Editing Encephalization quotient (section)

== Perspective on intelligence measures ==
{{More citations needed section|date=May 2020}}
Encephalization quotient was developed in an attempt to provide a way of correlating an animal's physical characteristics with perceived intelligence. It improved on the previous attempt, [[brain-to-body mass ratio]], so it has persisted. Subsequent work, notably Roth,<ref name="Roth2005">{{cite journal|last1=Roth|first1=Gerhard|last2=Dicke|first2=Ursula|date=May 2005|title=Evolution of the brain and intelligence|journal=Trends in Cognitive Sciences|volume=9|issue=5|pages=250–7|doi=10.1016/j.tics.2005.03.005|pmid=15866152|s2cid=14758763}}</ref> found EQ to be flawed and suggested brain size was a better predictor, but that has problems as well.{{Unbalanced opinion|date=May 2020}}

Currently the best predictor for intelligence across all animals is [[forebrain]] neuron count.<ref name=houzel2017>{{cite journal |last1=Herculano-Houzel |first1=Suzana |title=Numbers of neurons as biological correlates of cognitive capability |journal=Current Opinion in Behavioral Sciences |year=2017 |volume=16 |pages=1–7 | doi=10.1016/j.cobeha.2017.02.004 |s2cid=53172110 }}</ref> This was not seen earlier because neuron counts were previously inaccurate for most animals. For example, human brain neuron count was given as 100 billion for decades before [[Suzana Herculano-Houzel|Herculano-Houzel]]<ref name=houzel2009>{{cite journal |last1=Herculano-Houzel |first1=Suzana |title=The human brain in numbers: a linearly scaled-up primate brain |journal=Frontiers in Human Neuroscience |year=2009 |volume=3 |pages=31 |doi=10.3389/neuro.09.031.2009 |pmc=2776484 |pmid=19915731 |doi-access=free }}</ref><ref>{{cite book |last1=Herculano-Houzel |first1=Suzana |title=The Human Advantage: How our brains became remarkable |year=2017 |publisher=MIT Press|isbn=978-0-262-53353-9}}{{pn|date=February 2020}}</ref> found a more reliable method of counting brain cells.

It could have been anticipated that EQ might be superseded because of both the number of exceptions and the growing complexity of the formulae it used. (See the rest of this article.){{Unbalanced opinion|date=May 2020}} The simplicity of counting neurons has replaced it.{{citation needed|date=May 2020}} The concept in EQ of comparing the brain capacity exceeding that required for body sense and motor activity may yet live on to provide an even better prediction of intelligence, but that work has not been done yet.{{fact|date=February 2020}}{{Unbalanced opinion|date=May 2020}}

=== Variance in brain sizes ===
Body size accounts for 80–90% of the variance in brain size, between species, and a relationship described by an allometric equation: the regression of the logarithms of brain size on body size. The distance of a species from the regression line is a measure of its encephalization.<ref name=Finlay2009/> The scales are logarithmic, distance, or residual, is an encephalization quotient (EQ), the ratio of actual brain size to expected brain size. Encephalization is a characteristic of a species.

Rules for brain size relates to the number brain neurons have varied in evolution, then not all mammalian brains are necessarily built as larger or smaller versions of a same plan, with proportionately larger or smaller numbers of neurons. Similarly sized brains, such as a cow or chimpanzee, might in that scenario contain very different numbers of neurons, just as a very large cetacean brain might contain fewer neurons than a gorilla brain. Size comparison between the human brain and non-primate brains, larger or smaller, might simply be inadequate and uninformative – and our view of the human brain as outlier, a special oddity, may have been based on the mistaken assumption that all brains are made the same (Herculano-Houzel, 2012).<ref name=Herculano2012/>{{citation needed |reason=Entire paragraph is full of spelling/grammatical errors. Source material should be referenced to obtain the correct verbiage. |date=December 2019}}

=== Limitations and possible improvements over EQ ===
There is a distinction between brain parts that are necessary for the maintenance of the body and those that are associated with improved cognitive functions. These brain parts, although functionally different, all contribute to the overall weight of the brain. Jerison (1973) has for this reason considered 'extra neurons', neurons that contribute strictly to cognitive capacities, as more important indicators of intelligence than pure EQ. Gibson et al. (2001) reasoned that bigger brains generally contain more 'extra neurons' and thus are better predictors of cognitive abilities than pure EQ among primates.<ref>Jerison, H.J., 1973. Evolution of the brain and [[intelligence]] Academic Press.[HTE].{{pn|date=February 2020}}</ref><ref name="roth">{{cite book |doi=10.1016/B978-0-444-53860-4.00020-9 |pmid=22230639 |isbn=9780444538604 |chapter=Evolution of the brain and intelligence in primates |title=Evolution of the Primate Brain |series=Progress in Brain Research |year=2012 |last1=Roth |first1=Gerhard |last2=Dicke |first2=Ursula |volume=195 |pages=413–430 }}</ref>

Factors such as the recent evolution of the [[cerebral cortex]] and different degrees of brain folding ([[gyrification]]), which increases the surface area (and volume) of the cortex, are [[correlation|positively correlated]] to intelligence in humans.<ref>{{cite journal |last1=Haier |first1=Richard J. |last2=Jung |first2=Rex E. |last3=Yeo |first3=Ronald A. |last4=Head |first4=Kevin |last5=Alkire |first5=Michael T. |title=Structural brain variation and general intelligence |journal=NeuroImage |date=September 2004 |volume=23 |issue=1 |pages=425–433 |doi=10.1016/j.neuroimage.2004.04.025 |pmid=15325390 |s2cid=29426973 }}</ref><ref>{{cite journal |last1=Gregory |first1=Michael D. |last2=Kippenhan |first2=J. Shane |last3=Dickinson |first3=Dwight |last4=Carrasco |first4=Jessica |last5=Mattay |first5=Venkata S. |last6=Weinberger |first6=Daniel R. |last7=Berman |first7=Karen F. |title=Regional Variations in Brain Gyrification Are Associated with General Cognitive Ability in Humans |journal=Current Biology |date=May 2016 |volume=26 |issue=10 |pages=1301–1305 |doi=10.1016/j.cub.2016.03.021 |pmid=27133866 |pmc=4879055 |bibcode=2016CBio...26.1301G }}</ref>

In a meta-analysis, Deaner et al. (2007) tested absolute brain size (ABS), cortex size, cortex-to-brain ratio, EQ, and corrected relative brain size (cRBS) against global cognitive capacities. They have found that, after normalization, only ABS and neocortex size showed significant correlation to cognitive abilities. In primates, ABS, neocortex size, and N<small>c</small> (the number of cortical neurons) correlated fairly well with cognitive abilities. However, there were inconsistencies found for N<small>c</small>. According to the authors, these inconsistencies were the result of the faulty assumption that N<small>c</small> increases linearly with the size of the cortical surface. This notion is incorrect because the assumption does not take into account the variability in [[Cerebral cortex#Thickness|cortical thickness]] and cortical neuron density, which should influence N<small>c</small>.<ref>{{cite journal |title=Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates |journal=Brain Behav Evol |year=2007 |volume=70 |issue=2 |pages=115–124 |doi=10.1159/000102973 |pmid=17510549 |last1=Deaner |first1=Robert O. |last2=Isler |first2=Karin |last3=Burkart |first3=Judith |last4=Van Schaik |first4=Carel |citeseerx=10.1.1.570.7146 |s2cid=17107712 }}</ref><ref name="roth"/>

According to Cairo (2011), EQ has flaws to its design when considering individual data points rather than a species as a whole. It is inherently biased given that the cranial volume of an obese and underweight individual would be roughly similar, but their body masses would be drastically different. Another difference of this nature is a lack of accounting for sexual dimorphism. For example, the female human generally has smaller cranial volume than the male; however, this does not mean that a female and male of the same body mass would have different cognitive abilities. Considering all of these flaws, EQ should not be viewed as a valid metric for intraspecies comparison.<ref name="Cairo O. 2011 108"/>

The notion that encephalization quotient corresponds to intelligence has been disputed by Roth and Dicke (2012). They consider the absolute [[List of animals by number of neurons#Forebrain (cerebrum or pallium)|number of cortical neurons]] and [[Neural circuit|neural connection]]s as better correlates of cognitive ability.<ref>{{cite book |doi=10.1016/B978-0-12-374236-0.10001-X |isbn=9780123742360 |chapter=Brain Evolution |title=The Human Nervous System |year=2012 |last1=Herculano-Houzel |first1=Suzana |pages=2–13 }}</ref> According to Roth and Dicke (2012), mammals with relatively high cortex volume and neuron packing density (NPD) are more intelligent than mammals with the same brain size. The human brain stands out from the rest of the mammalian and vertebrate [[taxa]] because of its large cortical volume and high NPD, [[Nerve conduction velocity|conduction velocity]], and [[Connectome#Primary challenge for macroscale connectomics%3A determining parcellations of the brain|cortical parcellation]]. All aspects of human intelligence are found, at least in its primitive form, in other nonhuman primates, mammals, or vertebrates, with the exception of [[syntactical]] language. Roth and Dicke consider syntactical language an "intelligence amplifier".<ref name="roth"/>