The size of the brain is a frequent topic of study within the fields of anatomy and evolution. Brain size is sometimes measured by weight and sometimes by volume (via MRI scans or by skull volume). Neuroimaging intelligence testing can be used to study brain size. One question that has been frequently investigated is the relation of brain size to intelligence.
The balance of findings for human brain size, largely based on participants of European ancestry, indicates an average adult brain volume of 1130 cubic centimeters (cm3) for women and 1260 cm3 for men. There is, however, substantial variation; a study of 46 adults aged 22–49 years and of mainly European descent found an average brain volume of 1273.6 cm3 for men, ranging from 1052.9 to 1498.5 cm3, and 1131.1 cm3 for women, ranging from 974.9 to 1398.1 cm3.
The right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically of more similar size. The adult human brain weighs on average about 1.5 kg (3.3 lb) with a volume of around 1130 cubic centimetres cm3 in women and 1260 cm3 in men, although there is substantial individual variation.
From early primates to hominids and finally to Homo sapiens, the brain has grown. The volume of the human brain 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. The increase in brain size stopped with neanderthals. Since then, the average brain size has been shrinking over the past 28,000 years. The cranial capacity has decreased from around 1,550 cm3 to around 1,440 cm3 in males while the female cranial capacity has shrunk from around 1,500 cm3 to around 1,240 cm3. Other sources with bigger sample sizes of modern Homo sapiens find approximately the same cranial capacity for males but a higher cranial capacity of around 1330 cm3 in females.
In recent years, experiments have been conducted drawing conclusions to brain size in association to the gene mutation that causes MCPH, a neural developmental disorder that affects cerebral cortical volume.
A number of studies have found correlation between variation in brain size in cranial capacity and geographic ancestry in humans. 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).
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. 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".
Overall, there is a background of similarity between adult brain volume measures of people of differing ages and sexes. Nevertheless, underlying structural asymmetries do exist. There is variation in child development in the size of different brain structures between individuals and genders. A human 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 age of 40, after which it begins declining at 5% per decade, speeding up around 70. Average adult male brain size is 1,345 gram, while an adult female has an average brain weight of 1,222 gram. Males have been found to have on average greater cerebral, cerebellar and cerebral cortical lobar volumes, except possibly left parietal. The gender differences in size vary by more specific brain regions. Studies have tended to indicate that men have a relatively larger amygdala and hypothalamus, while women have a relatively larger caudate and hippocampi. When covaried for intracranial volume, height, and weight, the balance of Kelly (2007) indicates women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There is high variability between individuals in these studies, however.
However, Yaki (2011) found no statistically significant gender differences in the gray matter ratio for most ages (grouped by decade), except in the 3rd and 6th decades of life in the sample of 758 women and 702 men aged 20–69. The average male in their third decade (ages 20–29) had a significantly higher gray matter ratio than the average female of the same age group. In contrast, among subjects in their sixth decade, the average woman had a significantly larger gray matter ratio, though no meaningful difference was found among those in their 7th decade of life.
Total cerebral and gray matter volumes peak during the ages from 10–20 years (earlier in girls than boys), whereas white matter and ventricular volumes increase. There is a general pattern in neural development of childhood peaks followed by adolescent declines (e.g. synaptic pruning). Consistent with adult findings, average cerebral volume is approximately 10% larger in boys than girls. However, such differences should not be interpreted 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, and of note is the high variability of brain size even in narrowly defined groups, for example children at the same age may have as much as a 50% differences in total brain volume. Young girls have on average relative larger hippocampal volume, whereas the amygdalae are larger in boys.
Significant dynamic changes in brain structure take place through adulthood and aging, 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. 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. Overall white matter volume does not appear to decline with age, although there is variation between brain regions.
Adult twin studies have indicated high heritability estimates for overall brain size in adulthood (between 66% and 97%). The effect varies regionally within the brain, however, 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. Genes may cause the association between brain structure and cognitive functions, or the latter may influence the former during life. A number of candidate genes have been identified or suggested, but they await replication.
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. The majority of MRI studies report moderate correlations around 0.3 to 0.4 between brain volume and intelligence. The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampi, and the cerebellum, but only account for a relatively small amount of variance in IQ, which suggests that while brain size may be related to human intelligence, other factors also play a role. In addition, brain volumes do not correlate strongly with other and more specific cognitive measures. In men, IQ correlates more with gray matter volume in the frontal lobe and parietal lobe, which is roughly involved in sensory integration and attention, whereas in women it correlates with gray matter volume in the frontal lobe and Broca's area, which is involved in language.
Research measuring brain volume, P300 auditory evoked potentials, and intelligence shows a dissociation, such that both brain volume and speed of P300 correlate with measured aspects of intelligence, but not with each other. Evidence conflicts on the question of whether brain size variation also predicts intelligence between siblings, with some studies finding moderate correlations and others finding none. A recent review by Nesbitt, Flynn et al. (2012) point out that crude brain size is unlikely to be a good measure of IQ, for example brain size also differs between men and women, but without well documented differences in IQ.
A discovery in recent years is that the structure of the adult human brain changes when a new cognitive or motor skill, including vocabulary, is learned. Structural neuroplasticity (increased gray matter volume) has been demonstrated in adults after three months of training in a visual-motor skill, with the qualitative change (i.e. learning of a new task) appearing more critical for the brain to change its structure than continued training of an already-learned task. Such changes (e.g. revising for medical exams) have been shown to last for at least 3 months without further practicing; other examples include learning novel speech sounds, musical ability, navigation skills and learning to read mirror-reflected words.
The largest brains are those of sperm whales, weighing about 8 kg (18 lb). An elephant's brain weighs just over 5 kg (11 lb), a bottlenose dolphin's 1.5 to 1.7 kg (3.3 to 3.7 lb), whereas a human brain is around 1.3 to 1.5 kg (2.9 to 3.3 lb). Brain size tends to vary according to body size. The relationship is not proportional, though: the brain-to-body mass ratio varies. The largest ratio found is in the shrew. Averaging brain weight across all orders of mammals, it follows a power law, with an exponent of about 0.75. There are good reasons to expect a power law: for example, the body-size to body-length relationship follows a power law with an exponent of 0.33, and the body-size to surface-area relationship follows a power law with an exponent of 0.67. The explanation for an exponent of 0.75 is not obvious; however, it is worth noting that several physiological variables appear to be related to body size by approximately the same exponent—for example, the basal metabolic rate.
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. 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. A standard formula for assessing an animal's brain size compared to what would be expected from its body size is known as the encephalization quotient. The encephalization quotient for humans is approximately 4.6.
When the mammalian brain increases in size, not all parts increase at the same rate. 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.The evolution of Homo sapiens over the past two million years has been marked by a steady increase in brain size, but much of it can be accounted for by corresponding increases in body size. There are, however, many departures from the trend that are difficult to explain in a systematic way: in particular, the appearance of modern man about 100,000 years ago was marked by a decrease in body size at the same time as an increase in brain size. Even so, it is noteworthy that Neanderthals, which went extinct about 40,000 years ago, had larger brains than modern Homo sapiens.
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. 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.
While humans have the largest encephalization quotient of extant animals, it is not out of line for a primate. Gorillas are out of line, having a smaller brain to body ratio than would be expected. Some other anatomical trends are correlated in the human evolutionary path with brain size: the basicranium becomes more flexed with increasing brain size relative to basicranial length.
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, and this in turn is used as a rough indicator of the potential intelligence of the organism. 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. 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.
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. 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. 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.
However, larger cranial capacity is not always indicative of a more intelligent organism, since larger capacities are required for controlling a larger body, or in many cases are an adaptive feature for life in a colder environment. For instance, among modern Homo Sapiens, northern populations have a 20% larger visual cortex than those in the southern latitude populations, and this potentially explains the population differences in brain size (and roughly cranial capacity). Neurological functions are determined more by the organization of the brain rather than the volume. Individual variability is also important when considering cranial capacity, for example the average Neanderthal cranial capacity for females was 1300 cm3 and 1600 cm3 for males 
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. 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.
Examples of cranial capacity
- Orangutans: 275–500 cm3 (16.8–30.5 cu in)
- Chimpanzees: 275–500 cm3 (16.8–30.5 cu in)
- Gorillas: 340–752 cm3 (20.7–45.9 cu in)
- Neanderthals: 1,500–1,800 cm3 (92–110 cu in)
- Australopithecus afarensis; 438 cm3
- Australopithecus africanus 452 cm3
- Paranthropus boisei 521 cm3
- Paranthropus robustus 530 cm3
- Brain-to-body mass ratio
- Craniometry — includes historical discussion
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