reaction time

there is no question there is a rough correlation between neuron count and intelligence capability in a general way from looking at the biological species "spectrum" outside of humans. see wikipedia list of animals by number of neurons. eg in wide differences such as comparing insects vs primates etcetera. there is also a rough correlation of neuron count with body mass and total cell count in animals. however within a single species, small variations in neuron count probably do not have much effect on overall intelligence. also it is known that homo neanderthalensis, the ~30k yr precursor to [homo sapiens] had a larger brain cavity and therefore presumably more neurons but apparently less intellectual capacity than homo sapiens, possibly one factor in its extinction due to competitive evolutionary pressures. even various gorilla species have larger brain mass and conceivably correspondingly larger amounts of neurons (under a rough assumption that neuron density per mass would not vary strongly).

it is also thought that brain size has been affected by evolutionary pressure wrt smaller skull size which has to comfortably fit through the birth canal as a constraint. another aspect to study is the enteric nervous system in the human which has a large number of neurons but does not seem to exhibit a large measurable "intelligence" in the classic sense of guiding the animal behavior, but conceivably uses significant processing to manage gastrointestinal control.

another aspect of the question to consider: programmed cell death in the developing nervous system. large numbers of neurons die during normal natal development in "critical windows" and this seems to show that biologically, total number of neurons is apparently not a key measure or controller of intelligence (otherwise evolutionary pressures would tend to decrease this). the nervous system appears to grow "extra" neurons more than "necessary" for smooth functioning of the human organism.

The social brain hypothesis and its implications for social evolution

The social brain hypothesis was proposed as an explanation for the fact that primates have unusually large brains for body size compared to all other vertebrates: Primates evolved large brains to manage their unusually complex social systems. Although this proposal has been generalized to all vertebrate taxa as an explanation for brain evolution, recent analyses suggest that the social brain hypothesis takes a very different form in other mammals and birds than it does in anthropoid primates. In primates, there is a quantitative relationship between brain size and social group size (group size is a monotonic function of brain size), presumably because the cognitive demands of sociality place a constraint on the number of individuals that can be maintained in a coherent group. In other mammals and birds, the relationship is a qualitative one: Large brains are associated with categorical differences in mating system, with species that have pairbonded mating systems having the largest brains. It seems that anthropoid primates may have generalized the bonding processes that characterize monogamous pairbonds to other non-reproductive relationships ('friendships'), thereby giving rise to the quantitative relationship between group size and brain size that we find in this taxon. This raises issues about why bonded relationships are cognitively so demanding (and, indeed, raises questions about what a bonded relationship actually is), and when and why primates undertook this change in social style.

Ask a Neuroscientist: Does a bigger brain make you smarter?

“There is a puzzling question I have been meaning to ask some science experts, but I didn’t know where to turn to. I just learned recently that some humans have larger brains than others. Is there a correlation between having a large brain and intelligence?” — Apioth


You bring up a controversial question!

The relationship between brain size and intelligence, both amongst humans and between different species, has never been particularly well-defined. Humans like to believe that our exceptional cognitive abilities must indicate that we are the kings of the animal kingdom in terms of brain size, or at least that we have the largest brains relative to our body size. As nature would have it, both of these common assumptions are incorrect. Whales and elephants have much bigger brains than humans, and we have about the same brain-to-body mass ratio as mice. Since it would be against human nature to admit defeat, scientists have crafted a third measure of brain size called the encephalization quotient, which is the ratio of actual brain mass relative to the predicted brain mass for an animal’s size (based off the assumption that larger animals require slightly less brain matter relative to their size compared to very small animals). By this metric, at least, humans come out on top, with an EQ of 7.5 far surpassing the dolphin’s 5.3 and the mouse’s measly 0.5.

Okay, so despite the uncertain relationship between brain size and cognitive abilities between different species, can brain size predict anything about intelligence amongst humans? Does having a gigantic brain mean that you’re smarter, as cartoons like Pinky and the Brain and Jimmy Neutron Boy Genius would have us believe?

Some studies claim the answer is yes.

The emergence of magnetic resonance imaging (MRI) has made it possible to compare brain sizes of living humans, and in the ongoing hunt for a physical metric of intelligence, several researchers eagerly sought to correlate MRI measures of brain volume with IQ. Ten years ago, a meta-analysis that examined the results from 26 imaging studies concluded that the correlation between IQ and brain volume is consistently in the 0.3-0.4 range. More recently, a genome-wide association study which included 20,000 human subjects was widely reported by the media to have discovered an “IQ gene.” According to their results, certain variations in the HMGA2 gene, which codes for a protein that helps regulate DNA transcription and cell growth, are correlated with increased intracranial volume as well as enhanced IQ.

To be honest, I find these correlation a bit unsettling. Clearly, there is more to intelligence than brain size, or classic geniuses like Albert Einstein, who had an average-sized brain, would have been out of luck! It is important think about how we should actually define intelligence, and to keep in mind that the studies cited above only show a correlation between brain size and a person’s score on an intelligence quotient test. Although IQ is historically the most widely used intelligence measure, by no means does it account for all aspects of human intelligence, nor is it an entirely consistent readout of cognitive ability between individuals. Furthermore, a closer look at the results of the gene-association study reveal that most of the relationship the authors found between HMGA2 gene variations and cranial size could be accounted for by the fact that the gene is also correlated with human height. Correlational studies have only established a weak to moderate linear relationship between brain size an intelligence, which is enough fuel to ensure that the brain size and intelligence hypothesis doesn’t burn out, but does little to explain the true basis of human cognitive capacity.

Luckily, there is much more to a brain when you look at it under a microscope, and most neuroscientists now believe that the complexity of cellular and molecular organization of neural connections, or synapses, is what truly determines a brain’s computational capacity. This view is supported by findings that intelligence is more correlated with frontal lobe volume and volume of gray matter, which is dense in neural cell bodies and synapses, than sheer brain size. Other research comparing proteins at synapses between different species suggests that what makes up synapses at the molecular level has had a huge impact on intelligence throughout evolutionary history. So, although having a big brain is somewhat predictive of having big smarts, intelligence probably depends much more on how efficiently different parts of your brain communicate with each other.

The Paradox of the Elephant Brain

W e have long deemed ourselves to be at the pinnacle of cognitive abilities among animals. But that is different from being at the pinnacle of evolution in a number of very important ways. As Mark Twain pointed out in 1903, to presume that evolution has been a long path leading to humans as its crowning achievement is just as preposterous as presuming that the whole purpose of building the Eiffel Tower was to put that final coat of paint on its tip. Moreover, evolution is not synonymous with progress, but simply change over time. And humans aren’t even the youngest, most recently evolved species. For example, more than 500 new species of cichlid fish in Lake Victoria, the youngest of the great African lakes, have appeared since it filled with water some 14,500 years ago.

Still, there is something unique about our brain that makes it cognitively able to ponder even its own constitution and the reasons for its own presumption that it reigns over all other brains. If we are the ones putting other animals under the microscope, and not the other way around, 1 then the human brain must have something that no other brain has.

Hello Handsome: Since the late 1960s, psychologists have speculated whether the ability to recognize oneself in a mirror was indicative of intelligence and self-awareness. James Balog / Getty Images

Sheer mass would be the obvious candidate: If the brain is what generates conscious cognition, having more brain should only mean more cognitive abilities. But here the elephant in the room is, well, the elephant—a species that is larger-brained than humans, but not equipped with behaviors as complex and flexible as ours. Besides, equating larger brain size with greater cognitive capabilities presupposes that all brains are made the same way, starting with a similar relationship between brain size and number of neurons. But my colleagues and I already knew that all brains were not made the same. Primates have a clear advantage over other mammals, which lies in an evolutionary turn of events that resulted in the economical way in which neurons are added to their brain, without the massive increases in average cell size seen in other mammals.

We also knew how many neurons different brains were made of, and so we could rephrase “more brain” and test it. Sheer number of neurons would be the obvious candidate, regardless of brain size, because if neurons are what generates conscious cognition, then having more neurons should mean more cognitive capabilities. Indeed, even though cognitive differences among species were once thought to be qualitative, with a number of cognitive capabilities once believed to be exclusive to humans, it is now recognized that the cognitive differences between humans and other animals are a matter of degree. That is, they are quantitative, not qualitative, differences.

Did the African elephant brain, more than three times as heavy as ours, really have more neurons than our brain?

The Genius of Learning

By Lauren R. Weinstein

Lauren R. Weinstein is a cartoonist based in New Jersey. She is currently working on a graphic novel tentatively entitled How to Draw a Nose. Her previous books include Girl Stories and The Goddess. READ MORE

Our tool use is impressively complex, and we even design tools to make other tools—but chimpanzees use twigs as tools to dig for termites, monkeys learn to use rakes to reach for food that is out of sight, and crows not only shape wires to use as tools to get food, but also keep them safe for later reuse. Alex, the African gray parrot owned by psychologist Irene Pepperberg, learned to produce words that symbolize objects, and chimpanzees and gorillas, though they cannot vocalize for anatomical reasons, learn to communicate with sign language. Chimpanzees can learn hierarchical sequences: They play games where they must touch squares in the ascending order of the numbers previously shown, and they do it as well and as fast as highly trained humans. Chimpanzees and elephants cooperate to secure food that is distant and can’t be reached by their efforts alone. Chimpanzees, but also other primates, appear to infer others’ mental state, a requirement for showing deceitful behavior. Even birds seem to have knowledge of other individuals’ mental state, as magpies will overtly cache food in the presence of onlookers and then retrieve and move it to a secret location as soon as the onlookers are gone. Chimpanzees and gorillas, elephants, dolphins, and also magpies appear to recognize themselves in the mirror, using it to inspect a visible mark placed on their heads.

These are fundamental discoveries that attest to the cognitive capacities of nonhuman species—but such one-of-a-kind observations do not serve the types of cross-species comparisons we need to make if we are to find out what it is about the brain that allows some species to achieve cognitive feats that are outside the reach of others. And here we run into another problem, the biggest one at this point: how to measure cognitive capabilities in a large number of species and in a way that generates measurements that are comparable across all those species.

A 2014 study tested for self-control, a cognitive ability that relies on the prefrontal, associative part of the cerebral cortex, among a number of animal species—mostly primates, but also small rodents, doglike carnivores, the Asian elephant, and a variety of bird species. They found that the best correlate with correct performance in the test of self-control was absolute brain volume—except for the Asian elephant, which, despite being the largest-brained in the set, failed miserably at the task. A number of reasons come to mind, from “It did not care about the food or the task” to “It enjoyed annoying its caretakers by not performing.” (I like to think that the reason why it’s so hard to train monkeys to do things that are easily learned by humans is their dismay at the obviousness of the task: “C’mon, you want me to move to do just that? Gimme something more challenging to do! Gimme videogames!”)

Brainiac: Suzana Herculano-Houzel seeks to learn exactly what it is about the human brain that allows it to perform much more complex maneuvers than other animal brains seem to. Here, she gives a TED Talk. James Duncan Davidson, courtesy of TED

The most interesting possibility to me, however, is that the African elephant might not have all the prefrontal neurons in the cerebral cortex that it takes to solve self-control decision tasks like the ones in the study. Once we had recognized that primate and rodent brains are made differently, with different numbers of neurons for their size, we had predicted that the African elephant brain might have as few as 3 billion neurons in the cerebral cortex and 21 billion neurons in the cerebellum, compared to our 16 billion and 69 billion, despite its much larger size—if it was built like a rodent brain.

On the other hand, if it was built like a primate brain, then the African elephant brain might have a whopping 62 billion neurons in the cerebral cortex and 159 billion neurons in the cerebellum. But elephants are neither rodents nor primates, of course they belong to the superorder Afrotheria, as do a number of small animals like the elephant shrew and the golden mole we had already studied—and determined that their brains did, in fact, scale very much like rodent brains.

Here was a very important test, then: Did the African elephant brain, more than three times as heavy as ours, really have more neurons than our brain? If it did, then my hypothesis that cognitive powers come with sheer absolute numbers of neurons would be disproved. But if the human brain still had many more neurons than the much larger African elephant brain, then that would support my hypothesis that the simplest explanation for the remarkable cognitive abilities of the human species is the remarkable number of its brain neurons, equaled by none other, regardless of the size of the brain. In particular, I expected the number of neurons to be larger in the human than in the African elephant cerebral cortex.

The logic behind my expectation was the cognitive literature that had long hailed the cerebral cortex (or, more precisely, the prefrontal part of the cerebral cortex) as the sole seat of higher cognition—abstract reasoning, complex decision making, and planning for the future. However, nearly all of the cerebral cortex is connected to the cerebellum through loops that tie cortical and cerebellar information processing to each other, and more and more studies have been implicating the cerebellum in the cognitive functions of the cerebral cortex, with the two structures working in tandem. And, because these two structures together accounted for the vast majority of all neurons in the brain, cognitive capabilities should correlate equally well with the number of neurons in the whole brain, in the cerebral cortex, and in the cerebellum.

Which is why our findings for the African elephant brain turned out to be better than expected.

Brain Soup by the Gallon

The brain hemisphere of an African elephant weighs more than 2.5 kilograms, which meant that it would obviously have to be cut into hundreds of smaller pieces for processing and counting since turning brains into soup to determine the number of neurons inside works with chunks of no more than 3 to 5 grams of tissue at a time. I wanted the cutting to be systematic, instead of haphazard. We had previously used a deli slicer to turn a human brain hemisphere into one such full series of thin cuts. The slicer was wonderful for separating cortical gyri—but it had one major drawback: Too much of the human brain matter stayed on its circular blade, precluding estimates of the total number of cells in the hemisphere. If we wanted to know the total number of neurons in the elephant brain hemisphere, we had to cut it by hand, and in thicker slices, to minimize eventual losses to the point of making them negligible.

Why spend $100,000 when a handheld butcher knife would do the job well enough?

And so the day started at the hardware store, where my daughter and I (school vacation having just started) went looking for L-brackets to serve as solid, flat, regular frames for cutting the elephant hemisphere, plus the longest knife I could hold in one hand. (Here was an opportunity not to be missed for a young teenager, who years later could say, “Hey, Mom, remember the day we sliced up an elephant brain?”) We first sawed off the structural reinforcements of the L-brackets then made the elephant brain fit inside. Sure, there are fancy $100,000 machines that would do the job to perfection, but why spend that much money when a handheld butcher knife would do the job well enough?

I laid the hemisphere flat on the bench top, framed inside the two L-brackets. A student held the frames in position while I held the hemisphere down with my left hand and sliced firmly but gently through the brain with the right, in back-and-forth movements. Several cuts later, also into the back half as well as the cerebellum, and we had a completely sliced elephant brain “loaf” lying flat on our benchtop: 16 sections through the cortical hemisphere, eight through the cerebellum, plus the entire brainstem and the gigantic, 20-gram olfactory bulb (10 times the mass of a rat brain) lying separately.

Counting Neurons: Suzana Herculano-Houzel and her students cross-sectioned an elephant brain, shown here, to determine the number of neurons it has and compare that with what’s found in the human brain. Courtesy of the author

Next, we had to separate the internal structures—striatum, thalamus, hippocampus—from the cortex, then cut the cortex into smaller pieces for processing, then separate each of these pieces into gray and white matter. In all, we had 381 pieces of tissue, most of which were still several times larger than the 5 grams we could process at one time. It was by far the most tissue we had processed. One person working alone and processing one piece of tissue per day would need well over one year—nonstop—to finish the job. This clearly had to be a team effort, especially if I wanted to have the results in no more than six months. But, even with a small army of undergraduates, it was taking too long: two months went by and only one-tenth of the brain hemisphere had been processed. Something had to be done.

Capitalism came to the rescue. I did some math and realized I had some $2,500 to spare—roughly $1 per gram of tissue to be processed. I gathered the team and made them an offer: Anybody could help, and everyone would be rewarded financially by the same amount. Small partnerships quickly formed, with one student doing the grinding, the other doing the counting, and both sharing the proceeds. It worked wonders. My husband would visit the lab and comment, in awe, on the crowd of students at the bench, chatting animatedly while working away (until then, they mostly worked in shifts, it being a small lab). Jairo Porfírio took over the large batches of antibody stains, I did all the neuron counts at the microscope—and in just under six months we had the entire African elephant brain hemisphere processed, as planned.

Lo and behold, the African elephant brain had more neurons than the human brain. And not just a few more: a full three times the number of neurons, 257 billion to our 86 billion neurons. But—and this was a huge, immense “but”—a whopping 98 percent of those neurons were located in the cerebellum, at the back of the brain. In every other mammal we had examined so far, the cerebellum concentrated most of the brain neurons, but never much more than 80 percent of them. The exceptional distribution of neurons within the elephant brain left a relatively meager 5.6 billion neurons in the whole cerebral cortex itself. Despite the size of the African elephant cerebral cortex, the 5.6 billion neurons in it paled in comparison to the average 16 billion neurons concentrated in the much smaller human cerebral cortex.

So here was our answer. No, the human brain does not have more neurons than the much larger elephant brain—but the human cerebral cortex has nearly three times as many neurons as the over twice as large cerebral cortex of the elephant. Unless we were ready to concede that the elephant, with three times more neurons in its cerebellum (and, therefore, in its brain), must be more cognitively capable than we humans, we could rule out the hypothesis that total number of neurons in the cerebellum was in any way limiting or sufficient to determine the cognitive capabilities of a brain.

Only the cerebral cortex remained, then. Nature had done the experiment that we needed, dissociating numbers of neurons in the cerebral cortex from the number of neurons in the cerebellum. The superior cognitive capabilities of the human brain over the elephant brain can simply—and only—be attributed to the remarkably large number of neurons in its cerebral cortex.

While we don’t have the measurements of cognitive capabilities required to compare all mammalian species, or at least those for which we have numbers of cortical neurons, we can already make a testable prediction based on those numbers. If the absolute number of neurons in the cerebral cortex is the main limitation to the cognitive capabilities of a species, then my predicted ranking of species by cognitive abilities based on number of neurons in the cerebral cortex would look like this:

which is more intuitively reasonable than the current ranking based on brain mass, which places animals such as the giraffe above many primate species, like this:

As it turns out, there is a simple explanation for how the human brain, and it alone, can be at the same time similar to others in its evolutionary constraints, and yet so different to the point of endowing us with the ability to ponder our own material and metaphysical origins. First, we are primates, and this bestows upon humans the advantage of a large number of neurons packed into a small cerebral cortex. And second, thanks to a technological innovation introduced by our ancestors, we escaped the energetic constraint that limits all other animals to the smaller number of cortical neurons that can be afforded by a raw diet in the wild.

So what do we have that no other animal has? A remarkable number of neurons in the cerebral cortex, the largest around, attainable by no other species, I say. And what do we do that absolutely no other animal does, and which I believe allowed us to amass that remarkable number of neurons in the first place? We cook our food. The rest—all the technological innovations made possible by that outstanding number of neurons in our cerebral cortex, and the ensuing cultural transmission of those innovations that has kept the spiral that turns capacities into abilities moving upward—is history.

Suzana Herculano-Houzel is a Brazilian neuroscientist. She is an associate professor and the head of the Laboratory of Comparative Anatomy, Institute of Biomedical Sciences, Federal University of Rio de Janeiro.

Excerpted from The Human Advantage: A New Understanding of How Our Brain Became Remarkable by Suzana Herculano-Houzel published this month by The MIT Press. All rights reserved.

The Human Brain is a Linearly Scaled-Up Primate Brain in its Number of Neurons. What Now?

Cognitive abilities, brain size and number of neurons

To conclude that the human brain is a linearly scaled-up primate brain, with just the expected number of neurons for a primate brain of its size, is not to state that it is unremarkable in its capabilities. However, as studies on the cognitive abilities of non-human primates and other large-brained animals progress, it becomes increasingly likely that humans do not have truly unique cognitive abilities, and hence must differ from these animals not qualitatively, but rather in the combination and extent of abilities such as theory of mind, imitation and social cognition (Marino et al., 2009). Quantitative changes in the neuronal composition of the brain could therefore be a main driving force that, through the exponential combination of processing units, and therefore of computational abilities, leads to events that may look like “jumps” in the evolution of brains and intelligence (Roth and Dicke, 2005). Such quantitative changes are likely to be warranted by increases in the absolute (rather than relative) numbers of neurons in relevant cortical areas and, coordinately, in the cerebellar circuits that interact with them (Ramnani, 2006). Moreover, viewing the human brain as a linearly scaled-up primate brain in its cellular composition does not diminish the role that particular neuroanatomical arrangements, such as changes in the relative size of functional cortical areas (for instance, Semendeferi et al., 2001 Rilling and Seligman, 2002), in the volume of prefrontal white matter (Schoenemann et al., 2005) or in the size of specific portions of the cerebellum (Ramnani, 2006) may play in human cognition. Rather, such arrangements should contribute to brain function in combination with the large number of neurons in the human brain. Our analysis of numbers of neurons has so far been restricted to large brain divisions, such as the entire cerebral cortex and the ensemble of brainstem, diencephalon and basal ganglia, but an analysis of the cellular scaling of separate functional cortical areas and the related subcortical structures is underway. Such data should allow us to address important issues such as mosaic evolution through concerted changes in the functionally related components of distributed systems, and the presumed increase in relative number of neurons in systems that increase in importance (Barton and Harvey, 2000 Barton, 2006).

If cognitive abilities among non-human primates scale with absolute brain size (Deaner et al., 2007) and brain size scales linearly across primates with its number of neurons (Herculano-Houzel et al., 2007), it is tempting to infer that the cognitive abilities of a primate, and of other mammals for that matter, are directly related to the number of neurons in its brain. In this sense, it is interesting to realize that, if the same linear scaling rules are considered to apply to great apes as to other primates, then similar three-fold differences in brain size and in brain neurons alike apply to humans compared to gorillas, and to gorillas compared to baboons. This, however, is not to say that any cognitive advantages that the human brain may have over the gorilla and that the gorilla may have over the baboon are equally three-fold – although these differences are difficult to quantify. Since neurons interact combinatorially through the synapses they establish with one another, and further so as they interact in networks, the increase in cognitive abilities afforded by increasing the number of neurons in the brain can be expected to increase exponentially with absolute number of neurons, and might even be subject to a thresholding effect once critical points of information processing are reached. In this way, the effects of a three-fold increase in numbers of neurons may be much more remarkable when comparing already large brains, such as those of humans and gorillas, than when comparing small brains, such as those of squirrel monkeys and galagos.

Intraspecific variability in size, numbers and abilities

One final caveat to keep in mind when studying scaling of numbers of brain neurons, particularly in regard to cognition, is that relationships observed across species need not apply to comparisons across individuals of the same species. Not only the extent of intraspecific variation is much smaller (on the order of 10�%) than interspecific variation (which spans five orders of magnitude within mammals Tower, 1954 Stolzenburg et al., 1989), but also the mechanisms underlying interspecific and intraspecific variation are also likely to differ. Our own preliminary data suggest that, indeed, variations in brain size across rats of the same age are not correlated with variations in numbers of neurons (Morterá and Herculano-Houzel, unpublished observations). There is no justification, therefore, to extend the linear correlation between brain size and number of neurons across primates to a putative correlation across persons of different brain sizes (which might be used, inappropriately, as grounds for claims that larger-brained individuals have more neurons, and are therefore “smarter”, than smaller-brained persons). In fact, although men have been reported to have more neurons in the cerebral cortex than women (Pakkenberg and Gundersen, 1997 Pelvig et al., 2008), there is no significant correlation between brain size and general cognitive ability within families (Schoenemann et al., 2000). Across these individuals, other factors such as variations in number and identity of synaptic connections within and across structures, building on a statistically normal, albeit variable, number of neurons, and depending on genetics and life experiences such as learning, are more likely to be determinant of the individual cognitive abilities (see, for instance, Mollgaard et al., 1971 Black et al., 1990 Irwin et al., 2000 Draganski et al., 2004).

Concluding remarks: our place in nature

Novel quantitative data on the cellular composition of the human brain and its comparison to other primate brains strongly indicate that we need to rethink our notions about the place that the human brain holds in nature and evolution, and rewrite some of the basic concepts that are taught in textbooks. Accumulating evidence (Deacon, 1997 Roth and Dicke, 2005 Deaner et al., 2007) indicates that an alternative view of the source of variations in cognitive abilities across species merits investigation: one that disregards body and brain size and examines absolute numbers of neurons as a more relevant parameter instead. Now that these numbers can be determined in various brains and their structures, direct comparisons can be made across species and orders, with no assumptions about body𠄻rain size relationships required. Complementarily, however, it now becomes possible to examine how numbers of neurons in the brain, rather than brain size, relate to body mass and surface as well as metabolism, parameters that have been considered relevant in comparative studies (Martin, 1981 Fox and Wilczynski, 1986 MacLarnon, 1996 Schoenemann, 2004), in order to establish what mechanisms underlie the loosely correlated scaling of body and brain.

According to this now possible neuron-centered view, rather than to the body-centered view that dominates the literature (see Gazzaniga, 2008, for a comprehensive review), the human brain has the number of neurons that is expected of a primate brain of its size a cerebral cortex that is exactly as large as expected for a primate brain of 1.5 kg just as many neurons as expected in the cerebral cortex for the size of this structure and, despite having a relatively large cerebral cortex (which, however, a rodent brain of 1.5 kg would also be predicted to have), this enlarged cortex holds just the same proportion of brain neurons in humans as do other primate cortices (and rodent cortices, for that matter). This final observation calls for a reappraisal of the view of brain evolution that concentrates on the expansion of the cerebral cortex, and its replacement with a more integrated view of coordinate evolution of cellular composition, neuroanatomical structure, and function of cerebral cortex and cerebellum (Whiting and Barton, 2003).

Other �ts” that deserve updating are the ubiquitous quote of 100 billion neurons (a value that lies outside of the margin of variation found so far in human brains Azevedo et al., 2009), and, more strikingly, the widespread remark that there are 10× more glial cells than neurons in the human brain. As we have shown, glial cells in the human brain are at most 50% of all brain cells, which is an important finding since it is one more brain characteristic that we share with other primates (Azevedo et al., 2009).

Finally, if being considered the bearer of a linearly scaled-up primate brain does not sound worthy enough for the animal that considers himself the most cognitively able on Earth, one can note that there are, indeed, two advantages to the human brain when compared to others – even if it is not an outlier, nor unique in any remarkable way. First, the human brain scales as a primate brain: this economical property of scaling alone, compared to rodents, assures that the human brain has many more neurons than would fit into a rodent brain of similar size, and possibly into any other similar-sized brain. And second, our standing among primates as the proud owners of the largest living brain assures that, at least among primates, we enjoy the largest number of neurons from which to derive cognition and behavior as a whole. It will now be interesting to determine whether humans, indeed, have the largest number of neurons in the brain among mammals as a whole.

Elephants Have The Most Neurons. Why Aren't They The Smartest Animals?

Why aren't elephants the smartest animals since they have the most neurons? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Fabian van den Berg, Neuropsychologist, on Quora:

Why aren't elephants the smartest animals since they have the most neurons?

We often hear 'bigger is better' which might be true for pay-checks but not for other things. I’m of course talking about brains, what else? Nature has an astounding diversity of life, each with a unique brain. Some of those brains grow to be massive organs, like that of the African Elephant with a 5kg brain (11lbs) and 257 billion neurons. Some brains stay tiny, like that of roundworms which comes in at only a fraction of a gram with about 300 neurons in total. Humans rank in between, with a 1.4kg (3lbs) brain and give or take 86 billion neurons.

That begs the question, if humans are outranked by animals such as elephants, why are we the self-proclaimed smartest creature on earth? How is it that an elephant with almost 3 times the number of neurons isn’t laughing at our struggle with quantum mechanics?

Like a late night news-report, the reason might surprise you. To put it bluntly, humans aren’t all that special. Like mentioned above, we don’t have the biggest brain with the most neurons. Nor do we have the brain with the biggest surface area dolphins beat us there with their amazingly complex brain folds. We get a bit closer if we take body size into account, but we’d lose from a marmoset (a sort of small monkey which honestly isn’t all that bright). A new measure was developed called the ‘encephalisation quotient’ (EQ), which takes into account that the relationship between brain and body size isn’t linear. It’s a whole formula, but it gave us what we needed for our ego, we were on top! Based on our size we have a brain that is 7 times larger than it should be. Sounds great for us, but the measure failed a bit for other animals. The rhesus monkey should be smarter than a gorilla if we were to believe their EQ, which isn’t the case. That puts us back to square one.

Humans don’t stand out that much in general, except when it comes to intelligence. Absolute brain size isn’t what makes us smart, neither is surface area, EQ, or neuron density. Then why is it that an elephant, with a huge brain and more neurons, isn’t as smart or even smarter than a human? This is where neuroscience and biology get a bit tricky, an example might help.

Consider the fastest supercomputer in the world. At the time of writing, that is the Summit made by IBM. It has an impressive 9.216 CPUs, 27.648 GPUs and can make 200 quadrillion calculations per second. For comparison, it would take every person on earth working together, doing 1 calculation per second for almost a year to do what this machine can do in 1 second. It is set to model the universe, explore cancer, and figure out genetics on a scale we cannot imagine. But can it run Minecraft? No it cannot. Yet my old i7 quad-core laptop can run Minecraft just fine. Weird isn’t it, an immense computer with more memory and processing power than fits in my apartment can’t run a simple game that my rickety laptop can? So much for “super” computers.

The truth is, the thing isn’t designed to run Minecraft. It’s made to run those complex astronomical and biological models, while my laptop is designed to run games and various other tasks useful to me. I’m sure with some fiddling you can get any game running on those systems, but you’d definitely get in trouble for that. When comparing brains, the absolute neuron count isn’t the only thing we need to look at. Just like absolute processing power isn’t the only thing you look for when you need to play Minecraft. What’s in a machine, how it’s connected, how it interfaces, all change depending on a computer’s purpose.

Human brains and Elephant brain are different in more ways than one. Different parts have different concentrations of neurons for example. Despite having three times as many neurons, elephants only have a third as many neurons in their cerebral cortex. The cortex just so happens to be the part of the brain we associate with a lot of “higher cognitive functions” and intelligence. All those elephant brain cells are concentrated in other areas, like the cerebellum which is used for movements (that trunk does look very capable).

The way the brain is put together is another factor. We estimate that Neanderthals had bigger brains than us they had the capacity for a 1600cm3 brain. When researchers recently grew some Neanderthal brain-matter, we saw that they were very different from our own. Human mini-brains were nice, smooth spheres, whereas Neanderthal brains were more like popcorn. The consequences are still not clear, but it does bring us to this point: brains are complicated. Brains aren’t homogenous masses of neurons and support cells. Brains have structure to them, neurons form columns and layers, have specific pathways to send and receive specific information. The way neurons are structured and connected affects what and how they process information. Different animals have different needs, different senses, and different bodies. Brains are formed to deal with all of that. An elephant needs to control its trunk to get food, not solve math problems to get good grades.

As mentioned in the beginning, nature has an astounding diversity of life and brains. Those brains have been sculpted by evolution over millions of years, and evolution doesn’t care about intelligence as much as we do. Evolution is a process without goals instead it takes more of a “good enough” approach. An organism has to function within its environment. For our elephant, an elephant brain is absolutely perfect for doing elephant things, it’s the pinnacle of elephantness.

Humans had different survival tactics and evolutionary challenges. We didn’t have claws and weren’t very big and strong, instead we were smart and social. In evolutionary terms we bet everything on our brain, which is reflected by our cerebral cortex. Unlike other measurements, our cerebral cortex usually comes out on top compared to other animals. Even when compared to other primates, our cortex is astounding (more so in organization than size). It does require a lot of fuel, making it very reasonable to assume we beat other primates in the intelligence game because we started cooking. But that’s a story for another day.

Intelligence is an elusive concept we don’t really know for sure what makes one species smarter than another. It’ll be a while before we have definitive answers, but we do know it has to do with a lot of factors. Brain size, number of neurons, number of connections, different structures, densities, how they are connected, they all play a role. No single measure can explain why some animals are smarter than others, let alone why some humans are smarter than others.

An elephant is not as intelligent as a human, because an elephant brain is formed and wired to do elephant things. Just like a supercomputer isn’t made to play Minecraft, but rather focuses on simulating supernovae. Human brains do human things instead of elephant things in fact we make terrible elephants.

It’s not the size of the brain that matters it’s how you use it.

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Social intelligence turns out to be especially important in crisis situations. Consider the experience of workers at a large Canadian provincial health care system that had gone through drastic cutbacks and a reorganization. Internal surveys revealed that the frontline workers had become frustrated that they were no longer able to give their patients a high level of care. Notably, workers whose leaders scored low in social intelligence reported unmet patient-care needs at three times the rate—and emotional exhaustion at four times the rate—of their colleagues who had supportive leaders. At the same time, nurses with socially intelligent bosses reported good emotional health and an enhanced ability to care for their patients, even during the stress of layoffs (see the sidebar “The Chemistry of Stress”). These results should be compulsory reading for the boards of companies in crisis. Such boards typically favor expertise over social intelligence when selecting someone to guide the institution through tough times. A crisis manager needs both.

The Chemistry of Stress

When people are under stress, surges in the stress hormones adrenaline and cortisol strongly affect their reasoning and cognition. At low levels, cortisol facilitates thinking and other mental functions, so well-timed pressure to perform and targeted critiques of subordinates certainly have their place. When a leader’s demands become too great for a subordinate to handle, however, soaring cortisol levels and an added hard kick of adrenaline can paralyze the mind’s critical abilities. Attention fixates on the threat from the boss rather than the work at hand memory, planning, and creativity go out the window. People fall back on old habits, no matter how unsuitable those are for addressing new challenges.

Poorly delivered criticism and displays of anger by leaders are common triggers of hormonal surges. In fact, when laboratory scientists want to study the highest levels of stress hormones, they simulate a job interview in which an applicant receives intense face-to-face criticism—an analogue of a boss’s tearing apart a subordinate’s performance. Researchers likewise find that when someone who is very important to a person expresses contempt or disgust toward him, his stress circuitry triggers an explosion by stress hormones and a spike in heart rate of 30 to 40 beats per minute. Then, because of the interpersonal dynamic of mirror neurons and oscillators, the tension spreads to other people. Before you know it, the destructive emotions have infected an entire group and inhibited its performance.

Leaders are themselves not immune to the contagion of stress. All the more reason they should take the time to understand the biology of their emotions.

Bird brains are dense&mdashwith neurons

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Birds are smart. They use tools, engage in social learning, plan for the future, and do a variety of other things that were once thought to be exclusively the stuff of primates. But hundreds of millions of years of evolution separate mammals and birds, and structurally, their brains look very distinct. Plus there's the whole size thing. If you look at a bird's head, it's clear that there's not a whole lot of space for mental hardware in it. So how do the birds manage with smaller brains?

Further Reading

While other studies have tackled a lot of the structural differences, a new one released this week in PNAS shows that, to some extent, size doesn't matter. Its authors show that birds pack neurons into their brains at densities well above densities in mammals' brains, putting some relatively compact bird brains into the same realm as those of primates when it comes to total cell counts.

And the funny thing is, we probably should have known this was the case.

If you look at a typical avian brain without knowing much about brains, you'll mostly be impressed by the size (or lack of it). Some of the heaviest brains in birds are found in the macaws, and those weigh in at under 25 grams. The raven, a large bird with a well-deserved reputation for intelligence, has a brain that is typically around 15g. That's in the same neighborhood as a rabbit.

If you know your way around some neuroanatomy, however, other things will stand out. Many of the structures we associate with higher cognition in mammals (and especially in primates) either aren't clearly there or look rather different in birds, which suggests that bird cognition has to be radically different from the cognition in mammals.

But as we have identified the proteins that act as key regulators of mammalian brain development, we have discovered that the same proteins are all there in birds, too. Tracking their expression as the brain develops has allowed us to determine that some of the brain structures that look physically different in birds and mammals actually have the same developmental history and express the same suite of genes when mature. Finally, manipulating the activity of these genes affects bird and mammalian brains in similar ways.

So, all the same basic pieces seem to be there in both birds and mammals, which leaves the issue of raw horsepower. Mammalian brains are simply so much bigger that it seems inevitable that they could get more done.

But size isn't everything. Neural capabilities seem to be based on the number of neurons present, as well as the number of connections they can establish. Could birds simply cram more neurons into the same amount of physical space and thus get more done with a smaller brain?

We should have expected that answer to be yes. It turns out that flying animals tend to reduce the size of their genomes compared to their non-flying kin. This is the case for both bats and birds. One consequence of this smaller genome is that the cells that carry these genomes end up smaller as well. That tendency has been used to argue that the group of dinosaurs that evolved into birds had already been experiencing a shrinking genome for millions of years beforehand.

Further Reading

But we could just as easily have applied that logic to neurons. If birds' cells are smaller, more cells can be squeezed into the same volume. Under those circumstances, a small brain wouldn't be as much of a liability as it appears. But logic only gets you so far, so a team of researchers set out to try to count all the neurons in the brains of a range of birds, mostly from the songbirds, corvids, and parrots.

Small songbirds, which weigh as little as 4.5g, really do have small brains. Their brains can weigh as little as a third of a gram and only contain about 100 million neurons. But the heavier birds can have brains that weigh more than a dozen grams and pack in more than 2 billion neurons. On average, birds have twice as many neurons per unit mass as mammals do. So a bird called the goldcrest, which Wikipedia introduces as "a very small passerine bird," weighs a bit more than 10 percent of your average mouse but has more than double the neurons.

The largest parrot brains, by contrast, weigh in at 20g, even though parrot body mass is similar to the heaviest songbirds. The parrot brain also has more than 3 billion neurons. In fact, when it comes to the largest corvids and parrots, the authors write that "their total numbers of neurons are comparable to those of small monkeys or much larger ungulates."

Those cells also have an interesting distribution: as more cells are added, they're preferentially added to a region of the brain called the pallium, which in humans handles things like spatial reasoning, language, and memory. As a result, this area has an impressive number of cells. Ravens and keas (a type of parrot that does live in the fjords) have more neurons in the pallium than a Capuchin monkey. A macaw has more than a rhesus monkey.

As with size and weight, there's no simple relationship between the number of neurons in a brain and its capabilities. But the work certainly presents an argument that we shouldn't assume the thought processes of birds have to be limited by their brains' size. And the authors even suggest there might be some advantages with more neurons packed closer together, signals shouldn't typically have to travel as far before reaching their destination. Thus, birds might perform information processing a bit more quickly than mammals.

Neurons By Race

With all of my recent articles on neurons and brain size, I’m now asking the following question: do neurons differ by race? The races of man differ on most all other variables, why not this one?

As we would have it, there are racial differences in total brain neurons.In 1970, an anti-hereditarian (Tobias) estimated the number of “excess neurons” available to different populations for processing bodily information, which Rushton (1988 1997: 114) averaged to find: 8,550 for blacks, 8,660 for whites and 8,900 for Asians (in millions of excess neurons). A difference of 100-200 million neurons would be enough to explain away racial differences in achievement, for one. Two, these differences could also explain differences in intelligence. Rushton (1997: 133) writes:

This means that on this estimate, Mongoloids, who average 1,364 cm3 have 13.767 billion cortical neurons (13.767 x 109 ). Caucasoids who average 1,347 cm3 have 13.665 billion such neurons, 102 million less than Mongoloids. Negroids who average 1,267 cm3 , have 13.185 billion cerebral neurons, 582 million less than Mongoloids and 480 million less than Caucasoids.

Of course, Rushton’s citation of Jerison, I will leave alone now that we know that encephilazation quotient has problems. Rushton (1997: 133) writes:

The half-billion neuron difference between Mongoloids and Negroids are probably all “excess neurons” because, as mentioned, Mongoloids are often shorter in height and lighter in weight than Negroids. The Mongoloid-Negroid difference in brain size across so many estimation procedures is striking

Of course, small differences in brain size would translate to differences differences neuronal count (in the hundreds of millions), which would then affect intelligence.

Moreover, since whites have a greater volume in their prefrontal cortex (Vint, 1934). Using Herculano-Houzel’s favorite definition for intelligence, from MIT physicist Alex Wissner-Gross:

The ability to plan for the future, a significant function of prefrontal regions of the cortex, may be key indeed. According to the best definition I have come across so far, put forward by MIT physicist Alex Wissner-Gross, intelligence is the ability to make decisions that maximize future freedom of action—that is, decisions that keep most doors open for the future. (Herculano-Houzel, 2016: 122-123)

You can see the difference in behavior and action in the races how one race has the ability to make decisions to maximize future ability of action—and those peoples with a smaller prefrontal cortex won’t have this ability (or it will be greatly hampered due to its small size and amount of neurons it has).

With a smaller, less developed frontal lobe and less overall neurons in it than a brain belonging to a European or Asian, this may then account for overall racial differences in intelligence. The few hundred million difference in neurons may be the missing piece to the puzzle here.Neurons transmit information to other nerves and muscle cells. Neurons have cell bodies, axons and dendrites. The more neurons (that’s also packed into a smaller brain, neuron packing density) in the brain, the better connectivity you have between different areas of the brain, allowing for fast reaction times (Asians beat whites who beat blacks, Rushton and Jensen, 2005: 240).

Remember how I said that the brain uses a certain amount of watts well I’d assume that the different races would use differing amount of power for their brain due to differing number of neurons in them. Their brain is not as metabolically expensive. Larger brains are more intelligent than smaller brains ONLY BECAUSE there is a higher chance for there to be more neurons in the larger brain than the smaller one. With the average cranial capacity (blacks: 1267 cc, 13,185 million neurons whites: 1347 cc, 13,665 million neurons, and Asians: 1,364, 13,767 million neurons). (Rushton and Jensen, 2005: 265, table 3) So as you can see, these differences are enough to account for racial differences in achievement.

A bigger brain would mean, more likely, more neurons which would then be able to power the brain and the body more efficiently. The more neurons one has, the more likely it it that they are intelligent as they have more neuronal pathways. The average cranial capcities of the races show that there are neuronal differences between them, which these neuronal differences then are the cause for racial differences, with the brain size itself being only a proxy, not an actual indicator of intelligence. The brain size doesn’t matter as much as the amount of neurons in the brain.

A difference in the brain of 100 grams is enough to account for 550 million cortical neurons (!!) (Jensen, 1998b: 438). But that ignores sex differences and neuronal density. However, I’d assume that there will be at least small differences in neuron count, especially from Rushton’s data from Race, Evolution and Behavior. Jensen (1998) also writes on page 439:

I have not found any investigation of racial differences in neuron density that, as in the case of sex differences, would offset the racial difference in brain weight or volume.

So neuronal density by brain weight is a great proxy.

Racial differences in intelligence don’t come down to brain size they come down to total neuron amount in the brain differences in size in certain parts of the brain critical to intelligence and amount of neurons in those critical portions of the brain. I’ve yet to come across a source talking about the different number of neurons in the brain by race, but when I do I will update this article. From what we know, we can make the assumption that blacks have less packing density as well as a smaller number of neurons in their PFC and cerebral cortex. Psychopathy is associated with abnormalities in the PFC maybe, along with less intelligence, blacks would be more likely to be psychopathic? This also echoes what Richard Lynn says about Race and Psychopathic Personality:

There is a difference between blacks and whites—analogous to the difference in intelligence—in psychopathic personality considered as a personality trait. Both psychopathic personality and intelligence are bell curves with different means and distributions among blacks and whites. For intelligence, the mean and distribution are both lower among blacks. For psychopathic personality, the mean and distribution are higher among blacks. The effect of this is that there are more black psychopaths and more psychopathic behavior among blacks.

Neuronal differences and size of the PFC more than account for differences in psychopathy rates as well as differences in intelligence and scholastic achievement. This could, in part, explain the black-white IQ gap. Since the total number of neurons in the brain dictates, theoretically speaking, how well an organism can process information, and blacks have a smaller PFC (related to future time preference) and since blacks have less cortical neurons than Whites or Asians, this is one large reason why black are less intelligent, on average, than the other races of Man.


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