Brain size
The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution. Measuring brain size and cranial capacity is relevant both to humans and other animals, and can be done by weight or volume via MRI scans, by skull volume, or by neuroimaging intelligence testing.
The relationship between brain size and intelligence has been a controversial and frequently investigated question. In 2021 scientists from Stony Brook University and the Max Planck Institute of Animal Behavior published findings showing that the brain size to body size ratio of different species has changed over time in response to a variety of conditions and events.
As Kamran Safi, researcher at the Max Planck Institute of Animal Behavior and the study's senior author writes:
"Sometimes, relatively big brains can be the end result of a gradual decrease in body size to suit a new habitat or way of moving—in other words, nothing to do with intelligence at all."
Humans
In humans, 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. In men the average weight is about and in women about. The volume is around in men and in women, although there is substantial individual variation. Yet another study found that adult human brain weight is 1300–1400 g for adult humans and 350–400 g for newborn humans. There is a range of volume and weights, and not just one number that one can definitively rely on. Variation between humans of similar age is smaller than that between species. The mechanisms of interspecific and intraspecific variation also differ.Variation and evolution
From early primates to hominids and finally to Homo sapiens, the brain gets progressively larger - with the exception of extinct Neanderthals whose brain size exceeded that of modern Homo sapiens. The volume of the human brain has increased as humans have evolved, starting from about in Homo habilis up to in Homo neanderthalensis, which was the hominid with the biggest brain size. Some data suggest that the average brain size has decreased since then. One study concludes that the decrease "was surprisingly recent, occurring in the last 3,000 years". However, a reanalysis of the same data suggests that brain size has not decreased, and that the conclusion was made using datasets that are too dissimilar to support quantitative comparison.Richerson and Boyd point out disadvantages to large brain-size.
Proponents of recent changes in brain size draw attention to the gene mutation that causes microcephaly, a neural developmental disorder that affects cerebral cortical volume. Similarly, sociocultural explanations draw attention to externalization of knowledge and group decision-making, partly via the advent of social systems of distributed cognition, social organization, division of labor and sharing of information as possible causes.
| Name | Brain size |
| Homo habilis | 550–687 |
| Homo ergaster | 700–900 |
| Homo erectus | 600–1250 |
| Homo heidelbergensis | 1100–1400 |
| Homo neanderthalensis | 1200–1750 |
| Homo sapiens | 1400 |
| Homo floresiensis | 417 |
''H. floresiensis''' small brain
Homo floresiensis is a hominin from the island of Flores in Indonesia with fossils dating from 60,000-100,000 years ago. Despite its relatively derived position in the hominin phylogeny, CT imaging of its skull reveals that its brain volume was only, less than that of even Homo habilis, which is believed to have gone extinct far earlier. The reason for this regression in brain size is believed to be island syndrome, in which the brains of insular species become smaller due to reduced predation-risk. A smaller brain is beneficial as it reduces the basal metabolic rate without significant increases in predation risk.Hydrocephalus
Exceptional cases of hydrocephalus, such as that reported by John Lorber in 1980 and by a study with rats, suggest that relatively high levels of intelligence and relatively normal functioning are possible even with very small brains. It is unclear what conclusions to draw from such reports – such as about brain capacities, redundancies, mechanics and size requirements.Biogeographic variation
Efforts to find racial or ethnic variation in brain size are generally considered to be a pseudoscientific endeavor and have traditionally been tied to scientific racism and attempts to demonstrate a racial intellectual hierarchy.The majority of efforts to demonstrate this have relied on indirect data that assessed skull measurements as opposed to direct brain observations. These are considered scientifically discredited.
A large-scale 1984 survey of global variation in skulls has concluded that variation in skull and head sizes is unrelated to race, but rather climatic heat preservation, stating "We find little support for the use of brain size in taxonomic assessment. Racial taxonomies which include cranial capacity, head shape, or any other trait influenced by climate confound ecotypic and phyletic causes. For Pleistocene hominids, we doubt that the volume of the braincase is any more taxonomically 'valuable' than any other trait."
Sex
A human baby's brain at birth averages and increases, during the first year of life, to about, after which the growth rate declines. Brain volume peaks at the teenage years, and after the age of 40 it begins declining at 5% per decade, speeding up around 70. Average adult male brain weight is, while an adult female has an average brain weight of. 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, Kelly 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 found no statistically significant gender differences in the gray matter ratio for most ages, 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 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, whereas white matter and ventricular volumes increase. There is a general pattern in neural development of childhood peaks followed by adolescent declines. 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. However, multiple studies have found a higher synaptic density in males: a 2008 study reported that men had a significantly higher average synaptic density of 12.9 × 108 per cubic millimeter, whereas in women it was 8.6 × 108 per cubic millimeter, a 33% difference. Other studies have found an average of 4 billion more neurons in the male brain, corroborating this difference, as each neuron has on average 7,000 synaptic connections to other neurons.
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.
Genetic contribution
Adult twin studies have indicated high heritability estimates for overall brain size in adulthood. The effect varies regionally within the brain, however, with high heritabilities of frontal lobe volumes, moderate estimates in the hippocampi, 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.Intelligence
Studies demonstrate a correlation between brain size and intelligence, 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 hippocampus, 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, as some studies find moderate correlations and others find none. A recent review by Nesbitt, Flynn et al. points out that crude brain size is unlikely to be a accurate measure of IQ. Brain size is known to differ between men and women, for example, but without well documented differences in IQ. A 2017 study found that the brains of women have a higher density of grey matter, which could compensate for the loss of volume.
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 has been demonstrated in adults after three months of training in a visual-motor skill, as the qualitative change appear more critical for the brain to change its structure than continued training of an already-learned task. Such changes 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.