Brain size

In humans, the right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically closer in size. The adult human brain weighs on average about 1.3 kg.{{cite book |title=Carpenter's Human Neuroanatomy |last=Parent |first=A |author2=Carpenter MB |publisher=Williams & Wilkins |year=1995 |isbn=978-0-683-06752-1 |chapter=Ch. 1}} In men the average weight is about 1370 g and in women about 1200 g.{{cite journal |last1=Harrison |first1=Paul J. |last2=Freemantle |first2=Nick |last3=Geddes |first3=John R. |title=Meta-analysis of brain weight in schizophrenia |journal=Schizophrenia Research |date=November 2003 |volume=64 |issue=1 |pages=25–34 |doi=10.1016/s0920-9964(02)00502-9 |pmid=14511798 |s2cid=3102745 }}{{Contradictory inline|reason=If for men the average is 1370 g and in women about 1200 g, the overall adult average cannot be 1.5 kg.|date=March 2024}} The volume is around 1260 cm³ in men and 1130 cm³ in women, although there is substantial individual variation.{{cite journal |last1=Cosgrove |first1=Kelly P. |last2=Mazure |first2=Carolyn M. |last3=Staley |first3=Julie K. |title=Evolving Knowledge of Sex Differences in Brain Structure, Function, and Chemistry |journal=Biological Psychiatry |date=October 2007 |volume=62 |issue=8 |pages=847–855 |doi=10.1016/j.biopsych.2007.03.001 |pmid=17544382 |pmc=2711771 }} Yet another study found that adult human brain weight is 1300–1400 g for adult humans and 350–400 g for newborn humans. There is a range of volume and weights, and not just one number that one can definitively rely on. Variation between humans of similar age is smaller than that between species. The mechanisms of interspecific and intraspecific variation also differ.

{{See also|Evolution of the brain}}

File:Modern human cranial size over the last 300 ka using data consolidated into 100-year means.jpg

File:Modern human cranial size over the last 30 ka using data consolidated into 100-year means.jpg

From early primates to hominids and finally to Homo sapiens, the brain gets progressively larger - with the exception of extinct Neanderthals whose brain size exceeded that of modern Homo sapiens. The volume of the human brain has increased as humans have evolved (see Homininae), starting from about 600 cm³ in Homo habilis up to 1680 cm³ in Homo neanderthalensis, which was the hominid with the biggest brain size.{{cite web | url =http://www.infoplease.com/encyclopedia/society/neanderthal-man.html | title =Neanderthal man | publisher =infoplease}} Some data suggest that the average brain size has decreased since then.{{cite web |url =https://www.discovermagazine.com/the-sciences/if-modern-humans-are-so-smart-why-are-our-brains-shrinking |url-access =limited |title =If Modern Humans Are So Smart, Why Are Our Brains Shrinking? |first =Kathleen |last =McAuliffe |work =DiscoverMagazine.com |date =2011-01-20 |access-date =2014-03-05}} One study concludes that the decrease "was surprisingly recent, occurring in the last 3,000 years".{{cite journal |last1=Henneberg |first1=Maciej |title=Decrease of human skull size in the Holocene |journal=Human Biology |date=1988 |pages=395–405 |jstor=41464021 |volume=60 |issue=3 |pmid=3134287}} However, a reanalysis of the same data suggests that brain size has not decreased, and that the conclusion was made using datasets that are too dissimilar to support quantitative comparison.{{cite news |last1 =Corless |first1 =Victoria |title =No, the human brain did not shrink |url =https://www.advancedsciencenews.com/no-the-human-brain-did-not-shrink/ |access-date =21 August 2022 |work =Advanced Science News |date =18 August 2022}}{{cite journal |last1 =Villmoare |first1 =Brian |last2 =Grabowski |first2 =Mark |title =Did the transition to complex societies in the Holocene drive a reduction in brain size? A reassessment of the DeSilva et al. (2021) hypothesis | journal=Frontiers in Ecology and Evolution | date=2022 | volume=10 | doi=10.3389/fevo.2022.963568 | issn=2296-701X |doi-access=free |hdl=10852/99818 |hdl-access =free }}

Richerson and Boyd point out disadvantages to large brain-size.

{{cite book

|last1 = Richerson

|first1 = Peter

|author-link1 = Peter Richerson

|last2 = Boyd

|first2 = Robert

|author-link2 = Robert Boyd (anthropologist)

|date = 26 June 2008

|orig-date = 2004

|chapter = Culture is an Adaptation

|title = Not By Genes Alone: How Culture Transformed Human Evolution

|url = https://books.google.com/books?id=dU-KtEVgK6sC

|publication-place = Chicago

|publisher = University of Chicago Press

|page = 135

|isbn = 9780226712130

|access-date = 27 November 2024

|quote = All other things being equal, selection should ruthlessly favor small brains, because large brains are costly. [...] In addition to metabolic requirements, there are other significant costs of big brains, such as increased difficulty at birth, greater vulnerability to head trauma, increased potential for developmental snafus, and the time and trouble to fill them with usable information. In effect, all animals are under stringent selection pressure to be as stupid as they can get away with.

}}

Proponents of recent changes in brain size draw attention to the gene mutation that causes microcephaly, a neural developmental disorder that affects cerebral cortical volume.{{cite journal | last1=Kouprina | first1=Natalay | last2=Pavlicek | first2=Adam | last3=Mochida | first3=Ganeshwaran H | last4=Solomon | first4=Gregory | last5=Gersch | first5=William | last6=Yoon | first6=Young-Ho | last7=Collura | first7=Randall | last8=Ruvolo | first8=Maryellen | last9=Barrett | first9=J. Carl | last10=Woods | first10=C. Geoffrey | last11=Walsh | first11=Christopher A | last12=Jurka | first12=Jerzy | last13=Larionov | first13=Vladimir | title=Accelerated Evolution of the ASPM Gene Controlling Brain Size Begins Prior to Human Brain Expansion | journal=PLOS Biology | date=23 March 2004 | volume=2 | issue=5 | pages=e126 | doi=10.1371/journal.pbio.0020126 | pmid=15045028 | pmc=374243 | doi-access=free }} Similarly, sociocultural explanations draw attention to externalization of knowledge and group decision-making, partly via the advent of social systems of distributed cognition, social organization, division of labor and sharing of information as possible causes.{{cite news | title=When and why did human brains decrease in size 3,000 years ago? Ants may hold clues | url=https://phys.org/news/2021-10-human-brains-decrease-size-years.html | access-date=15 November 2021 | work=phys.org | language=en}}{{cite news | last1=Baraniuk | first1=Chris | title=Why human brains were bigger 3,000 years ago | url=https://www.bbc.com/future/article/20220503-why-human-brains-were-bigger-3000-years-ago | access-date=2 September 2022 | work=BBC | language=en}}{{cite journal | last1=DeSilva | first1=Jeremy M. | last2=Traniello | first2=James F. A. | last3=Claxton | first3=Alexander G. | last4=Fannin | first4=Luke D. | title=When and Why Did Human Brains Decrease in Size? A New Change-Point Analysis and Insights From Brain Evolution in Ants | journal=Frontiers in Ecology and Evolution | date=2021 | volume=9 | pages=712 | doi=10.3389/fevo.2021.742639 | issn=2296-701X |doi-access=free }}

File:Trends in hominin brain size evolution.jpg

File:Specimens of an analysis on human brain size over 9.8-million-years.jpg

Homo floresiensis is a hominin from the island of Flores in Indonesia with fossils dating from 60,000-100,000 years ago.{{cite journal |title =Revised stratigraphy and chronology for Homo floresiensis at Liang Bua in Indonesia |journal =Nature |date = 30 March 2016 |last1 =Sutikna |first1 =Thomas |last2 =Tocheri |first2 =Matthew W. |doi =10.1038/nature17179 |display-authors =etal |pmid =27027286 |volume =532 |issue =7599 |pages =366–9|bibcode =2016Natur.532..366S |s2cid =4469009 |url =https://ro.uow.edu.au/smhpapers/3778 }} Despite its relatively derived position in the hominin phylogeny, CT imaging of its skull reveals that its brain volume was only 417 cm³,{{cite journal |last1 =Falk |first1 =Dean |last2 =Hildebolt |first2 =Charles |last3 =Smith |first3 =Kirk |last4 =Morwood |first4 =M. J. |last5 =Sutikna |first5 =Thomas |last6 =Brown |first6 =Peter |last7 =Jatmiko |last8 =Saptomo |first8 =E. Wayhu |last9 =Brunsden |first9 =Barry |last10 =Prior |first10 =Fred |title= The Brain of LB1, Homo floresiensis |date =8 Apr 2005 |volume =308 |issue =5719 |pages =242–245 |doi =10.1126/science.1109727 |journal =Science |pmid =15749690 |bibcode =2005Sci...308..242F |s2cid =43166136 |url =https://www.science.org/doi/10.1126/science.1109727}} less than that of even Homo habilis, which is believed to have gone extinct far earlier (around 1.65 million years ago.{{cite journal |author1 =F. Spoor |author2 =P. Gunz |author3 =S. Neubauer |author4 =S. Stelzer |author5 =N. Scott |author6 =A. Kwekason |author7 =M. C. Dean | year = 2015 | title = Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo | journal = Nature | volume = 519 |issue= 7541 | pages =83–86 | doi = 10.1038/nature14224 | pmid =25739632 | bibcode =2015Natur.519...83S|s2cid =4470282 }}). The reason for this regression in brain size is believed to be island syndrome,{{cite journal |last1 = Baeckens |first1 = Simon |last2 = Van Damme |first2 = Raoul|date = 20 April 2020|title =The island syndrome |journal = Current Biology |volume = 30 |issue =8 |pages = R329–R339 |doi =10.1016/j.cub.2020.03.029|pmid = 32315628 |doi-access = free |bibcode = 2020CBio...30.R338B }} in which the brains of insular species become smaller due to reduced predation-risk. A smaller brain is beneficial as it reduces the basal metabolic rate without significant increases in predation risk.{{cite journal |last1 =Herculano-Houzel |first1 =Suzana |title =Scaling of Brain Metabolism with a Fixed Energy Budget per Neuron: Implications for Neuronal Activity, Plasticity and Evolution |date =1 March 2011 |doi =10.1371/journal.pone.0017514 |journal =PLOS ONE |volume =6 |issue =3|pages =e17514 |pmid =21390261 |pmc =3046985 |bibcode =2011PLoSO...617514H |doi-access =free }}

Exceptional cases of hydrocephalus, such as that reported by John Lorber in 1980 and by a study with rats,{{cite news |last1 =Bracci |first1 =Aria |title =A rat had basically no brain—but it could still see, hear, smell and feel |url =https://medicalxpress.com/news/2020-01-rat-basically-brainbut.html |access-date =22 November 2021 |work =Northeastern University |language =en}}{{cite journal |last1 =Ferris |first1 =C. F. |last2 =Cai |first2 =X. |last3=Qiao |first3=J. |last4=Switzer |first4=B. |last5=Baun |first5=J. |last6=Morrison |first6=T. |last7=Iriah |first7 =S. |last8 =Madularu |first8 =D. |last9 =Sinkevicius |first9 =K. W. |last10 =Kulkarni |first10 =P. |title =Life without a brain: Neuroradiological and behavioral evidence of neuroplasticity necessary to sustain brain function in the face of severe hydrocephalus |journal =Scientific Reports |date =11 November 2019 |volume =9 |issue =1 |pages =16479 |doi =10.1038/s41598-019-53042-3 |pmid =31712649 |pmc =6848215 |bibcode =2019NatSR...916479F |language =en |issn =2045-2322}} suggest that relatively high levels of intelligence and relatively normal functioning are possible even with very small brains.{{cite journal |last1 =Forsdyke |first1 =Donald R. |title =Wittgenstein's Certainty is Uncertain: Brain Scans of Cured Hydrocephalics Challenge Cherished Assumptions |journal =Biological Theory |date =1 December 2015 |volume =10 |issue =4 |pages =336–342 |doi =10.1007/s13752-015-0219-x |s2cid =9240791 |language =en |issn =1555-5550}}{{cite news |title =Remarkable story of maths genius who had almost no brain |url =https://www.irishtimes.com/news/remarkable-story-of-maths-genius-who-had-almost-no-brain-1.1026845 |access-date =22 November 2021 |newspaper =The Irish Times |language =en}} It is unclear what conclusions to draw from such reports – such as about brain capacities, redundancies, mechanics and size requirements.

Efforts to find racial or ethnic variation in brain size are generally considered to be a pseudoscientific endeavor{{Cite web|url=https://www.discovermagazine.com/planet-earth/lost-research-notes-clear-up-racial-bias-debate-in-old-skull-size-study|title=Lost Research Notes Clear up Racial Bias Debate in Old Skull Size Study}}{{Cite magazine|url=https://www.wired.co.uk/article/superior-the-return-of-race-science-angela-saini|title = The disturbing return of scientific racism|magazine = Wired UK}}{{cite journal |last1=Mitchell |first1=Paul Wolff |title=The fault in his seeds: Lost notes to the case of bias in Samuel George Morton's cranial race science |journal=PLOS Biology |date=4 October 2018 |volume=16 |issue=10 |pages=e2007008 |doi=10.1371/journal.pbio.2007008 |pmid=30286069 |pmc=6171794 |s2cid=52919024 |doi-access=free }} and have traditionally been tied to scientific racism and attempts to demonstrate a racial intellectual hierarchy.Gould, S. J. (1981). The Mismeasure of Man. New York: W. W. Norton & Company.{{page needed|date=November 2020}}{{cite journal |last1=Graves |first1=Joseph L. |title=Great Is Their Sin: Biological Determinism in the Age of Genomics |journal=The Annals of the American Academy of Political and Social Science |date=September 2015 |volume=661 |issue=1 |pages=24–50 |doi=10.1177/0002716215586558 |s2cid=146963288 }}{{cite journal |last1=Kaplan |first1=Jonathan Michael |last2=Pigliucci |first2=Massimo |last3=Banta |first3=Joshua Alexander |title=Gould on Morton, Redux: What can the debate reveal about the limits of data? |journal=Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences |date=1 August 2015 |volume=52 |pages=22–31 |doi=10.1016/j.shpsc.2015.01.001 |pmid=25666493 |url=https://philarchive.org/rec/KAPGOM }}

The majority of efforts to demonstrate this have relied on indirect data that assessed skull measurements as opposed to direct brain observations. These are considered scientifically discredited.{{cite journal |last1=Kamin |first1=Leon J. |last2=Omari |first2=Safiya |title=Race, Head Size, and Intelligence |journal=South African Journal of Psychology |date=September 1998 |volume=28 |issue=3 |pages=119–128 |doi=10.1177/008124639802800301 |s2cid=53117248 }}

A large-scale 1984 survey of global variation in skulls has concluded that variation in skull and head sizes is unrelated to race, but rather climatic heat preservation, stating "We find little support for the use of brain size in taxonomic assessment (other than with paleontological extremes over time). Racial taxonomies which include cranial capacity, head shape, or any other trait influenced by climate confound ecotypic and phyletic causes. For Pleistocene hominids, we doubt that the volume of the braincase is any more taxonomically 'valuable' than any other trait."{{cite journal |last1=Beals |first1=Kenneth L. |last2=Smith |first2=Courtland L. |last3=Dodd |first3=Stephen M. |last4=Angel |first4=J. Lawrence |last5=Armstrong |first5=Este |last6=Blumenberg |first6=Bennett |last7=Girgis |first7=Fakhry G. |last8=Turkel |first8=Spencer |last9=Gibson |first9=Kathleen R. |last10=Henneberg |first10=Maciej |last11=Menk |first11=Roland |last12=Morimoto |first12=Iwataro |last13=Sokal |first13=Robert R. |last14=Trinkaus |first14=Erik |title=Brain Size, Cranial Morphology, Climate, and Time Machines [and Comments and Reply] |journal=Current Anthropology |date=June 1984 |volume=25 |issue=3 |pages= 312 |doi=10.1086/203138 |s2cid=86147507 | url=http://syslearn.oregonstate.edu/instruction/anth/smith/TimeMach1984.pdf }}

File:Brain weight age.gif

A human baby's brain at birth averages 369 cm³ and increases, during the first year of life, to about 961 cm³, after which the growth rate declines. Brain volume peaks at the teenage years,{{cite journal |last1=Giedd |first1=Jay N. |last2=Blumenthal |first2=Jonathan |last3=Jeffries |first3=Neal O. |last4=Castellanos |first4=F. X. |last5=Liu |first5=Hong |last6=Zijdenbos |first6=Alex |last7=Paus |first7=Tomáš |last8=Evans |first8=Alan C. |last9=Rapoport |first9=Judith L. |title=Brain development during childhood and adolescence: a longitudinal MRI study |journal=Nature Neuroscience |date=October 1999 |volume=2 |issue=10 |pages=861–863 |doi=10.1038/13158 |pmid=10491603 |s2cid=204989935 }} and after the age of 40 it begins declining at 5% per decade, speeding up around 70.{{Cite journal|title=Ageing and the brain|journal=Postgraduate Medical Journal|url=http://ncbi.nlm/|last=Peters|first=R.|pmc=2596698|pmid=16461469|doi=10.1136/pgmj.2005.036665|volume=82|issue=964|year=2006|pages=84–8|access-date=2019-09-12|archive-date=2013-07-15|archive-url=https://web.archive.org/web/20130715163844/http://www.ncbi.nlm/|url-status=dead}} Average adult male brain weight is {{convert|1345|g}}, while an adult female has an average brain weight of {{convert|1222|g}}.{{Cite book|url = https://books.google.com/books?id=u1bwQj7qdsYC|title = Reader in Gender archaeology|access-date = 2014-09-21|publisher = Routlegde|author1=Kelley Hays |author2=David S. |isbn = 9780415173605|year = 1998}} (This does not take into account neuron density nor brain-to-body mass ratio; men on average also have larger bodies than women.) Males have been found to have on average greater cerebral, cerebellar and cerebral cortical lobar volumes, except possibly left parietal.{{cite journal |last1=Carne |first1=Ross P. |last2=Vogrin |first2=Simon |last3=Litewka |first3=Lucas |last4=Cook |first4=Mark J. |title=Cerebral cortex: An MRI-based study of volume and variance with age and sex |journal=Journal of Clinical Neuroscience |date=January 2006 |volume=13 |issue=1 |pages=60–72 |doi=10.1016/j.jocn.2005.02.013 |pmid=16410199 |s2cid=20486422 }} The gender differences in size vary by more specific brain regions. Studies have tended to indicate that men have a relatively larger amygdala and hypothalamus, while women have a relatively larger caudate and hippocampi. When covaried for intracranial volume, height, and weight, Kelly (2007) indicates women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There is high variability between individuals in these studies, however.

However, Yaki (2011) found no statistically significant gender differences in the gray matter ratio for most ages (grouped by decade), except in the 3rd and 6th decades of life in the sample of 758 women and 702 men aged 20–69.{{Cite journal | last1 = Taki | first1 = Y. | last2 = Thyreau | first2 = B. | last3 = Kinomura | first3 = S. | last4 = Sato | first4 = K. | last5 = Goto | first5 = R. | last6 = Kawashima | first6 = R. | last7 = Fukuda | first7 = H. | editor1-last = He | editor1-first = Yong | title = Correlations among Brain Gray Matter Volumes, Age, Gender, and Hemisphere in Healthy Individuals | doi = 10.1371/journal.pone.0022734 | journal = PLOS ONE | volume = 6 | issue = 7 | pages = e22734 | year = 2011 | pmid = 21818377| pmc =3144937 | bibcode = 2011PLoSO...622734T | doi-access = free }} The average male in their third decade (ages 20–29) had a significantly higher gray matter ratio than the average female of the same age group. In contrast, among subjects in their sixth decade, the average woman had a significantly larger gray matter ratio, though no meaningful difference was found among those in their 7th decade of life.

Total cerebral and gray matter volumes peak during the ages from 10–20 years (earlier in girls than boys), whereas white matter and ventricular volumes increase. There is a general pattern in neural development of childhood peaks followed by adolescent declines (e.g. synaptic pruning). Consistent with adult findings, average cerebral volume is approximately 10% larger in boys than girls. However, such differences should not be interpreted as imparting any sort of functional advantage or disadvantage; gross structural measures may not reflect functionally relevant factors such as neuronal connectivity and receptor density, and of note is the high variability of brain size even in narrowly defined groups, for example children at the same age may have as much as a 50% differences in total brain volume.{{cite journal |last1=Giedd |first1=Jay N. |title=The Teen Brain: Insights from Neuroimaging |journal=Journal of Adolescent Health |date=April 2008 |volume=42 |issue=4 |pages=335–343 |doi=10.1016/j.jadohealth.2008.01.007 |pmid=18346658 }} Young girls have on average relative larger hippocampal volume, whereas the amygdalae are larger in boys. However, multiple studies{{cite journal |last1=Rabinowicz |first1=Theodore |last2=Petetot |first2=Jean MacDonald-Comber |last3=Gartside |first3=Peter S. |last4=Sheyn |first4=David |last5=Sheyn |first5=Tony |last6=de Courten-Myers |first6=Gabrielle M. |title=Structure of the Cerebral Cortex in Men and Women |journal=Journal of Neuropathology & Experimental Neurology |date=January 2002 |volume=61 |issue=1 |pages=46–57 |doi=10.1093/jnen/61.1.46 |id={{ProQuest|229729071}} |pmid=11829343 |s2cid=16815298 |doi-access=free }}{{cite journal |last1=Alonso-Nanclares |first1=L. |last2=Gonzalez-Soriano |first2=J. |last3=Rodriguez |first3=J. R. |last4=DeFelipe |first4=J. |title=Gender differences in human cortical synaptic density |journal=Proceedings of the National Academy of Sciences of the United States of America |date=23 September 2008 |volume=105 |issue=38 |pages=14615–14619 |doi=10.1073/pnas.0803652105 |jstor=25464278 |pmid=18779570 |pmc=2567215 |bibcode=2008PNAS..10514615A |doi-access=free }} have found a higher synaptic density in males: a 2008 study reported that men had a significantly higher average synaptic density of 12.9 × 108 per cubic millimeter, whereas in women it was 8.6 × 108 per cubic millimeter, a 33% difference. Other studies have found an average of 4 billion more neurons in the male brain,{{cite journal |last1=Pakkenberg |first1=Bente |last2=Gundersen |first2=Hans Jørgen G. |title=Neocortical neuron number in humans: Effect of sex and age |journal=Journal of Comparative Neurology |date=1997 |volume=384 |issue=2 |pages=312–320 |doi=10.1002/(SICI)1096-9861(19970728)384:2<312::AID-CNE10>3.0.CO;2-K |pmid=9215725 |s2cid=25706714 }} corroborating this difference, as each neuron has on average 7,000 synaptic connections to other neurons.

Significant dynamic changes in brain structure take place through adulthood and aging, with substantial variation between individuals. In later decades, men show greater volume loss in whole brain volume and in the frontal lobes, and temporal lobes, whereas in women there is increased volume loss in the hippocampi and parietal lobes. Men show a steeper decline in global gray matter volume, although in both sexes it varies by region with some areas exhibiting little or no age effect. Overall white matter volume does not appear to decline with age, although there is variation between brain regions.{{cite journal |last1=Good |first1=Catriona D. |last2=Johnsrude |first2=Ingrid S. |last3=Ashburner |first3=John |last4=Henson |first4=Richard N.A. |last5=Friston |first5=Karl J. |last6=Frackowiak |first6=Richard S.J. |title=A Voxel-Based Morphometric Study of Ageing in 465 Normal Adult Human Brains |journal=NeuroImage |date=July 2001 |volume=14 |issue=1 |pages=21–36 |doi=10.1006/nimg.2001.0786 |pmid=11525331 |s2cid=6392260 |url=http://pdfs.semanticscholar.org/1043/8bd0e1231dc8bf12ec3f407b5a6a7aa026c9.pdf |archive-url=https://web.archive.org/web/20201117094046/http://pdfs.semanticscholar.org/1043/8bd0e1231dc8bf12ec3f407b5a6a7aa026c9.pdf |url-status=dead |archive-date=2020-11-17 }}

Adult twin studies have indicated high heritability estimates for overall brain size in adulthood (between 66% and 97%). The effect varies regionally within the brain, however, with high heritabilities of frontal lobe volumes (90-95%), moderate estimates in the hippocampi (40-69%), and environmental factors influencing several medial brain areas. In addition, lateral ventricle volume appears to be mainly explained by environmental factors, suggesting such factors also play a role in the surrounding brain tissue. Genes may cause the association between brain structure and cognitive functions, or the latter may influence the former during life. A number of candidate genes have been identified or suggested, but they await replication.{{cite journal |last1=Peper |first1=Jiska S. |last2=Brouwer |first2=Rachel M. |last3=Boomsma |first3=Dorret I. |last4=Kahn |first4=René S. |last5=Hulshoff Pol |first5=Hilleke E. |title=Genetic influences on human brain structure: A review of brain imaging studies in twins |journal=Human Brain Mapping |date=June 2007 |volume=28 |issue=6 |pages=464–473 |doi=10.1002/hbm.20398 |pmid=17415783 |pmc=6871295 }}{{cite journal |last1=Zhang |first1=Jianzhi |title=Evolution of the human ASPM gene, a major determinant of brain size |journal=Genetics |date=December 2003 |volume=165 |issue=4 |pages=2063–2070 |doi=10.1093/genetics/165.4.2063 |pmid=14704186 |pmc=1462882 }}

{{See also|Neuroscience and intelligence#Brain size}}

Studies demonstrate a correlation between brain size and intelligence, larger brains predicting higher intelligence. It is however not clear if the correlation is causal.{{cite journal |last1=Nisbett |first1=Richard E. |last2=Aronson |first2=Joshua |last3=Blair |first3=Clancy |last4=Dickens |first4=William |last5=Flynn |first5=James |last6=Halpern |first6=Diane F. |last7=Turkheimer |first7=Eric |title=Intelligence: New findings and theoretical developments |journal=American Psychologist |date=February 2012 |volume=67 |issue=2 |pages=130–159 |doi=10.1037/a0026699 |pmid=22233090 |s2cid=7001642 |url=https://pdfs.semanticscholar.org/c03f/f20904c35a370534a9d3710453dd6dc7a2d2.pdf |archive-url=https://web.archive.org/web/20191230083128/https://pdfs.semanticscholar.org/c03f/f20904c35a370534a9d3710453dd6dc7a2d2.pdf |url-status=dead |archive-date=2019-12-30 }} The majority of MRI studies report moderate correlations around 0.3 to 0.4 between brain volume and intelligence.{{cite journal |last1=Mcdaniel |first1=M |title=Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence |journal=Intelligence |date=July 2005 |volume=33 |issue=4 |pages=337–346 |doi=10.1016/j.intell.2004.11.005 }}{{cite journal |last1=Luders |first1=Eileen |last2=Narr |first2=Katherine L. |last3=Thompson |first3=Paul M. |last4=Toga |first4=Arthur W. |title=Neuroanatomical correlates of intelligence |journal=Intelligence |date=March 2009 |volume=37 |issue=2 |pages=156–163 |doi=10.1016/j.intell.2008.07.002 |pmid=20160919 |pmc=2770698 }} The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampus, and the cerebellum, but only account for a relatively small amount of variance in IQ, which suggests that while brain size may be related to human intelligence, other factors also play a role.{{cite journal |last1=Hoppe |first1=Christian |last2=Stojanovic |first2=Jelena |title=High-Aptitude Minds |journal=Scientific American Mind |date=August 2008 |volume=19 |issue=4 |pages=60–67 |doi=10.1038/scientificamericanmind0808-60 }} In addition, brain volumes do not correlate strongly with other and more specific cognitive measures.{{cite journal |last1=Allen |first1=John S. |last2=Damasio |first2=Hanna |last3=Grabowski |first3=Thomas J. |title=Normal neuroanatomical variation in the human brain: An MRI-volumetric study |journal=American Journal of Physical Anthropology |date=August 2002 |volume=118 |issue=4 |pages=341–358 |doi=10.1002/ajpa.10092 |pmid=12124914 }} In men, IQ correlates more with gray matter volume in the frontal lobe and parietal lobe, which is roughly involved in sensory integration and attention, whereas in women it correlates with gray matter volume in the frontal lobe and Broca's area, which is involved in language.

Research measuring brain volume, P300 auditory evoked potentials, and intelligence shows a dissociation, such that both brain volume and speed of P300 correlate with measured aspects of intelligence, but not with each other.{{cite journal |last1=Egan |first1=Vincent |last2=Chiswick |first2=Ann |last3=Santosh |first3=Celestine |last4=Naidu |first4=K. |last5=Rimmington |first5=J.Ewen |last6=Best |first6=Jonathan J.K. |title=Size isn't everything: A study of brain volume, intelligence and auditory evoked potentials |journal=Personality and Individual Differences |date=September 1994 |volume=17 |issue=3 |pages=357–367 |doi=10.1016/0191-8869(94)90283-6 }}{{cite journal |last1=Egan |first1=Vincent |last2=Wickett |first2=John C. |last3=Vernon |first3=Philip A. |title=Brain size and intelligence: erratum, addendum, and correction |journal=Personality and Individual Differences |date=July 1995 |volume=19 |issue=1 |pages=113–115 |doi=10.1016/0191-8869(95)00043-6 }} Evidence conflicts on the question of whether brain size variation also predicts intelligence between siblings, as some studies find moderate correlations and others find none. A recent review by Nesbitt, Flynn et al. (2012) points out that crude brain size is unlikely to be a accurate measure of IQ. Brain size is known to differ between men and women, for example (men on average have larger bodies than women), but without well documented differences in IQ. A 2017 study found that the brains of women have a higher density of grey matter, which could compensate for the loss of volume.{{cite web |title=Penn Study Finds Gray Matter Density Increases During Adolescence - Penn Medicine |url=https://www.pennmedicine.org/news/news-releases/2017/may/penn-study-finds-gray-matter-density-increases-during-adolescence |website=www.pennmedicine.org |access-date=31 March 2024}}

A discovery in recent years is that the structure of the adult human brain changes when a new cognitive or motor skill, including vocabulary, is learned.{{cite journal |last1=Lee |first1=H. |last2=Devlin |first2=J. T. |last3=Shakeshaft |first3=C. |last4=Stewart |first4=L. H. |last5=Brennan |first5=A. |last6=Glensman |first6=J. |last7=Pitcher |first7=K. |last8=Crinion |first8=J. |last9=Mechelli |first9=A. |last10=Frackowiak |first10=R. S. J. |last11=Green |first11=D. W. |last12=Price |first12=C. J. |title=Anatomical Traces of Vocabulary Acquisition in the Adolescent Brain |journal=Journal of Neuroscience |date=31 January 2007 |volume=27 |issue=5 |pages=1184–1189 |doi=10.1523/JNEUROSCI.4442-06.2007 |pmid=17267574 |pmc=6673201 |s2cid=10268073 |doi-access=free }} Structural neuroplasticity (increased gray matter volume) has been demonstrated in adults after three months of training in a visual-motor skill, as the qualitative change (i.e. learning of a new task) appear more critical for the brain to change its structure than continued training of an already-learned task. Such changes (e.g. revising for medical exams) have been shown to last for at least 3 months without further practicing; other examples include learning novel speech sounds, musical ability, navigation skills and learning to read mirror-reflected words.{{cite journal |last1=Driemeyer |first1=Joenna |last2=Boyke |first2=Janina |last3=Gaser |first3=Christian |last4=Büchel |first4=Christian |last5=May |first5=Arne |title=Changes in Gray Matter Induced by Learning—Revisited |journal=PLOS ONE |date=23 July 2008 |volume=3 |issue=7 |pages=e2669 |doi=10.1371/journal.pone.0002669 |pmid=18648501 |pmc=2447176 |bibcode=2008PLoSO...3.2669D |s2cid=13906832 |doi-access=free }}{{cite journal |last1=Ilg |first1=R. |last2=Wohlschlager |first2=A. M. |last3=Gaser |first3=C. |last4=Liebau |first4=Y. |last5=Dauner |first5=R. |last6=Woller |first6=A. |last7=Zimmer |first7=C. |last8=Zihl |first8=J. |last9=Muhlau |first9=M. |title=Gray Matter Increase Induced by Practice Correlates with Task-Specific Activation: A Combined Functional and Morphometric Magnetic Resonance Imaging Study |journal=Journal of Neuroscience |date=16 April 2008 |volume=28 |issue=16 |pages=4210–4215 |doi=10.1523/JNEUROSCI.5722-07.2008 |pmid=18417700 |pmc=6670304 |s2cid=8454258 |doi-access=free }}

The largest brains are those of sperm whales, weighing about {{convert|8|kg|abbr=on}}. An elephant's brain weighs just over {{convert|5|kg|abbr=on}}, a bottlenose dolphin's {{convert|1.5|to|1.7|kg|abbr=on}}, whereas a human brain is around {{convert|1.3|to|1.5|kg|abbr=on}}. Brain size tends to vary according to body size. The relationship is not proportional, though: the brain-to-body mass ratio varies. The largest ratio found is in the shrew.{{cite web|url=http://www.kk.org/thetechnium/archives/2004/11/brains_of_white.php|title=The Technium: Brains of White Matter|author=Kevin Kelly|work=kk.org}} Averaging brain weight across all orders of mammals, it follows a power law, with an exponent of about 0.75.{{cite journal |last1=Armstrong |first1=E |title=Relative brain size and metabolism in mammals |journal=Science |date=17 June 1983 |volume=220 |issue=4603 |pages=1302–1304 |doi=10.1126/science.6407108 |pmid=6407108 |bibcode=1983Sci...220.1302A }} There are good reasons to expect a power law: for example, the body-size to body-length relationship follows a power law with an exponent of 0.33, and the body-size to surface-area relationship follows a power law with an exponent of 0.67. The explanation for an exponent of 0.75 is not obvious; however, it is worth noting that several physiological variables appear to be related to body size by approximately the same exponent—for example, the basal metabolic rate.{{cite journal |last1=Savage |first1=V. M. |last2=Gillooly |first2=J. F. |last3=Woodruff |first3=W. H. |last4=West |first4=G. B. |last5=Allen |first5=A. P. |last6=Enquist |first6=B. J. |last7=Brown |first7=J. H. |title=The predominance of quarter-power scaling in biology |journal=Functional Ecology |date=April 2004 |volume=18 |issue=2 |pages=257–282 |doi=10.1111/j.0269-8463.2004.00856.x |doi-access=free |bibcode=2004FuEco..18..257S }}

This power law formula applies to the "average" brain of mammals taken as a whole, but each family (cats, rodents, primates, etc.) departs from it to some degree, in a way that generally reflects the overall "sophistication" of behavior.{{cite book |last1=Jerison |first1=Harry J. |title=Evolution of the Brain and Intelligence |date=1973 |publisher=Academic Press |isbn=978-0-12-385250-2 }}{{page needed|date=November 2020}} Primates, for a given body size, have brains 5 to 10 times as large as the formula predicts. Predators tend to have relatively larger brains than the animals they prey on; placental mammals (the great majority) have relatively larger brains than marsupials such as the opossum. A standard measure for assessing an animal's brain size compared to what would be expected from its body size is known as the encephalization quotient. The encephalization quotient for humans is between 7.4-7.8.{{cite journal |vauthors=Roth G, Dicke U |title=Evolution of the brain and intelligence |journal=Trends Cogn. Sci. (Regul. Ed.) |volume=9 |issue=5 |pages=250–7 |date=May 2005 |pmid=15866152 |doi=10.1016/j.tics.2005.03.005 |s2cid=14758763 }}

When the mammalian brain increases in size, not all parts increase at the same rate.{{cite journal |last1=Finlay |first1=Barbara L. |last2=Darlington |first2=Richard B. |last3=Nicastro |first3=Nicholas |title=Developmental structure in brain evolution |journal=Behavioral and Brain Sciences |date=April 2001 |volume=24 |issue=2 |pages=263–278 |doi=10.1017/S0140525X01003958 |pmid=11530543 |s2cid=20978251 |url=http://pdfs.semanticscholar.org/61f5/d2349949e6171891491b45829c09c173b36a.pdf |archive-url=https://web.archive.org/web/20190225164141/http://pdfs.semanticscholar.org/61f5/d2349949e6171891491b45829c09c173b36a.pdf |url-status=dead |archive-date=2019-02-25 }} In particular, the larger the brain of a species, the greater the fraction taken up by the cortex. Thus, in the species with the largest brains, most of their volume is filled with cortex: this applies not only to humans, but also to animals such as dolphins, whales or elephants. The evolution of Homo sapiens over the past two million years has been marked by a steady increase in brain size, but much of it can be accounted for by corresponding increases in body size.{{cite journal |last1=Kappelman |first1=John |title=The evolution of body mass and relative brain size in fossil hominids |journal=Journal of Human Evolution |date=March 1996 |volume=30 |issue=3 |pages=243–276 |doi=10.1006/jhev.1996.0021 |bibcode=1996JHumE..30..243K }} There are, however, many departures from the trend that are difficult to explain in a systematic way: in particular, the appearance of modern man about 100,000 years ago was marked by a decrease in body size at the same time as an increase in brain size. Even so, it is noteworthy that Neanderthals, which became extinct about 40,000 years ago, had larger brains than modern Homo sapiens.{{cite book |doi=10.1093/acprof:oso/9780198523901.003.0003 |chapter=Toward a synthetic theory of human brain evolution |title=Origins of the Human Brain |year=1996 |last1=Holloway |first1=Ralph L. |pages=42–54 |isbn=978-0-19-852390-1 }}

Not all investigators are happy with the amount of attention that has been paid to brain size. Roth and Dicke, for example, have argued that factors other than size are more highly correlated with intelligence, such as the number of cortical neurons and the speed of their connections.{{cite journal |last1=Roth |first1=G |last2=Dicke |first2=U |title=Evolution of the brain and intelligence |journal=Trends in Cognitive Sciences |date=May 2005 |volume=9 |issue=5 |pages=250–257 |doi=10.1016/j.tics.2005.03.005 |pmid=15866152 |s2cid=14758763 }} Moreover, they point out that intelligence depends not just on the amount of brain tissue, but on the details of how it is structured. It is also well known that crows, ravens, and grey parrots are quite intelligent even though they have small brains.

While humans have the largest encephalization quotient of extant animals, it is not out of line for a primate.{{cite news |last1=Motluk |first1=Alison |title=Size isn't everything: The big brain myth |url=https://www.newscientist.com/article/mg20727711-300-size-isnt-everything-the-big-brain-myth/ |work=New Scientist |date=28 July 2010 }}{{cite journal |last1=Azevedo |first1=Frederico A.C. |last2=Carvalho |first2=Ludmila R.B. |last3=Grinberg |first3=Lea T. |last4=Farfel |first4=José Marcelo |last5=Ferretti |first5=Renata E.L. |last6=Leite |first6=Renata E.P. |last7=Filho |first7=Wilson Jacob |last8=Lent |first8=Roberto |last9=Herculano-Houzel |first9=Suzana |title=Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain |journal=The Journal of Comparative Neurology |date=10 April 2009 |volume=513 |issue=5 |pages=532–541 |doi=10.1002/cne.21974 |pmid=19226510 |s2cid=5200449 | quote = We find that the adult male human brain contains on average 86.1 ± 8.1 billion NeuN-positive cells (“neurons”) and 84.6 ± 9.8 billion NeuN-negative (“nonneuronal”) cells. [...] These findings challenge the common view that humans stand out from other primates in their brain composition and indicate that, with regard to numbers of neuronal and nonneuronal cells, the human brain is an isometrically scaled-up primate brain.}} Some other anatomical trends are correlated in the human evolutionary path with brain size: the basicranium becomes more flexed with increasing brain size relative to basicranial length.{{cite journal |last1=Ross |first1=Callum |last2=Henneberg |first2=Maciej |title=Basicranial flexion, relative brain size, and facial kyphosis inHomo sapiens and some fossil hominids |journal=American Journal of Physical Anthropology |date=December 1995 |volume=98 |issue=4 |pages=575–593 |doi=10.1002/ajpa.1330980413 |pmid=8599387 }}

Cranial capacity is a measure of the volume of the interior of the skull of those vertebrates who have a brain. The most commonly used unit of measure is the cubic centimetre (cm³). The volume of the cranium is used as a rough indicator of the size of the brain, and this in turn is used as a rough indicator of the potential intelligence of the organism. Cranial capacity is often tested by filling the cranial cavity with glass beads and measuring their volume, or by CT scan imaging.{{cite journal |last1=Logan |first1=Corina J. |last2=Clutton-Brock |first2=Tim H. |title=Validating methods for estimating endocranial volume in individual red deer (Cervus elaphus) |journal=Behavioural Processes |date=January 2013 |volume=92 |pages=143–146 |doi=10.1016/j.beproc.2012.10.015 |pmid=23137587 |s2cid=32069068 |url=https://www.repository.cam.ac.uk/handle/1810/261261 }}{{cite journal |last1=Logan |first1=Corina J. |last2=Palmstrom |first2=Christin R. |title=Can endocranial volume be estimated accurately from external skull measurements in great-tailed grackles (Quiscalus mexicanus)? |journal=PeerJ |date=11 June 2015 |volume=3 |pages=e1000 |doi=10.7717/peerj.1000 |pmid=26082858 |pmc=4465945 |doi-access=free }} A more accurate way of measuring cranial capacity, is to make an endocranial cast and measure the amount of water the cast displaces. In the past there have been dozens of studies done to estimate cranial capacity on skulls. Most of these studies have been done on dry skull using linear dimensions, packing methods or occasionally radiological methods.{{citation needed|date=August 2019}}

Knowledge of the volume of the cranial cavity can be important information for the study of different populations with various differences like geographical, racial, or ethnic origin. Other things can also affect cranial capacity such as nutrition.{{cite journal |last1=Rushton |first1=J. Philippe |last2=Jensen |first2=Arthur R. |title=Thirty years of research on race differences in cognitive ability |journal=Psychology, Public Policy, and Law |date=2005 |volume=11 |issue=2 |pages=235–294 |doi=10.1037/1076-8971.11.2.235 |citeseerx=10.1.1.186.102 }} It is also used to study correlating between cranial capacity with other cranial measurements and in comparing skulls from different beings. It is commonly used to study abnormalities of cranial size and shape or aspects of growth and development of the volume of the brain.{{citation needed|date=August 2019}} Cranial capacity is an indirect approach to test the size of the brain. A few studies on cranial capacity have been done on living beings through linear dimensions.{{citation needed|date=August 2019}}

However, larger cranial capacity is not always indicative of a more intelligent organism, since larger capacities are required for controlling a larger body, or in many cases are an adaptive feature for life in a colder environment. For instance, among modern Homo sapiens, northern populations have a 20% larger visual cortex than those in the southern latitude populations, and this potentially explains the population differences in human brain size (and roughly cranial capacity).{{cite news|url=http://www.bbc.co.uk/news/science-environment-14279729|title=BBC News - Dark winters 'led to bigger human brains and eyeballs'|work=BBC News|date=27 July 2011}}{{cite web|url=https://www.theguardian.com/science/2011/jul/27/higher-latitudes-bigger-eyes-brains|title=People at darker, higher latitudes evolved bigger eyes and brains|author=Alok Jha|work=the Guardian|date=27 July 2011}} Neurological functions are determined more by the organization of the brain rather than the volume. Individual variability is also important when considering cranial capacity, for example the average Neanderthal cranial capacity for females was 1300 cm³ and 1600 cm³ for males.Stanford, C., Allen, J.S., Anton, S.C., Lovell, N.C. (2009). Biological Anthropology: the Natural History of Humankind. Toronto: Pearson Canada. p. 301 Neanderthals had larger eyes and bodies relative to their height, thus a disproportionately large area of their brain was dedicated to somatic and visual processing, functions not normally associated with intelligence. When these areas were adjusted to match anatomically modern human proportions it was found Neanderthals had brains 15-22% smaller than in anatomically-modern humans.{{cite journal |last1=Pearce |first1=Eiluned |last2=Stringer |first2=Chris |last3=Dunbar |first3=R. I. M. |title=New insights into differences in brain organization between Neanderthals and anatomically modern humans |journal=Proceedings of the Royal Society B: Biological Sciences |date=7 May 2013 |volume=280 |issue=1758 |pages=20130168 |doi=10.1098/rspb.2013.0168 |pmid=23486442 |pmc=3619466 }} When the neanderthal version of the NOVA1 gene is inserted into stem cells it creates neurons with fewer synapses than stem cells containing the human version.{{cite journal |last1=Cohen |first1=Jon |title=Exclusive: Neanderthal 'minibrains' grown in dish |journal=Science |date=20 June 2018 |url=https://www.science.org/content/article/exclusive-neanderthal-minibrains-grown-dish }}

Parts of a cranium found in China in the 1970s show that the young man had a cranial capacity of around 1700{{nbsp}}cm{{sup|3}} at least 160,000 years ago. This is greater than the average of modern humans.

In an attempt to use cranial capacity as an objective indicator of brain size, the encephalization quotient (EQ) was developed in 1973 by Harry Jerison. It compares the size of the brain of the specimen to the expected brain size of animals with roughly the same weight.Campbell, G.C., Loy, J.D., Cruz-Uribe, K. (2006). Humankind Emerging: Ninth Edition. Boston: Pearson. p346 This way a more objective judgement can be made on the cranial capacity of an individual animal. A large scientific collection of brain endocasts and measurements of cranial capacity has been compiled by Holloway.Holloway, Ralph L., Yuan, M. S., and Broadfield, D.C. (2004). The Human Fossil Record: Brain Endocasts: The Paleoneurological Evidence. New York. John Wiley & Sons Publishers (http://www.columbia.edu/~rlh2/PartII.pdf and http://www.columbia.edu/~rlh2/available_pdfs.html for further references).

Examples of cranial capacity

Apes

• Orangutans: {{convert|275|-|500|cm3|abbr = on}}
• Chimpanzees: {{convert|275|-|500|cm3|abbr = on}}
• Gorillas: {{convert|340|-|752|cm3|abbr = on}}

Hominids

• Anatomically-modern human: average 1473{{nbsp}}cm{{sup|3}}
• Neanderthals: 1500–1740{{nbsp}}cm{{sup|3}}
• Xujiayao 6 (160 to 200 ka ago): ca. 1700{{nbsp}}cm{{sup|3}}{{cite journal |last1=Michael Marshall |title=160,000-year-old fossil may be the first Denisovan skull we've found |journal=New Scientist |date=Feb 5, 2022 |url=https://www.newscientist.com/article/2305830-160000-year-old-fossil-may-be-the-first-denisovan-skull-weve-found/}}{{cite journal |display-authors=etal|last1=Xiu-Jie Wu |title=Evolution of cranial capacity revisited: A view from the late Middle Pleistocene cranium from Xujiayao, China |journal=Journal of Human Evolution |date=Feb 2022 |volume=163 |page=103119 |doi=10.1016/j.jhevol.2021.103119|pmid=35026677 |bibcode=2022JHumE.16303119W |s2cid=245858877 }}
• Homo erectus; 850–1100 cm³
• Australopithecus anamensis; 365–370 cm³ {{cite journal |last1=Haile-Selassie |first1=Yohannes |last2=Melillo |first2=Stephanie M. |last3=Vazzana |first3=Antonino |last4=Benazzi |first4=Stefano |last5=Ryan |first5=Timothy M. |title=A 3.8-million-year-old hominin cranium from Woranso-Mille, Ethiopia |journal=Nature |date=12 September 2019 |volume=573 |issue=7773 |pages=214–219 |doi=10.1038/s41586-019-1513-8 |pmid=31462770 |hdl=11585/697577 |hdl-access=free |bibcode=2019Natur.573..214H |s2cid=201656331 }}
• Australopithecus afarensis; 438 cm³ {{Cite book|title = THE EVOLUTION OF THE HUMAN HEAD|last = Lieberman|first = Daniel|pages =433 }}
• Australopithecus africanus 452 cm³ {{Cite book|title = THE EVOLUTION OF THE HUMAN HEAD|last = Lieberman|first = Daniel|pages =435 }}
• Paranthropus boisei 521 cm³
• Paranthropus robustus 530 cm³

• Brain-to-body mass ratio
• Encephalization quotient
• List of animals by number of neurons
• Craniometry — includes historical discussion
• Neuroscience and intelligence
• Human brain

{{reflist|colwidth=40em}}

• {{cite magazine|last1=Jabr|first1=Ferris|title=How Humans Ended Up With Freakishly Huge Brains|url=http://www.wired.com/2015/11/how-humans-ended-up-with-freakishly-huge-brains/|access-date=29 November 2015|magazine=Wired|date=28 November 2015|ref=jabr2015}}

{{animal cognition}}

Category:Brain