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 remains a controversial although frequently investigated question.
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 1370 g and in women about 1200 g. The volume is around 1260 cm3 in men and 1130 cm3 in women, although there is substantial individual variation. Yet another study argued 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, as with body mass. It is also important to note that variation between individuals is not as important as variation within species, as overall the differences are much smaller. The mechanisms of interspecific and intraspecific variation also differ.
Variation and evolution
From early primates to hominids and finally to Homo sapiens, the brain is progressively larger, with exception of extinct Neanderthals whose brain size exceeded modern Homo sapiens. The volume of the human brain has increased as humans have evolved (see Homininae), starting from about 600 cm3 in Homo habilis up to 1680 cm3 in Homo neanderthalensis, which was the hominid with the biggest brain size. Some data suggest that the average brain size has decreased since then, including a study concluding 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.
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.
=== 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 417 cm3, less than that of even Homo habilis, which is believed to have gone extinct far earlier (around 1.65 million years ago.). 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. This is beneficial as it reduces the basal metabolic rate without significant increases in predation risk.
Hydrocephalus
Exceptional cases of hydrocephalus, such as what was 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 could be drawn 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.Gould, S. J. (1981). The Mismeasure of Man. New York: W. W. Norton & Company.
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.