This article needs attention from an expert in biology.(September 2009)
The size of the brain is a frequent topic of study within the fields of anatomy and evolution. Brain size can be measured by weight or by volume (via MRI scans or by skull volume). The relationship between brain size and intelligence is frequently a topic of research.
Studies on human brain size, largely based on participants of European ancestry, tend to find an average adult brain volume of 1260 cubic centimeters (cm3) for men and 1130 cm3 for women. There is, however, substantial variation between individuals; one study of 46 adults, aged 22–49 years and of mainly European descent, found an average brain volume of 1273.6 cm3 for men, with a range of 1052.9 to 1498.5 cm3, and 1131.1 cm3 for women, with a range of 974.9 to 1398.1 cm3.
The right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically closer in size. The adult human brain weighs on average about 1.5 kg (3.3 lb). The average weight is about 1370 g in men and about 1200 g in women.
Changes over time
Brain size has increased considerably over the course of humans' recent evolutionary history. Homo erectus, a relative of humans, had a brain size of 1,100 cm3. Homo floresiensis, with a brain size of 380 cm3. Neanderthals had a slightly larger brain than modern humans, perhaps due to larger visual systems.
Some studies suggest that the average brain size has been decreasing over the past 28,000 years. Others suggest that the cranial capacity for males is unchanged, but that the cranial capacity of females has increased.
Average cranial capacity in humans varies significantly depending on geographic ancestry in humans, in the range of 1,200 to 1,450 cm3 between populations. Larger cranial volume is associated with climatic region, the largest averages being found in populations of Siberia and the Arctic. For this reason, Beals et al. (1984) proposed that the primary reason for the variation is climatic adaptation, favoring 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).
Changes over the lifespan
A 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 years, after which it begins to decline by 5% per decade, speeding up at around 70 years. Total cerebral and gray matter volumes peak between 10 and 20 years (earlier in girls than in boys) of age, whereas white matter and ventricular volumes increase. There is a general pattern in which neural development peaks in childhood and declines in adolescence, a process known as synaptic pruning. Overall white matter volume does not appear to decline with age, although there is variation among brain regions.
The average brain weight in adult males is 1,345 grams; in adult females, 1,222 grams. Males have been found to have, on average, greater cerebral, cerebellar, and cerebral cortical lobar volumes, except possibly left parietal.
When covaried for intracranial volume, height, and weight, one study found that women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There was high variability among individuals, however. Yaki (2011) found no statistically significant gender differences in the gray matter ratio for most ages in a sample of 758 women and 702 men aged 20–69.
Consistent with findings in adults, average cerebral volume is approximately 10% larger in boys than in girls. However, such differences should not be construed 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. Moreover, brain volumes, even in narrowly defined groups (e.g. children of the same age), may vary by as much as 50%. Young girls have, on average, larger hippocampi, whereas young boys have larger amygdalas.
Significant dynamic changes in brain structure take place throughout adulthood, 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.
In the 19th century, American anthropologist Samuel George Morton reported that whites had the greatest average cranial capacity, followed, in descending order, by Native Americans and blacks. Stephen Jay Gould argued in 1978 and in his subsequent book the Mismeasure of Man that Morton unconsciously misrepresented his data and that when it was properly interpreted, it showed no significant racial differences in cranial capacity. This claim was criticized in a 2011 paper, which concluded that Morton did not manipulate his results, unconsciously or otherwise. This paper has, in turn, been criticized for making unwarranted conclusions as to whether Gould's claims of bias were correct or not.
J. Philippe Rushton published multiple studies in the 1980s to 2000s claiming that average brain size was lowest in blacks (Negroids) and highest in East Asians (Mongoloids), with whites (Caucasoids) in between the two. His work in this area has been criticized for relying on flawed studies, for failing to consider explanations other than genetics for the observed differences, and for ignoring other studies with contradictory conclusions. Nathan Brody has also argued that the evidence regarding racial differences in brain size is not conclusive, and that, even if one accepts it, this difference does not support a genetic hypothesis regarding racial differences in intelligence. Critics of the hereditarian position also note that the difference in mean brain size between blacks and whites is smaller than 1 standard deviation and is insufficient to explain the vast majority of the black-white IQ gap.
Adult twin studies indicate that heritability of overall brain size in adulthood is high (between 66% and 97%). Infant brain volumes are also highly heritable, with heritability of total brain volume in neonates of around .8-.9.
Heritability varies regionally within the brain, 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.
Early studies yielded suggestive candidate genes. Much larger genome-wide studies have now yielded highly replicable associations for at least 8 genes linked to cortical and subcortical brain volume in a study of over 32,000 humans.
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. In healthy adults, the correlation of total brain volume and IQ is ~ 0.4  The correlation appears to be related to the known small correlation of height with intelligence, which can be explained almost entirely by greater brain volume.
Studies have sought particular regions that are more correlated with IQ than whole-brain volume. While consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampus, and the cerebellum, unique variation in these regions account for a relatively small amount of variance in IQ. The search for specific brain regions that correlate highly with cognitive measures designed to be specific has not yielded clear results.
There may be sex differences in the volume-IQ association. Some evidence suggests that while IQ correlates equally with frontal lobe volume, it may correlate more with parietal lobe volumes in men and with Broca's area in women, corresponding to spatial versus language specializations.
Small studies have attempted to link brain volume with functional measures such as P300 auditory evoked potentials but finding no association. Studies attempting to related sibling differences in IQ to differences in brain volume are hampered by relatively small sample sizes and the noisy nature of such difference scores, yielding weak evidence for cross-trait cross-sib correlations.
The largest brains are those of sperm whales, weighing about 8 kg (18 lb), and killer whales, weighing about 12–15 lb (5.4–6.8 kg). An elephant's brain weighs just over 5 kg (11 lb) and a bottlenose dolphin's 1.5 to 1.7 kg (3.3 to 3.7 lb).
Brain size tends to vary according to body size. The relationship is not proportional, however; 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. 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.
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.
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 grey parrots are quite intelligent even though they have small brains.
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 volume of the cranium is used as a rough indicator of the size of the brain, although due to the thickness of the membranes that surround the brain, brain volume is less than cranial capacity. 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. However, this method of measuring cranial capacity must be validated in each species to know whether it is an accurate representation of the braincase.
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, such as nutrition, can also affect cranial capacity.
The volume of the human braincase has increased as humans have evolved (see Homininae), starting from about 600 cm3 in Homo habilis up to 1740 cm3 in Homo neanderthalensis, which was the hominin with the biggest brain size.
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 et al. (2005).
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)
- Australopithecus afarensis: 445.8 cm3 (27.20 cu in)
- Australopithecus africanus: 491.2 cm3 (29.97 cu in)
- Homo habilis: 610.3 cm3 (37.24 cu in)
- Homo erectus: 1,092.9 cm3 (66.69 cu in)
- Homo erectus soloensis: 1,155.8 cm3 (70.53 cu in)
- Homo heidelbergensis: 1,262.8 cm3 (77.06 cu in)
- Homo neanderthalensis: 1,427.2 cm3 (87.09 cu in)
- Homo sapiens: 1,496.5 cm3 (91.32 cu in)
- Individual range, without cephalic disorder: 900–2,100 cm3 (55–128 cu in)
- Global group means range: 1,085–1,581 cm3 (66.2–96.5 cu in)
- Average: 1,353 cm3 (82.6 cu in)
- Brain-to-body mass ratio
- Encephalization quotient
- Craniometry — includes historical discussion
- Human brain
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