Posted on November 21, 2021

Brain Size Counts

Hippocrates, American Renaissance, July 2011

There is no doubt that when species are compared, a large brain confers greater intelligence than a small brain. Fish and reptiles have small brains and are not very bright. Cats and dogs have larger brains and are much brighter. Monkeys and apes have still larger brains and are brighter still. Humans have the largest brains — about three times larger than apes’ brains — and we are brightest of all, which is why we control the world.

Brain size is generally in proportion to body size. Elephants and sperm whales have larger brains than we do, but we are still smarter. Within broad limits, what matters for intelligence is the ratio of brain size to body size, or what is called the encephalization index. The table to the left gives the encephalization indices (EI) for various mammals. As a rule, the higher the EI, the greater the intelligence.

Species EI
Human 7.4 – 7.8
Bottlenose dolphin 4.14
Orca 2.57 – 3.3
Chimpanzee 2.2 – 2.5
Rhesus monkey 2.1
Whale 1.8
Elephant 1.13 – 2.36
Dog 1.2
Cat 1.00
Horse 0.9
Sheep 0.8
Mouse 0.5
Rat 0.4
Rabbit 0.4

It is a vexed question whether the association between brain size and intelligence holds for humans, and many anthropologists and psychologists have denied the association. For example, Professor Maciej Henneberg, professor of Anthropological and Comparative Anatomy at the University of Adelaide, has written that “there is ever more evidence accumulating against a direct relationship between cranial capacity and intellectual capacity.” Professor Diane Halpern, a past president of the American Psychological Association and currently Professor of Psychology at Claremont McKenna College, has written that “there is no evidence that larger brains are, in any way, better than smaller brains.”

Why are Professors Maciej Henneberg, Diane Halpern and many others so adamant about this? Could it be that if an association between brain size and intelligence is admitted, then it follows that races with larger brains are likely to be the more intelligent than those with smaller brains? This is a question many anthropologists and psychologists prefer to avoid.

The definitive study of race differences in brain size was carried out on approximately 20,000 crania by Professor Kenneth Beals and his colleagues at Oregon State University. Their results for endocranial volume, measured in cubic centimeters for the major races were as follows: North East Asians (Chinese, Japanese and Koreans): 1,416 cm; Europeans: 1,369cm; Native American Indians: 1,366cm; Southeast Asians: 1,332cm; Pacific Islanders: 1,317cm; South Asians: 1,293cm; Sub-Saharan Africans: 1,282cm; Bushmen: 1,270cm; Australian Aborigines: 1,225cm. These brain size differences correspond with intelligence differences derived from IQ tests given by Prof. Richard Lynn, who finds IQs of 105 for North East Asians,100 for Europeans, and so on downwards to 62 for Australian Aborigines and 54 for the Bushmen of the Kalahari desert.

Profs. Kenneth Beals and his colleagues were evidently somewhat embarrassed to find that North East Asians and Europeans had larger brains than the other races. Could this mean that they are more intelligent? The Oregon professors went out of their way to deny this. Of the possibility of a relationship between brain size and intellectual capacity they wrote: “No convincing case for such associations has ever been presented.”

Contrary to these assertions, the positive association between brain size and intelligence in humans has been shown in numerous studies beginning in the first decade of the 20th century. Professor Philip A. Vernon of the University of Western Ontario and his colleagues have summarized studies of the correlation between intelligence and head size, and the correlation between intelligence and the size of the brain itself. Every one of 54 studies that measured head size showed a positive relationship, with an overall correlation of 0.18. Research using CT (computerized axial tomography) and MRI (magnetic resonance imaging) gives a more accurate measure of brain size, and the 11 studies that used these methods found an overall correlation with intelligence of 0.40. Prof. Vernon and his colleagues conclude that brain size must be a determinant of intelligence because larger brains have more neurons and this gives them greater processing capacity.

A recent study has shown that it is not only in humans that a larger brain size helps solve new and difficult problems. Dr. Alexei Maklakov and his colleagues at the University of Uppsala found that this is also the case for birds. In a study of common species of passerine (perching) birds in and around 12 representative cities in France and Switzerland, they report that birds with large brains, such as crows, rooks, magpies, starlings and some breeds of sparrows and finch, have colonized towns and cities, while birds with small brains, such as larks, dippers, buntings and warblers, have not been able to do this and are found exclusively in the country.

They suggest that birds with larger brains are better able to adapt to the novel environments of towns and cities. For example, blue tits learn to peck through the tops of milk bottles left on doorsteps, and Japanese crows learn to drop nuts on roads and wait for an automobile to run over them and crack them open. When the road is clear they retrieve the nutmeats.

In a related study, Dr. Suzanne Shultz and her colleagues at the University of Oxford have found that in Britain, where the habitat for birds has been reduced in recent decades as a result of increasingly intensive farming, the numbers of birds has declined. The decline has been greatest among birds with smaller brains, apparently because they have been less successful in adapting to the changed environment. Larger brained birds have been able to migrate into towns and cities and thrive there.

The same relationship holds true for domesticated as opposed to wild species of mammals. Dogs have a lower encephalization index than wolves, and the same is true for domesticated pigs, horses, llamas, ferrets, etc. compared to wild variants. This is clearly a genetic trait because wild animals kept in captivity for several generations do not decrease in brain size, nor do domestic animals that go feral increase in brain size in just a few generations. Experiments have shown that wild animals are considerably smarter than their domestic kin; they have to be, since they do not depend on humans and have to fend for themselves. Many also have complex group behavior in the wild that is lost or diminished in captivity.

The amount of brain loss that results from domestication varies by species, but those that originally had high encephalization indices lose the most. Tame laboratory mice do not suffer any brain loss at all compared to their wild cousins. Tame guinea pigs have 5 to 7 percent less brain mass than wild guinea pigs, and lab rats have lost only about 9 percent through domestication.

The more intelligent wild animals give up more brain mass when they are domesticated: Llamas and alpacas — 19 percent; cats — 23 percent; dogs — 29 to 34 percent, depending on the breed; sheep — 30 percent; pigs — 34 percent. The decline in intelligence for a species is roughly in proportion to the percentage of brain mass that has been lost.

The message of all these studies is clear: A larger brain helps a species solve new problems. It can make the difference between survival and extinction when environments change. Brain size counts — even for birds.


Roth, G. and Dicke, U. Evolution of the brain and intelligence. TRENDS in Cognitive Sciences 9 (5): 250.

Beals, K. L., Smith, C. L. and Dodd, S. M. Brain size, cranial morphology, climate and time machines. Current Anthropology, 1984. 25, 301–330.

Henneberg, M. Comment on Beals Dodd and Smith. Current Anthropology, 1984. 25, 321–322.

Lynn, R. Race Differences in Intelligence: An Evolutionary Analysis. Augusta, GA: Washington Summit Publishers, 2006.

Maklakov, A. A., Immler, S. Gonzalez-Voyer, A. Rönn, J. & Kolm, N. Brains Sex and the city: big-brained passerine birds succeed in urban environments. Biology Letters, 2011, published online 27 April

Shultz, S., Bradbury, R. B., Evans, K. L., Gregory, R.D. & Blackburn, T. M. Brain size and resource specialization predict long-term population trends in British birds. Proceedings of the Royal Society, B, 2005, 272, 2305–2311.

Vernon, P.A., Wickett, J.C., Bazana, P.G. and Stelmack, R.M. The neuropsychology and neurophysiology of human intelligence. In R. J. Sternberg (Ed) Handbook of Intelligence. Cambridge, UK: Cambridge University Press, 2000.