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===Brain volume=== One of the main methods used to establish a relationship between intelligence and the brain is to use measures of brain volume.<ref name="Luders2009" /> The earliest attempts at estimating brain volume were done using measures of external head parameters, such as head circumference as a proxy for brain size.<ref name="Luders2009" /> More recent methodologies employed to study this relationship include post-mortem measures of brain weight and volume. These have their own limitations and strengths.<ref name="Witelson2006">{{cite journal | last1 = Witelson | first1 = S. F. | last2 = Beresh | first2 = H. | last3 = Kigar | first3 = D. L. | year = 2006 | title = Intelligence and brain size in 100 postmortem brains: sex, lateralization and age factors | journal = Brain | volume = 129 | issue = 2| pages = 386–398 | doi=10.1093/brain/awh696 | pmid=16339797| doi-access = free }}</ref> The advent of MRI as a non-invasive highly-accurate measure of living brain structure and function (using fMRI) made this the pre-dominant and preferred method for measuring brain volume.<ref name="Luders2009" /> Overall, larger brain size and volume is associated with better cognitive functioning and higher intelligence.<ref name=Luders2009 /> The specific regions that show the most robust correlation between volume and intelligence are the frontal, temporal and parietal lobes of the brain.<ref name="Andreasen1993">{{cite journal | last1 = Andreasen | first1 = N. C. | last2 = Flaum | first2 = M. | last3 = Victor Swayze | first3 = I. I. | last4 = O'Leary | first4 = D. S. | last5 = Alliger | first5 = R. | last6 = Cohen | first6 = G. | year = 1993 | title = Intelligence and brain structure in normal individuals | journal = Am J Psychiatry | volume = 150 | issue = 1| pages = 130–4 | doi=10.1176/ajp.150.1.130| pmid = 8417555 }}</ref><ref name="Flashman1997">{{cite journal | last1 = Flashman | first1 = L. A. | last2 = Andreasen | first2 = N. C. | last3 = Flaum | first3 = M. | last4 = Swayze | first4 = V. W. | year = 1997 | title = Intelligence and regional brain volumes in normal controls | journal = Intelligence | volume = 25 | issue = 3| pages = 149–160 | doi=10.1016/s0160-2896(97)90039-8}}</ref><ref name="MacLullich2002">{{cite journal | last1 = MacLullich | first1 = A. M. J. | last2 = Ferguson | first2 = K. J. | last3 = Deary | first3 = I. J. | last4 = Seckl | first4 = J. R. | last5 = Starr | first5 = J. M. | last6 = Wardlaw | first6 = J. M. | year = 2002 | title = Intracranial capacity and brain volumes are associated with cognition in healthy elderly men | journal = Neurology | volume = 59 | issue = 2| pages = 169–174 | doi=10.1212/wnl.59.2.169| pmid = 12136052 | s2cid = 46043963 }}</ref> A large number of studies have been conducted with uniformly positive correlations, leading to the generally safe conclusion that larger brains predict greater intelligence.<ref name="GrayThompson2004">{{cite journal | last1 = Gray | first1 = J. R. | last2 = Thompson | first2 = P. M. | year = 2004 | title = Neurobiology of intelligence: science and ethics | journal = Nature Reviews Neuroscience | volume = 5 | issue = 6| pages = 471–482 | doi=10.1038/nrn1405 | pmid=15152197| s2cid = 2430677 }}</ref><ref name="TogaThompson2005">{{cite journal | last1 = Toga | first1 = A. W. | last2 = Thompson | first2 = P. M. | year = 2005 | title = Genetics of brain structure and intelligence | journal = Annu. Rev. Neurosci. | volume = 28 | pages = 1–23 | doi=10.1146/annurev.neuro.28.061604.135655| pmid = 15651931 | s2cid = 780961 }}</ref> In healthy adults, the correlation of total brain volume and IQ is approximately 0.4 when high-quality tests are used.<ref name=":1">{{Cite journal|last1=Gignac|first1=Gilles E.|last2=Bates|first2=Timothy C.|title=Brain volume and intelligence: The moderating role of intelligence measurement quality|journal=Intelligence|volume=64|pages=18–29|doi=10.1016/j.intell.2017.06.004|year=2017|hdl=20.500.11820/a61135a7-6389-4f5c-9a4e-24403ba7e873|s2cid=84839916 |url=https://www.pure.ed.ac.uk/ws/files/39346142/GignacBatesI2017BrainVolumeAndIntelligence2.pdf|hdl-access=free}}</ref> A large scale study (n = 29k) using [[UK Biobank|the UK Biobank]] found a correlation of .275. The strength of this relationship did not depend on sex, contradicting some earlier studies.<ref>{{Cite journal|last1=Cox|first1=S. R.|last2=Ritchie|first2=S. J.|last3=Fawns-Ritchie|first3=C.|last4=Tucker-Drob|first4=E. M.|last5=Deary|first5=I. J.|date=2019-09-01|title=Structural brain imaging correlates of general intelligence in UK Biobank|journal=Intelligence|volume=76|pages=101376|doi=10.1016/j.intell.2019.101376|pmid=31787788|pmc=6876667|issn=0160-2896}}</ref> A study using a [[Sibling design|sibling-design]] in two medium sized samples found evidence of causality with an effect size of .19.<ref>{{Cite journal|last1=Lee|first1=James J.|last2=McGue|first2=Matt|last3=Iacono|first3=William G.|last4=Michael|first4=Andrew M.|last5=Chabris|first5=Christopher F.|date=July 2019|title=The causal influence of brain size on human intelligence: Evidence from within-family phenotypic associations and GWAS modeling|journal=Intelligence|volume=75|pages=48–58|doi=10.1016/j.intell.2019.01.011|pmid=32831433|pmc=7440690}}</ref> This study design rules out [[Confounding|confounders]] that vary between families, but not those that vary within families. Less is known about variation on scales less than total brain volume. A meta-analytic review by McDaniel found that the correlation between intelligence and in vivo brain size was larger for females (0.40) than for males (0.25).<ref name="McDaniel2005">{{cite journal | last1 = McDaniel | first1 = M. A. | year = 2005 | title = Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence | journal = Intelligence | volume = 33 | issue = 4| pages = 337–346 | doi=10.1016/j.intell.2004.11.005}}</ref> The same study also found that the correlation between brain size and Intelligence increased with age, with children showing smaller correlations.<ref name=McDaniel2005 /> It has been suggested that the link between larger brain volumes and higher intelligence is related to variation in specific brain regions: a whole-brain measure would under-estimate these links.<ref name=Andreasen1993 /> For functions more specific than general intelligence, regional effects may be more important. For instance evidence suggests that in adolescents learning new words, vocabulary growth is associated with gray matter density in bilateral posterior [[Supramarginal gyrus|supramarginal gyri]].<ref name="Lee2007">{{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 = Price | first7 = C. J. | year = 2007 | title = Anatomical traces of vocabulary acquisition in the adolescent brain | url =http://discovery.ucl.ac.uk/11653/1/11653.pdf | journal = The Journal of Neuroscience | volume = 27 | issue = 5| pages = 1184–1189 | doi=10.1523/jneurosci.4442-06.2007 | pmid=17267574| pmc = 6673201 }}</ref> Small studies have shown transient changes in gray-matter associated with developing a new physical skill (juggling) occipito-temporal cortex <ref name="Driemezer2008">{{cite journal | last1 = Driemeyer | first1 = J. | last2 = Boyke | first2 = J. | last3 = Gaser | first3 = C. | last4 = Büchel | first4 = C. | last5 = May | first5 = A. | year = 2008 | title = Changes in gray matter induced by learning—revisited | journal = PLOS ONE | volume = 3 | issue = 7| page = e2669 | doi=10.1371/journal.pone.0002669 | pmid=18648501 | pmc=2447176| bibcode = 2008PLoSO...3.2669D | doi-access = free }}</ref> Brain volume is not a perfect account of intelligence: the relationship explains a modest amount of variance in intelligence – 12% to 36% of the variance.<ref name=Witelson2006 /><ref name=Andreasen1993 /> The amount of variance explained by brain volume may also depend on the type of intelligence measured.<ref name=Witelson2006 /> Up to 36% of variance in verbal intelligence can be explained by brain volume, while only approximately 10% of variance in visuospatial intelligence can be explained by brain volume.<ref name=Witelson2006 /> A 2015 study by researcher [[Stuart J. Ritchie]] found that brain size explained 12% of the variance in intelligence among individuals.<ref name=":0">{{Cite journal|last1=Ritchie|first1=Stuart J.|last2=Booth|first2=Tom|last3=Valdés Hernández|first3=Maria del C.|last4=Corley|first4=Janie|last5=Maniega|first5=Susana Muñoz|last6=Gow|first6=Alan J.|last7=Royle|first7=Natalie A.|last8=Pattie|first8=Alison|last9=Karama|first9=Sherif|date=2015-01-01|title=Beyond a bigger brain: Multivariable structural brain imaging and intelligence|journal=Intelligence|volume=51|pages=47–56|doi=10.1016/j.intell.2015.05.001|issn=0160-2896|pmc=4518535|pmid=26240470}}</ref> These caveats imply that there are other major factors influencing how intelligent an individual is apart from brain size.<ref name=Luders2009 /> In a large meta-analysis consisting of 88 studies Pietschnig et al. (2015) estimated the correlation between brain volume and intelligence to be about correlation coefficient of 0.24 which equates to 6% variance.<ref name="Pietschnig 411–432">{{Cite journal|title = Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?|journal = Neuroscience & Biobehavioral Reviews|date = 2015-10-01|pages = 411–432|volume = 57|doi = 10.1016/j.neubiorev.2015.09.017|first1 = Jakob|last1 = Pietschnig|first2 = Lars|last2 = Penke|first3 = Jelte M.|last3 = Wicherts|first4 = Michael|last4 = Zeiler|first5 = Martin|last5 = Voracek|pmid=26449760|s2cid = 23180321}}</ref> Taking into account measurement quality, and sample type and IQ-range, the meta-analytic association of brain volume in appears to be ~ .4 in normal adults.<ref name=":1" /> Researcher Jakob Pietschnig argued that the strength of the positive association of brain volume and IQ remains robust, but has been overestimated in the literature. He has stated that "''It is tempting to interpret this association in the context of human cognitive evolution and species differences in brain size and cognitive ability, we show that it is not warranted to interpret brain size as an isomorphic proxy of human intelligence differences''".<ref name="Pietschnig 411–432"/>
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