Brain size

The size of the brain is a frequent topic of study within the fields of anatomy and evolution. Brain size can be measured by weight or by volume (via MRI scans or by skull volume). The relationship between brain size and intelligence is frequently a topic of research.

Humans

Studies on human brain size, largely based on participants of European ancestry, tend to find an average adult brain volume of 1260 cubic centimeters (cm3) for men and 1130 cm3 for women. There is, however, substantial variation between individuals;[1] one study of 46 adults, aged 22–49 years and of mainly European descent, found an average brain volume of 1273.6 cm3 for men, with a range of 1052.9 to 1498.5 cm3, and 1131.1 cm3 for women, with a range of 974.9 to 1398.1 cm3.[2]

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 1.5 kg (3.3 lb).[3] The average weight is about 1370 g in men and about 1200 g in women.[4]

Changes over time

Brain size has increased considerably over the course of humans' recent evolutionary history. Homo erectus, a relative of humans, had a brain size of 1,100 cm3. Homo floresiensis, with a brain size of 380 cm3.[5] Neanderthals had a slightly larger brain than modern humans,[6] perhaps due to larger visual systems.[7]

Some studies suggest that the average brain size has been decreasing over the past 28,000 years.[8][9] Others suggest that the cranial capacity for males is unchanged, but that the cranial capacity of females has increased.[10]

Biogeographic variation

A number of studies have found correlation between variation in brain size in cranial capacity and geographic ancestry in humans.[11][12] This variation in cranial capacity is believed to be primarily caused by climatic adaptation that favor large round heads in colder climates because they conserve heat and slender heads in warm climates closer to the equator (See Bergmann's rule and Allen's rule).[13]

The largest study done on the subject of geographic variation in brain size is the 1984 study Brain Size, Cranial Morphology, Climate, and Time Machines. The study found that human brain size varied with latitude of biogeographic ancestry.[11] The relationship between latitude and cranial size is described in the study as an example of Bergmann’s principle that crania are more spherical in cold climates because mass increases relative to surface area to conserve core temperatures and behaves independently of "race".

Changes over the lifespan

A 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 40 years, after which it begins to decline by 5% per decade, speeding up at around 70 years.[14] Total cerebral and gray matter volumes peak between 10 and 20 years (earlier in girls than in boys), whereas white matter and ventricular volumes increase. There is a general pattern in which neural development peaks in childhood and declines in adolescence, a process known as synaptic pruning. Overall white matter volume does not appear to decline with age, although there is variation among brain regions.[15]

Sex

Average brain weight for males and females over lifespan. From the study Changes in brain weights during the span of human life.

The average brain weight in adult males is 1,345 grams; in adult females, 1,222 grams.[16] Males have been found to have, on average, greater cerebral, cerebellar, and cerebral cortical lobar volumes, except possibly left parietal.[17]

When covaried for intracranial volume, height, and weight, one study found that women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There was high variability among individuals, however.[18] Yaki (2011) found no statistically significant gender differences in the gray matter ratio for most ages in a sample of 758 women and 702 men aged 20–69.[19]

Consistent with findings in adults, average cerebral volume is approximately 10% larger in boys than in girls. However, such differences should not be construed 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. Moreover, brain volumes, even in narrowly defined groups (e.g. children of the same age), may vary by as much as 50%.[20] Young girls have, on average, larger hippocampi, whereas young boys have larger amygdalas.[18]

Significant dynamic changes in brain structure take place throughout adulthood, 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.[18] 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.

Race

J. Philippe Rushton published multiple studies claiming that average brain size was lowest in blacks ("Negroids") and highest in Asians ("Mongoloids"), with whites ("Caucasoids") in between the two.[21][22][23] His work in this area has been criticized for relying on flawed studies, for failing to consider explanations other than genetics for the observed differences, and for ignoring other studies with contradictory conclusions.[24] Nathan Brody has also argued that the evidence regarding racial differences in brain size is not conclusive, and that even if one accepts it, this difference does not support a genetic hypothesis regarding racial differences in intelligence.[25] Critics of the hereditarian position also note that the difference in mean brain size between blacks and whites is smaller than 1 standard deviation, and is insufficient to explain the vast majority of the black-white IQ gap.[26]

According to a study published by Smith and Beals im 1990, based on measurements of approximately 20,000 crania from 87 populations worldwide, arctic indigenous peoples and East Asians have the biggest brains in the world, with an average volume of 1,443 cm3 and 1,416 cm3 respectively, followed by Europeans with an average volume of 1,369 cm3. Among the Europeans, Italians have the biggest brains (1,411 cm3).[27]

Genetic contribution

Adult twin studies indicate that heritability of overall brain size in adulthood is high (between 66% and 97%). Infant brain volumes are also highly heritable, with heritability of total brain volume in neonates of around .8-.9. [28]

Heritability varies regionally within the brain, 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.

Early studies yielded suggestive candidate genes.[29][30] Much larger genome-wide studies have now yielded highly replicable associations for at least 8 genes linked to cortical and subcortical brain volume in a study of over 32,000 humans.[31]

Intelligence

Studies demonstrate a correlation between brain size and intelligence, with larger brains predicting higher intelligence. It is however not clear if the correlation is causal.[32] The majority of MRI studies report moderate correlations around 0.3 to 0.4 between brain volume and intelligence.[33][34] In healthy adults, the correlation of total brain volume and IQ is ~ 0.4 [35]

Studies have sought particular regions that are more correlated with IQ than whole-brain volume. While consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampus, and the cerebellum, unique variation in these regions account for a relatively small amount of variance in IQ.[34][36] The search for specific brain regions that correlate highly with cognitive measures designed to be specific has not yielded clear results.[2]

There may be sex differences in the volume-IQ association. Some evidence suggests that while IQ correlates equally with frontal lobe volume, it may correlate more with parietal lobe volumes in men and with Broca's area in women, corresponding to spatial versus language specializations.[18]

Small studies have attempted to link brain volume with functional measures such as P300 auditory evoked potentials but finding no association.[37][38] Studies attempting to related sibling differences in IQ to differences in brain volume are hampered by relatively small sample sizes and the noisy nature of such difference scores, yielding weak evidence for cross-trait cross-sib correlations.[32]

Other animals

The largest brains are those of sperm whales, weighing about 8 kg (18 lb), and killer whales, weighing about 12–15 lb (5.4–6.8 kg). An elephant's brain weighs just over 5 kg (11 lb) and a bottlenose dolphin's 1.5 to 1.7 kg (3.3 to 3.7 lb).

Brain size tends to vary according to body size. The relationship is not proportional, however; the brain-to-body mass ratio varies. The largest ratio found is in the shrew.[39] Averaging brain weight across all orders of mammals, it follows a power law, with an exponent of about 0.75.[40] 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.[41] 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.

When the mammalian brain increases in size, not all parts increase at the same rate.[42] 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.

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.[43] 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 African gray parrots are quite intelligent even though they have small brains.

Cranial capacity

Cranial capacity is a measure of the volume of the interior of the cranium (also called the braincase or brainpan or skull) of those vertebrates who have both a cranium and a brain. The most commonly used unit of measure is the cubic centimetre or cm3. The volume of the cranium is used as a rough indicator of the size of the brain, although due to the thickness of the membranes that surround the brain, brain volume is less than cranial capacity. Cranial Capacity is often tested by filling the cranial cavity with particulate material (as mustard seed or small shot) and measuring the volume of the latter. However, this method of measuring cranial capacity must be validated in each species to know whether it is an accurate representation of the braincase.[44][45] 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.[46]

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.[47] 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.[48] 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.[49]

The volume of the human braincase has increased as humans have evolved (see Homininae), starting from about 600 cm3 in Homo habilis up to 1600 cm3 in Homo neanderthalensis, which was the hominid with the biggest brain size.[50][51]

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.[52] 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.[53]

Examples of cranial capacity

Apes

Early Hominids

Human races, not corrected for body size[27]

See also

Notes

  1. ↑ Cosgrove, KP; Mazure CM; Staley JK (2007). "Evolving knowledge of sex differences in brain structure, function, and chemistry". Biol Psychiat. 62 (8): 847–55. PMC 2711771 Freely accessible. PMID 17544382. doi:10.1016/j.biopsych.2007.03.001.
  2. 1 2 Allen et al., 2002
  3. ↑ Parent, A; Carpenter MB (1995). "Ch. 1". Carpenter's Human Neuroanatomy. Williams & Wilkins. ISBN 978-0-683-06752-1.
  4. ↑ Harrison, Paul J.; Freemantle, Nick; Geddes, John R. (2003-11-01). "Meta-analysis of brain weight in schizophrenia". Schizophrenia Research. 64 (1): 25–34. ISSN 0920-9964. PMID 14511798. doi:10.1016/s0920-9964(02)00502-9.
  5. ↑ Brown, P.; Sutikna, T.; Morwood, M. J.; Soejono, R. P.; Jatmiko; Saptomo, E. Wayhu; Due, Rokus Awe (October 2004). "A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia". Nature. 431 (7012): 1055–1061. PMID 15514638. doi:10.1038/nature02999.
  6. ↑ Holloway, 1995
  7. ↑ Pearce, Eiluned; Stringer, Chris; Dunbar, R. I. M. (2013-05-07). "New insights into differences in brain organization between Neanderthals and anatomically modern humans". Proceedings of the Royal Society of London B: Biological Sciences. 280 (1758): 20130168. ISSN 0962-8452. PMC 3619466 Freely accessible. PMID 23486442. doi:10.1098/rspb.2013.0168.
  8. ↑ "If Modern Humans Are So Smart, Why Are Our Brains Shrinking?". DiscoverMagazine.com. 2011-01-20. Retrieved 2014-03-05.
  9. ↑ Henneberg, Maciej (1988). "Decrease of human skull size in the Holocene". Human Biology. 60: 395–405. JSTOR 41464021.
  10. ↑ Rushton, John Philippe (1992). "Cranial capacity related to sex, rank, and race in a stratified random sample of 6,325 U.S. military personnel". Intelligence. 16 (3–4): 401–413. doi:10.1016/0160-2896(92)90017-l.
  11. 1 2 "Brain Size, Cranial Morphology, Climate, and Time Machines" (PDF). Kenneth L. Beals, Courtland L. Smith, and Stephen M. Dodd. 3 June 1984.
  12. ↑ "Morphological Adaptation to Climate in Modern Homo sapiens Crania: The Importance of Basicranial Breadth". Wioletta Nowaczewska, Pawe D browski1 and Lukasz KuŸmiñski.
  13. ↑ James Mielke, Lyle W. Konigsberg & John Relethford. 2006. Human biological variation". Oxford University Press 274-75
  14. ↑ Peters, R. (2006). "Ageing and the brain". ncbi.nlm. 82: 84–8. PMC 2596698 Freely accessible. PMID 16461469. doi:10.1136/pgmj.2005.036665.
  15. ↑ Good et al., 2001
  16. ↑ Kelley Hays; David S. Reader in Gender archaeology. Routlegde. Retrieved 2014-09-21.
  17. ↑ Carne et al., 2006
  18. 1 2 3 4 Cosgrove et al., 2007
  19. ↑ Taki, Y.; Thyreau, B.; Kinomura, S.; Sato, K.; Goto, R.; Kawashima, R.; Fukuda, H. (2011). He, Yong, ed. "Correlations among Brain Gray Matter Volumes, Age, Gender, and Hemisphere in Healthy Individuals". PLoS ONE. 6 (7): e22734. PMC 3144937 Freely accessible. PMID 21818377. doi:10.1371/journal.pone.0022734.
  20. ↑ Giedd, 2008
  21. ↑ Rushton, J.Philippe (January 1988). "Race differences in behaviour: A review and evolutionary analysis". Personality and Individual Differences. 9 (6): 1009–1024. doi:10.1016/0191-8869(88)90135-3.
  22. ↑ Rushton, J.Philippe; Rushton, Elizabeth W. (March 2003). "Brain size, IQ, and racial-group differences". Intelligence. 31 (2): 139–155. doi:10.1016/S0160-2896(02)00137-X.
  23. ↑ Rushton, J. Philippe; Ankney, C. Davison (March 1996). "Brain size and cognitive ability: Correlations with age, sex, social class, and race". Psychonomic Bulletin & Review. 3 (1): 21–36. doi:10.3758/BF03210739.
  24. ↑ Cain, Donald P.; Vanderwolf, C.H. (1990). "A critique of Rushton on race, brain size and intelligence". Personality and Individual Differences. 11 (8): 777–784. doi:10.1016/0191-8869(90)90185-T.
  25. ↑ Brody, Nathan (2003). "Jensen's Genetic Interpretation of Racial Differences in Intelligence: Critical Evaluation". The Scientific Study of General Intelligence: Tribute to Arthur Jensen. Elsevier Science. pp. 397–410.
  26. ↑ Wicherts, Jelte M.; Borsboom, Denny; Dolan, Conor V. (January 2010). "Evolution, brain size, and the national IQ of peoples around 3000 years B.C". Personality and Individual Differences. 48 (2): 104–106. doi:10.1016/j.paid.2009.08.020.
  27. 1 2 Smith, C. L., Beals, K. L. (1990). "Cultural correlates with cranial capacity". American Anthropologist. 92: 193–200.
  28. ↑ Gilmore, John H.; Schmitt, James Eric; Knickmeyer, Rebecca C.; Smith, Jeffrey K.; Lin, Weili; Styner, Martin; Gerig, Guido; Neale, Michael C. (2010-08-01). "Genetic and environmental contributions to neonatal brain structure: A twin study". Human Brain Mapping. 31 (8): 1174–1182. ISSN 1097-0193. doi:10.1002/hbm.20926.
  29. ↑ Peper, 2007
  30. ↑ Zhang, 2003
  31. ↑ Adams, H.H. (2016). "Novel genetic loci underlying human intracranial volume identified through genome-wide association.". Nature Neuroscience. 19: 1569–1582. doi:10.1038/nn.4398.
  32. 1 2 Nisbett et al. 2012b, p. 142.
  33. ↑ McDaniel, Michael (2005). "Big-brained people are smarter" (PDF). Intelligence. 33: 337–346. doi:10.1016/j.intell.2004.11.005.
  34. 1 2 Luders et al., 2008
  35. ↑ Gignac, Gilles E.; Bates, Timothy C. "Brain volume and intelligence: The moderating role of intelligence measurement quality". Intelligence. 64: 18–29. doi:10.1016/j.intell.2017.06.004.
  36. ↑ Hoppe & Stojanovic, 2008
  37. ↑ Egan et al., 1993
  38. ↑ Egan et al, 1995
  39. ↑ Kevin Kelly. "The Technium: Brains of White Matter". kk.org.
  40. ↑ Armstrong, 1983
  41. ↑ Jerison, Evolution of the Brain and Intelligence
  42. ↑ Finlay et al., 2001
  43. ↑ Roth & Dicke, 2005
  44. ↑ Logan & Clutton-Brock (2013). "Validating methods for estimating endocranial volume in individual red deer (Cervus elaphus)" (PDF). Behavioural Processes. 92: 143–146. doi:10.1016/j.beproc.2012.10.015.
  45. ↑ Logan & Palmstrom (2015). "Can endocranial volume be estimated accurately from external skull measurements in great-tailed grackles (Quiscalus mexicanus)?". PeerJ. 3: e1000. PMC 4465945 Freely accessible. PMID 26082858. doi:10.7717/peerj.1000.
  46. ↑ Ezejindu D.N., 1Chinweife K. C., 2 Ihentuge C.J., 2Uloleme G. C (2013). "Studies of Cranial Capacity between the Ages of 14 – 20 Yrs of Ogidi People of Anambra State, Nigeria." (PDF). Journal of Dental and Medical Sciences.
  47. ↑ J. Philippe Rushton; Arthur R. Jensen. "THIRTY YEARS OF RESEARCH ON RACE DIFFERENCES IN COGNITIVE ABILITY" (PDF). American Psychological Association.
  48. ↑ Ezejindu D.N.; Chinweife K. C.; Ihentuge C.J.; Uloleme G. C. "Studies of Cranial Capacity between the Ages of 14 – 20 Yrs of Ogidi People of Anambra State, Nigeria." (PDF). Journal of Dental and Medical Sciences.
  49. ↑ Ezejindu D.N., Chinweife K. C., Ihentuge C.J., 2Uloleme G. C. "Studies of Cranial Capacity between the Ages of 14 – 20 Yrs of Ogidi People of Anambra State, Nigeria." (PDF). Journal of Dental and Medical Sciences.
  50. ↑ "Neanderthal man". infoplease.
  51. ↑ Brown, Graham; Fairfax, Stephanie; Sarao, Nidhi. "Human Evolution". Tree of Life. Tree of Life Project. Retrieved 19 May 2016.
  52. ↑ Campbell, G.C., Loy, J.D., Cruz-Uribe, K. (2006). Humankind Emerging: Ninth Edition. Boston: Pearson. p346
  53. ↑ 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).

References

Further reading

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