[3] Encephalization quotient was developed in an attempt to provide a way of correlating an animal's physical characteristics with perceived intelligence.
Subsequent work, notably Roth,[4] found EQ to be flawed and suggested brain size was a better predictor, but that has problems as well.
[citation needed] 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.
[9][citation needed] 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.
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.
In primates, ABS, neocortex size, and Nc (the number of cortical neurons) correlated fairly well with cognitive abilities.
According to the authors, these inconsistencies were the result of the faulty assumption that Nc increases linearly with the size of the cortical surface.
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.
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.
[21] To determine the value of this factor, the brain and body weights of various mammals were plotted against each other, and the curve of such formula chosen as the best fit to that data.
[15] The driving theorization behind the development of EQ is that an animal of a certain size requires a minimum number of neurons for basic functioning, sometimes referred to as a grey floor.
[27] Animals with nutrient rich diets tend to have higher EQs, which is necessary for the energetically costly tissue of brain matter.
Arguments have been made that some carnivores may have higher EQ's due to their relatively enriched diets, as well as the cognitive capacity required for effectively hunting prey.
[30] Of the animals demonstrating the highest EQ's (see associated table), many are primarily frugivores, including apes, macaques, and proboscideans.
Specifically, frugivores must utilize a complex, trichromatic map of visual space to locate and pick ripe fruits and are able to provide for the high energetic demands of increased brain mass.
Eutheria low on the network of food chains can only develop a high RB (relative brain-mass) so long as they have small body masses.
Animals with very large flock size and/or complex social systems consistently score high EQ, with dolphins and orcas having the highest EQ of all cetaceans,[34] and humans with their extremely large societies and complex social life topping the list by a good margin.
[4] Birds generally have lower EQ than mammals, but parrots and particularly the corvids show remarkable complex behaviour and high learning ability.
Despite the jumping spider having a huge brain for its size, it is minuscule in absolute terms, and humans have a much higher EQ despite having a lower raw brain-to-body weight ratio.
[40] Biologist Stephen Jay Gould has noted that if one looks at vertebrates with very low encephalization quotients, their brains are slightly less massive than their spinal cords.
Behavioral complexity in living animals can to some degree be observed directly, making the predictive power of the encephalization quotient less relevant.
However, this technique is only limited to when there are both cranial and post-cranial remains associated with individual fossils, to allow for brain to body size comparisons.
[42] For example, remains of one Middle Pleistocene human fossil from Jinniushan province in northern China has allowed scientists to study the relationship between brain and body size using the Encephalization Quotient.
Paleo-neurological comparisons between Neanderthals and anatomically modern Homo sapiens (AMHS) via Encephalization quotient often rely on the use of endocasts, but this method has many drawbacks.
[43] The earliest Homo species were larger in brain size as compared to contemporary Australopithecus counterparts, with which they co-inhabited parts of Eastern and Southern Africa.
[43] Specifically, phrenologists paid attention to the external morphology of the skull, trying to relate certain lumps to corresponding aspects of personality.
In 1889, Sir Francis Galton, through a study on college students, attempted to quantify the relationship between brain size and intelligence.
[43] Due to Hitler's racial policies during World War II, studies on brain size and intelligence temporarily gained a negative reputation.
[43] However, with the advent of imaging techniques such as the fMRI and PET scan, several scientific studies were launched to suggest a relationship between encephalization and advanced cognitive abilities.
Harry J. Jerison, who invented the formula for encephalization quotient, believed that brain size was proportional to the ability of humans to process information.