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Cited by 1,781Primate Behavior and EcologyNeurogenesis and neuroplasticity mechanismsMemory and Neural Mechanisms

The human brain in numbers: a linearly scaled-up primate brain

Open access
Herculano-Houzel, Suzana·2009·Frontiers in Human Neuroscience
DOI 10.3389/neuro.09.031.2009PMC PMC2776484
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Abstract

The human brain has often been viewed as outstanding among mammalian brains: the most cognitively able, the largest-than-expected from body size, endowed with an overdeveloped cerebral cortex that represents over 80% of brain mass, and purportedly containing 100 billion neurons and 10x more glial cells. Such uniqueness was seemingly necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales. However, our recent studies using a novel method to determine the cellular composition of the brain of humans and other primates as well as of rodents and insectivores show that, since different cellular scaling rules apply to the brains within these orders, brain size can no longer be considered a proxy for the number of neurons in the brain. These studies also showed that the human brain is not exceptional in its cellular composition, as it was found to contain as many neuronal and non-neuronal cells as would be expected of a primate brain of its size. Additionally, the so-called overdeveloped human cerebral cortex holds only 19% of all brain neurons, a fraction that is similar to that found in other mammals. In what regards absolute numbers of neurons, however, the human brain does have two advantages compared to other mammalian brains: compared to rodents, and probably to whales and elephants as well, it is built according to the very economical, space-saving scaling rules that apply to other primates; and, among economically built primate brains, it is the largest, hence containing the most neurons. These findings argue in favor of a view of cognitive abilities that is centered on absolute numbers of neurons, rather than on body size or encephalization, and call for a re-examination of several concepts related to the exceptionality of the human brain.

Integrity signals

Registry

Crossref · 10.3389/neuro.09.031.2009

Cites

75 of 75 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

9 of 76 cited sources checked · 4 supported · 0 flagged · accuracy not yet measured

  • Premisecitation

    In the gray matter of the cerebral cortex, glial cells outnumber neurons by less than a factor of 2.

    “In the gray matter of the cerebral cortex, glial cells outnumber neurons by a factor of <2 (Sherwood et al., 2006; Pelvig et al., 2008).”
    Quote in the paperNot a substantial claim1 of 2 sources checked
    • cites Sherwood et al., 2006 source

      Table 2 (Results; page unavailable) reports glia–neuron ratios below 2 in several sampled frontal-cortex areas, including 1.65 in human area 9L. But the ratio is 2.19 in human area 4, and some sampled ratios are below 1, so the paper does not establish that glia outnumber neurons by a factor of less than 2 throughout cerebral cortical gray matter. (LLM, conf=0.90)

      Experimental — accuracy not yet measured
    • cites Pelvig et al., 2008 source
  • Premisecitation

    There is no significant correlation between brain size and general cognitive ability within families.

    “there is no significant correlation between brain size and general cognitive ability within families (Schoenemann et al., 2000)”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Schoenemann et al., 2000 source
  • Findingcitation

    The number of neurons in the cerebral cortex increases coordinately with the number of neurons in the cerebellum.

    “Instead, the number of neurons in the cerebral cortex increases coordinately with the number of neurons in the cerebellum (Herculano-Houzel, submitted).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Herculano-Houzel, submitted

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • Findingown data

    The human brain's cellular makeup is unexceptional: its neuronal and non-neuronal cell counts match those expected for a primate brain of the same size.

    “the human brain is not exceptional in its cellular composition, as it was found to contain as many neuronal and non-neuronal cells as would be expected of a primate brain of its size.”
    Quote in the paperSubstantial claim
  • Findingown data

    The fractional number of neurons in the cerebral cortex relative to the whole brain is not correlated with the relative size of the cerebral cortex.

    “this fractional number of neurons in the cerebral cortex relative to the whole brain is not correlated with the relative size of the cerebral cortex (Figure 2).”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim. 20
  • May combine two claims. 48

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Among birds, larger-brained corvids, parrots, and owls are considered the most intelligent.

    “Among birds, those that are larger-brained (corvids, parrots and owls) are also considered the most intelligent (Lefebvre et al., 2004).”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites Lefebvre et al., 2004 source

      The “Neural and Cognitive Correlates of Innovation in Birds” section identifies corvids, parrots, and owls among larger-brained birds and reports a general association between brain size and cognitive measures. It does not establish that these three groups are considered the most intelligent; in particular, the passage listing owls does not link them to superior cognition. Page numbers are unavailable in our copy. (LLM, conf=0.84)

      Experimental — accuracy not yet measured

  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationcitation

    Absolute brain mass and number of neurons must be considered because the exceeding number of neurons in a large brain should be larger than in a smaller brain of the same EQ.

    “Absolute brain mass and number of neurons, left out of the encephalization equation, must clearly be taken into consideration, since the “exceeding number of neurons” in a large brain should necessarily be larger than that in a smaller brain of same EQ (Herculano-Houzel, 2007).”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Herculano-Houzel, 2007 source

      The ‘Implications for Volume Comparisons Across Orders’ section says that comparisons of brain size should consider neuronal counts directly, not just brain volume. The paper also relates larger primate brain structures to greater numbers of neurons. It does not address encephalization quotient (EQ) or establish that a larger brain has more ‘exceeding’ neurons than a smaller brain with the same EQ; no page numbers are available in our copy. (LLM, conf=0.86)

      Experimental — accuracy not yet measured
    • Rests on an assumption: Cognitive abilities can be meaningfully compared across species.

  • May combine two claims.

    Interpretationcitation

    Because EQ is sensitive to the species included and the human brain conforms to primate scaling rules, the authors suggested that great apes such as orangutans and gorillas have bodies much larger than expected for primates of their brain size, rather than humans having a larger brain than expected.

    “we have suggested that, rather than humans having a larger brain than expected, it is the great apes such as orangutans and, more notably, gorillas that have bodies that are much larger than expected for primates of their brain size (Herculano-Houzel et al., 2007).”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Herculano-Houzel et al., 2007 source

      Split into 2 atoms — 0 confirmed, 2 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

      Experimental — accuracy not yet measured

  • May combine two claims.

    Premisecitation

    Primate brain size grows approximately isometrically with neuron number, with no systematic change in neuronal density or in the non-neuronal-to-neuronal ratio as brain size increases.

    “primate brain size increases approximately isometrically as a function of neuron number, with no systematic change in neuronal density or in the non-neuronal/neuronal ratio with increasing brain size”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Herculano-Houzel et al., 2007 source

  • May combine two claims.

    Findingcitation

    In Rodentia, brain size increases faster than neuron number, with neuronal density decreasing; with constant non-neuronal density, this implies average neuronal size increases rapidly as neurons become more numerous.

    “In the order Rodentia, we find that the brain increases in size faster than it gains neurons, with a decrease in neuronal densities which, in the presence of constant non-neuronal cell densities, implies that average neuronal size increases rapidly as neurons become more numerous (Herculano-Houzel et al., 2006).”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Herculano-Houzel et al., 2006 source

  • Classified as probably not a substantial claim.

    Premisecitation

    Different mammalian orders have traditionally been pooled in brain allometry studies as if their brains were built according to the same scaling rules.

    “Different mammalian orders have traditionally been pooled together in studies of brain allometry, as if their brains were built according to the same scaling rules (for example, Haug, 1987; Zhang and Sejnowski, 2000).”
    Quote in the paperSubstantial claim1 of 2 sources checked
    • cites Haug, 1987 source
    • cites Zhang and Sejnowski, 2000 source

  • May combine two claims.

    Findingcitation

    The cerebral cortex varies in relative size from 42% in the mouse to 82% in the human, but contains between 13 and 28% of all brain neurons in 15 of 18 species studied, ranging from 13% in moles to 41% in the squirrel monkey.

    “the cerebral cortex, despite varying in relative size from 42% (in the mouse) to 82% of brain mass (in the human), contains between 13 and 28% of all brain neurons in 15 of 18 species studied, ranging between 13% (in moles) and 41% (in the squirrel monkey; Herculano-Houzel et al., 2006, 2007; Sarko et al., 2009).”
    Quote in the paperSubstantial claim1 of 3 sources checked
    • cites Herculano-Houzel et al., 2006 source

      The Results (page not provided) report that cortical mass increases with brain size while the cortex contains a relatively stable 17.8 ± 3.4% of brain neurons across six rodent species. This supports the general pattern, but the paper does not establish the quoted 42%–82% mass range or the neuron percentages across 18 species, including humans, moles, and squirrel monkeys. (LLM, conf=0.88)

      Experimental — accuracy not yet measured
    • cites Herculano-Houzel et al., 2007

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites Sarko et al., 2009

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingown data

    An informal 2007 survey of senior neuroscientists found that most believed the human brain's cell numbers were already known as about 100 billion neurons and 10 times more glial cells, but none could cite an original reference.

    “an informal survey with senior neuroscientists that we ran in 2007 showed that most believed that the number of cells in the human brain was indeed already known: that we have about 100 billion neurons, outnumbered by about 10 times more glial cells – but none of the consulted scientists could cite an original reference for these numbers”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The human cerebral cortex contains only 19% of all brain neurons, a proportion similar to that in other mammals.

    “the so-called overdeveloped human cerebral cortex holds only 19% of all brain neurons, a fraction that is similar to that found in other mammals.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The human cerebral cortex represents 82% of brain mass but holds only 19% of all neurons in the human brain.

    “the human cerebral cortex, which represents 82% of brain mass, holds only 19% of all neurons in the human brain”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    Examination of the cerebellum of orangutans and one gorilla shows that the sizes of the cerebellum and cerebral cortex predicted from cerebellar cell number match their actual sizes, suggesting these animals' brains are built according to the same scaling rules as humans and other primates.

    “An examination of the cellular composition of the cerebellum of orangutans and one gorilla shows that the sizes of the cerebellum and cerebral cortex predicted for these species from the number of cells in the cerebellum match their actual sizes, which suggests that the brain of these animals indeed is built according to the same scaling rules that apply to humans and other primates (Herculano-Hou”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Herculano-Houzel and Kaas, in preparation

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingown data

    In larger rodent and primate brains, the absolute number of cerebral cortical neurons increases much faster than the number in the combined brainstem, diencephalon, and basal ganglia, and cerebellar neuron number increases similarly fast.

    “the absolute number of neurons in the rodent and primate cerebral cortex does increase much faster in larger brains compared to the number of neurons in the combined brainstem, diencephalon and basal ganglia, and is accompanied by a similarly fast increase in the number of neurons in the cerebellum (Figure 5).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    The human brain follows the economical, space-saving scaling rules characteristic of other primates, unlike rodents and probably whales and elephants.

    “compared to rodents, and probably to whales and elephants as well, it is built according to the very economical, space-saving scaling rules that apply to other primates”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    Roth and Dicke concluded that across species the factors correlating better with intelligence are the number of cortical neurons and conduction velocity.

    “led two authors to conclude that the “factors that correlate better with intelligence (across species) are the number of cortical neurons and conduction velocity, as the basis for information processing” (Roth and Dicke, 2005).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Roth and Dicke, 2005 source

  • May combine two claims.

    Premisecitation

    Many original articles, reviews, and textbooks state that the human brain has 100 billion neurons and 10 times more glial cells, usually without citing references.

    “Many original articles, reviews and textbooks affirm that we have 100 billion neurons and 10 times more glial cells (Kandel et al., 2000; Ullian et al., 2001; Doetsch, 2003; Nishiyama et al., 2005; Noctor et al., 2007; Allen and Barres, 2009), usually with no references cited.”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kandel et al., 2000

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites Ullian et al., 2001 source
    • cites Doetsch, 2003 source

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    Williams and Herrup estimated the human brain has about 85 billion neurons: 12-15 billion in the telencephalon, 70 billion cerebellar granule cells, and fewer than 1 billion in the brainstem.

    “These authors estimated the number of neurons in the human brain at about 85 billion: 12–15 billion in the telencephalon (Shariff, 1953), 70 billion in the cerebellum, as granule cells (based on Lange, 1975), and fewer than 1 billion in the brainstem.”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Williams and Herrup (1988) source
    • cites Shariff, 1953 source
    • cites Lange, 1975 source

  • May combine two claims.

    Interpretationreasoning

    Among primate brains built by economical scaling rules, the human brain is the largest and therefore has the most neurons.

    “among economically built primate brains, it is the largest, hence containing the most neurons.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    Because mammalian orders follow distinct rules for how cell numbers scale with brain mass, the mass of a brain does not indicate its neuron count.

    “since different cellular scaling rules apply to the brains within these orders, brain size can no longer be considered a proxy for the number of neurons in the brain.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Because cerebellar glial cells are outnumbered by neurons at least 25:1, the ubiquitous claim of 10 times more glial than neuronal cells in the whole human brain would require nearly one trillion glial cells in the remaining structures, which is unlikely since those structures are <10% of brain mass.

    “the only possible explanation for the ubiquitous quote of 10 times more glial than neuronal cells in the entire human brain would be the presence of nearly one trillion glial cells in the remaining structures alone – an unlikely scenario, since these structures represent <10% of total brain mass.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Andersen et al., 1992 source

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Premisenone given

    Monkeys have much smaller brains than ungulates but greater cognitive and behavioral flexibility; macaques outperform cows or horses despite the latter having 4-5 times larger brains.

    “Monkeys, for instance, possess brains that are much smaller than those of ungulates, but the higher cognitive and behavioral flexibility of monkeys over ungulates is anecdotally evident to any observer who compares the ingenious and complex abilities of macaques to those of cows or horses, even though the latter have 4–5× larger brains than macaques.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Cognitive abilities can be meaningfully compared across species.

  • Classified as probably not a substantial claim.

    Premisecitation

    Across species, the number of neurons and their relative abundance in different brain parts is widely considered a determinant of neural function and behavior.

    “Across species, the number of neurons and their relative abundance in different parts of the brain is widely considered to be a determinant of neural function and, consequently, of behavior (Williams and Herrup, 1988).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Williams and Herrup, 1988 source

  • May combine two claims.

    Interpretationown data

    The cellular scaling rules derived from a set of primate species extend to primates as a whole and can be used to infer the expected cellular composition of the human brain.

    “This is evidence that the cellular scaling rules considered here from a set of primate species also extend to primates as a whole, and can be used to infer the expected cellular composition of the human brain”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    In insectivores, the cerebellum scales linearly in size with neuron number, while the cerebral cortex scales hypermetrically as it gains neurons.

    “Across insectivore species, on the other hand, the cerebellum increases linearly in size as a function of its number of neurons (as in primates), while the cerebral cortex increases in size hypermetrically as it gains neurons (as in rodents; Sarko et al., 2009).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Sarko et al., 2009

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Interpretationcitation

    These findings corroborate previous studies showing neuronal density decreases and the glia-to-neuron ratio increases with brain size across mammalian taxa.

    “These findings corroborated previous studies describing neuronal density decreasing and the glia-to-neuron ratio increasing with increasing brain size across mammalian taxa (Tower and Elliot, 1952; Shariff, 1953; Friede, 1954; Tower, 1954; Hawking and Olszewski, 1957; Haug, 1987; Reichenbach, 1989; Stolzenburg et al., 1989).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Tower and Elliot, 1952 source
    • cites Shariff, 1953 source
    • cites Friede, 1954

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Interpretationown data

    The authors' quantitative studies on cellular scaling rules have shown that the assumption that brain size relates to neuron number similarly across orders is invalid and should no longer be applied.

    “our quantitative studies on the cellular scaling rules that apply to different mammalian orders have shown that this assumption is invalid and therefore should no longer be applied”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    For similar-sized brains, rodents perform more poorly than primates; the capuchin monkey, with a 52 g brain, has an outstanding behavioral, social, and cognitive repertoire compared to the capybara, with a 75 g brain.

    “With a brain of only 52 g, the behavioral, social and cognitive repertoire of the capuchin monkey is outstanding compared to the capybara, a giant Amazonian rodent (MacDonald, 1981), even though the latter has a larger brain of 75 g.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites MacDonald, 1981 source
    • Rests on an assumption: Cognitive abilities can be meaningfully compared across species.

  • May combine two claims.

    Premisecitation

    Capuchin monkeys have much larger EQs than gorillas but are outranked by gorillas in cognitive performance.

    “Capuchin monkeys, for instance, have much larger EQs than gorillas (Marino, 1998), but are outranked by these in cognitive performance (Deaner et al., 2007).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Marino, 1998 source
    • cites Deaner et al., 2007 source
    • Rests on an assumption: Cognitive abilities can be meaningfully compared across species.

  • Classified as probably not a substantial claim.

    Premisecitation

    If primates have big brains merely because they have small bodies, one cannot presume this represents an evolutionary trend driven by cognitive demands.

    “if primates have big brains merely because they have small bodies, we cannot presume that this represents an evolutionary trend driven by cognitive demands”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Deacon, 1997 source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Brain size varies across mammals by a factor of approximately 100,000.

    “Brain size varies across mammals by a factor of approximately 100,000 (Tower, 1954; Stolzenburg et al., 1989).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Tower, 1954 source
    • cites Stolzenburg et al., 1989 source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Compared to other mammalian orders, primate encephalization results from a shift in postcranial growth processes, not a modification of brain growth.

    “compared to other mammalian orders, primate encephalization is the result of a shift in postcranial growth processes, not a modification of brain growth (Deacon, 1997).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Deacon, 1997 source

  • May combine two claims.

    Premisecitation

    Compared to mammals as a whole, humans have the largest encephalization quotient, between 7 and 8.

    “Compared to mammals as a whole, humans have the largest EQ found for any mammal, of between 7 and 8 (Jerison, 1973)”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Jerison, 1973

  • May combine two claims.

    Premisereasoning

    The human brain mass represents about 2% of body mass, like other non-anthropoid primates.

    “like in other non-anthropoid primates, the human brain mass represents about 2% of body mass.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    A generic rodent brain of human-sized proportions, weighing 1.5 kg, would have only 12 billion neurons and 46 billion non-neuronal cells.

    “a generic rodent brain of human-sized proportions, weighing 1.5 kg: such a brain would have only 12 billion neurons, and a much larger number of 46 billion non-neuronal cells”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The distribution of cortical mass in humans may differ from other primates, giving regions such as area 10 relatively more neurons in the human cortex.

    “the distribution of cortical mass in humans may differ from that in other primates, endowing particularly relevant regions such as area 10 with relatively more neurons in the human cortex (Semendeferi et al., 2001).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Semendeferi et al., 2001 source

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Premisenone given

    Gorillas and orangutans overlap or exceed humans in body size, but their brains are about one-third the size of the human brain.

    “although gorillas and orangutans overlap or exceed humans in body size, their brains amount to only about one-third of the size of the human brain.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Compared to anthropoid primates only, humans have an encephalization quotient over 3, larger than that of any other primate or cetacean.

    “even if compared to anthropoid primates only, humans still have an EQ of over 3, a value that is larger than that obtained for any other primate or cetacean (Marino, 1998).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Marino, 1998 source

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Premisenone given

    The concept of encephalization presupposes that all brains scale the same way, so brain size and its deviation from expected are the only informative variables.

    “The very concept of encephalization presupposes that not only the brain scales as a function of body size, but that all brains scale the same way, such that the only informative (and sufficient) variable is brain size and its deviation from the expected.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    The human cerebral cortex accounts for 82% of brain mass.

    “its relatively large cerebral cortex, which accounts for 82% of brain mass.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    According to the encephalization quotient notion, small-brained animals with very large EQs should have more cognitive abilities than large-brained animals with smaller EQs.

    “according to this notion, small-brained animals with very large EQs should be expected to have more cognitive abilities than large-brained animals with smaller EQs.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Cognitive abilities can be meaningfully compared across species.

  • May combine two claims.

    Interpretationreasoning

    Primate brains scale in a more space-saving and economical way than rodent brains because average neuronal size stays invariant as primate brain size changes, whereas in rodents larger neuron numbers come with larger neurons.

    “By maintaining the average neuronal size (including all arborizations) invariant as brain size changes, primate brains scale in size in a much more space-saving, economical manner compared to the inflationary growth that occurs in rodents, in which larger numbers of neurons are accompanied by larger neurons.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    The assumption that brain size relates to number of neurons similarly across orders underlies most comparative studies to date.

    “This assumption, which was justifiable by the lack of direct estimates of the neuronal composition of the brain of different species, is so widespread that it implicitly or explicitly underlies most comparative studies to date (for example, Haug, 1987; Finlay and Darlington, 1995; Barton and Harvey, 2000; Clark et al., 2001).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Haug, 1987 source
    • cites Finlay and Darlington, 1995 source
    • cites Barton and Harvey, 2000 source

  • Classified as probably not a substantial claim.

    Premisecitation

    The human brain continues to grow as though in a larger body.

    “the human brain exhibits a further modification in that it continues to grow as though in a larger body (Deacon, 1997).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Deacon, 1997 source

  • May combine two claims.

    Interpretationreasoning

    Sheer size alone, or size relative to body size, is not an adequate parameter to qualify or disqualify the human brain as special.

    “sheer size alone, or in relation to body size, is not an adequate parameter to qualify, or disqualify, the human brain as “special””
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Among primates, humans have the largest living brain and therefore enjoy the largest number of neurons from which to derive cognition and behavior as a whole.

    “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”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Enlargement of the cerebral cortex is not exclusive to humans; both rodent and primate cellular scaling rules predict a similar expansion of cerebral cortex mass relative to whole brain, regardless of cortical neuron number.

    “the enlargement of the cerebral cortex is not, in principle, an exclusive feature of the human brain: a similar expansion of the mass of the cerebral cortex, relative to the whole brain, is predicted by both the rodent and primate cellular scaling rules, irrespective of the number of neurons contained in the cortex (Table 2).”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Interpretationreasoning

    If the same linear scaling rules apply to great apes as to other primates, then similar threefold differences in brain size and brain neuron number apply to humans compared with gorillas, and to gorillas compared with baboons.

    “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”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Azevedo et al. found that the adult male human brain, averaging 1.5 kg, has 86 billion neurons and 85 billion non-neuronal cells, deviating from expected values by 7% and 24%.

    “Azevedo et al. (2009), who found that the adult male human brain, at an average of 1.5 kg, has 86 billion neurons and 85 billion non-neuronal cells – numbers that deviate from the expected by 7 and 24% only.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Azevedo et al. (2009) source

  • May combine two claims.

    Interpretationreasoning

    The human brain scales as a primate brain, and this economical scaling 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.

    “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”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    A generic 1.5-kg primate brain should have a cerebral cortex of about 1.4 kg with 25 billion neurons, and a cerebellum weighing 120 g with 70 billion neurons.

    “This generic primate brain should have a cerebral cortex of about 1.4 kg, containing 25 billion neurons, and a cerebellum weighing 120 g, with 70 billion neurons (Table 2).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Given rodent cellular scaling rules, a hypothetical rodent brain with 86 billion neurons would be predicted to weigh 35 kg, beyond the largest known brain mass of 9 kg for the blue whale and probably physiologically unattainable.

    “a hypothetical rodent brain with 86 billion neurons, like the human brain, would be predicted to weigh overwhelming 35 kg – a value that is way beyond the largest known brain mass of 9 kg for the blue whale, and probably physiologically unattainable”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    As studies of non-human primate and other large-brained animal cognition progress, it becomes increasingly likely that humans do not have truly unique cognitive abilities and differ from these animals not qualitatively but in the combination and extent of abilities such as theory of mind, imitation, and social cognition.

    “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).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Marino et al., 2009

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Men have been reported to have more neurons in the cerebral cortex than women.

    “although men have been reported to have more neurons in the cerebral cortex than women (Pakkenberg and Gundersen, 1997; Pelvig et al., 2008)”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Pakkenberg and Gundersen, 1997

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites Pelvig et al., 2008 source

  • May combine two claims.

    Interpretationreasoning

    Because the power laws relating brain size and neuron number diverge between rodents and primates, primates can hold more neurons in the same brain volume, with higher neuronal densities than rodents.

    “Because of the diverging power laws that relate brain size and number of neurons across rodents and primates, the latter can hold more neurons in the same brain volume, with larger neuronal densities than found in rodents.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The increase in cognitive abilities afforded by increasing neuron number can be expected to increase exponentially with absolute neuron number, and might be subject to a thresholding effect once critical points of information processing are reached.

    “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”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Because neuronal density does not scale with brain size in primates but decreases with increasing brain size in rodents, the larger the brain, the larger the difference in neuron number between similar-sized rodent and primate brains.

    “Since neuronal density does not scale with brain size in primates, but decreases with increasing brain size in rodents, the larger the brain size, the larger is the difference in number of neurons across similar-sized rodent and primate brains.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    The human cerebral cortex averages 1233 g and 16 billion neurons, slightly below expectations for a 1.5-kg primate brain.

    “The human cerebral cortex, with an average 1233 g and 16 billion neurons, is slightly below expectations for a primate brain of 1.5 kg”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Glial cells in the human brain are at most 50% of all brain cells, a characteristic shared with other primates.

    “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)”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Azevedo et al., 2009 source

  • May combine two claims.

    Premisecitation

    The extent of intraspecific variation in brain size is much smaller, on the order of 10-50%, than interspecific variation, which spans five orders of magnitude within mammals.

    “Not only the extent of intraspecific variation is much smaller (on the order of 10–50%) than interspecific variation (which spans five orders of magnitude within mammals; Tower, 1954; Stolzenburg et al., 1989)”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Tower, 1954 source
    • cites Stolzenburg et al., 1989 source

  • May combine two claims.

    Premisecitation

    The human cerebellum averages 154 g and 69 billion neurons, matching or slightly exceeding expectations for a 1.5-kg primate brain.

    “while the human cerebellum, at 154 g and 69 billion neurons, matches or even slightly exceeds the expected (Table 2)”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    According to one estimate, the false killer whale and the African elephant would have about 11 billion neurons in the cerebral cortex, fewer than the 11.5 billion estimated for the human cerebral cortex, though only marginally so.

    “According to one estimate, the false killer whale and the African elephant would have about 11 billion neurons in the cerebral cortex, despite their large size – and fewer neurons than the 11.5 billion estimated by the same method for the human cerebral cortex, though only marginally so (Roth and Dicke, 2005).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Roth and Dicke, 2005 source

  • May combine two claims.

    Interpretationcitation

    According to primate cellular scaling rules, a generic 1.5-kg primate brain should have 93 billion neurons and 112 billion non-neuronal cells, so glial cells should make up at most half of all brain cells.

    “According to these rules, a generic primate brain of 1.5 kg should have 93 billion neurons, and 112 billion non-neuronal cells: glial cells, thus, should constitute at most half of all brain cells.”
    Quote in the paperSubstantial claim
61 of 61

  • The paper relies on this without support.

    Humans are more cognitively capable than larger-brained mammals such as elephants and whales.

    “Such uniqueness was seemingly necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales.”

  • The paper relies on this without support.

    Cognitive abilities can be meaningfully compared across species.

    “the higher cognitive and behavioral flexibility of monkeys over ungulates is anecdotally evident to any observer who compares the ingenious and complex abilities of macaques to those of cows or horses”

  • The paper relies on this without support.

    Non-neuronal cell densities are similar across species.

    “In view of the similar non-neuronal cell densities across species”
3 of 3

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Neural network parameters represent a computational analog to biological neural capacity, directly reflecting studies showing strong correlations between brain volume and total neuron count across species, humans, and mice brains.

    “Our neural network parameters represent a computational analog to biological neural capacity, directly reflecting the studies showing strong correlations between brain volume and total neuron count across species, humans, and mice brains (68–70).”

    From What if eye...? Computationally recreating vision evolution (2025) · cites this paper as [68]

    The “Cellular scaling rules for rodent, insectivore, and primate brains” section (page unavailable) relates brain size to neuron count within mammalian orders, including primates and rodents. But “Not all brains are created equal” (page unavailable) explicitly warns that brain size cannot be used indiscriminately as a proxy for neuron count across orders. The cited paper therefore supports an order-specific relationship, not the claim’s unqualified correlation across species; the neural-network analogy is the citing authors’ framing. (LLM, conf=0.86)

    Experimental — accuracy not yet measured

  • The average number of glial cells in the human brain is estimated to be 84.61 ± 9.83 × 10^9, with the population of glial cells in the neocortex estimated at from 18.2 to 38.6 × 10^9.

    “The average number of glial cells in the human brain is estimated to be 84.61 ± 9.83 × 109 (Herculano-Houzel, 2009), with the population of glial cells in the neocortex estimated at from 18.2 to 38.6 × 109”

    From Human Brain/Cloud Interface (2019) · cites this paper as Herculano-Houzel, 2009

    In “The cellular composition of the human brain” (page not provided), the paper reports about 85 billion non-neuronal cells in the adult male human brain, close to the cited estimate. It does not give the stated ±9.83 billion uncertainty there, and non-neuronal cells are not identified specifically as glial cells. The citation is attached to the whole-brain estimate, not the subsequent neocortex range. (LLM, conf=0.85)

    Experimental — accuracy not yet measured

  • Corvid brains are scaled-up songbird brains, just as human brains are to brains of nonhuman primates.

    “We suggest that corvid brains are scaled-up songbird brains, just as humans brains are to brains of nonhuman primates (38, 47)”

    From Birds have primate-like numbers of neurons in the forebrain (2016) · cites this paper as [47]

  • The relative homogeneity of the two largest neuronal structures (cerebellar cortex and cortex) is striking and suggests an evolutionary expansion of a canonical cortical blueprint.

    “the relative homogeneity of the two largest neuronal structures, with ~69 billion (cerebellar cortex) and ~16 billion (cortex) neurons out of the 86 billion neurons in the human brain33, is striking and suggests an evolutionary expansion of a canonical cortical blueprint34.”

    From An anatomically comprehensive atlas of the adult human brain transcriptome (2012) · cites this paper as [33]

  • The neocortex contains approximately 16.34 billion neurons and 164 trillion synapses.

    “The neocortex alone contains approximately 16.34 billion neurons (Herculano-Houzel, 2009) and 164 trillion synapses, points of communication between neurons (Tang et al., 2001).”

    From The Cellular and Molecular Landscapes of the Developing Human Central Nervous System (2016) · cites this paper as Herculano-Houzel, 2009

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Cortical mass is described as both exactly expected and below expected

    Minor

    The conclusion says the human cerebral cortex is exactly the expected size for a 1.5-kg primate brain, but the earlier results say its measured mass is below that expectation.

1 of 1

Cites

Flagged first, then by citations

18 of the 93 references couldn’t be matched to a known paper.

Cited by

Sorted by citations

Related

Authors
Herculano-Houzel, S.
Venue
Frontiers in Human Neuroscience
Publisher
Frontiers Media SA
Published
2009
Volume
3
Pages
31-31
ISSN
1662-5161 (ISSN-L)
DOI
10.3389/neuro.09.031.2009
PMC
PMC2776484
OpenAlex
W2005926822
Cited by
1,778
References
93
Citations by year
2026: 89 · 2025: 159 · 2024: 192 · 2023: 145 · 2022: 133 · 2021: 172 · 2020: 176 · 2019: 144 · 2018: 140 · 2017: 91 · 2016: 77 · 2015: 71 · 2014: 61 · 2013: 50 · 2012: 45
FWCI
33.51
Type
journal article
Language
en
Open access
Licence
cc-by
Locations
  • Frontiers in Human Neuroscience (publishedVersion, cc-by)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
  • Conselho Nacional de Desenvolvimento Científico e Tecnológico
  • Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro
Topics
  • Social Sciences › Psychology › Social Psychology › Primate Behavior and Ecology
  • Life Sciences › Neuroscience › Developmental Neuroscience › Neurogenesis and neuroplasticity mechanisms
  • Life Sciences › Neuroscience › Cognitive Neuroscience › Memory and Neural Mechanisms
Keywords
human brain, primate brain, brain size, neuron number, glial cells, cerebral cortex, encephalization, cognitive ability, rodents, elephants, whales

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-09-28.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-09-28.

Cited sources are checked by gpt-6-sol.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

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