There has been quite a bit of study of the relationships between brain size, body size, and other variables across a wide range of species, largely because the easiest way to study any object is to measure its size. Even for extinct species brain size can be estimated by measuring the cavity inside the skull. The story that emerges, however, is complex.

As might be expected, brain size tends to increase with body size (measured by weight, which is roughly equivalent to volume). The relationship is not a strict proportionality, though: averaging across all orders of mammals, it follows a power law, with an exponent of about 0.75.¹ There are good reasons for expecting 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 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.² 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.³ 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.⁴ 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.⁵ 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 notorious that Neanderthals, which went extinct about 40,000 years ago, had larger brains than modern homo sapiens.⁶

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.⁷ Moreover they point out that intelligence depends not just on the amount of brain tissue, but on the details of how it is structured.

Human brain size

The balance of findings, which have been largely on participants of European ancestry, indicate an average adult brain volume of 1130 cubic centimetres (cc) for women and 1260 cc for men. There is substantial variation however;⁸ a study of 46 adults aged 22-49 years and of mainly European descent, found an average brain volume of 1273.6cc for men, ranging from 1052.9 to 1498.5cc, and 1131.1cc for women, ranging from 974.9 to 1398.1cc.⁹ The right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically of more similar size. There is evidence of an increase in average brain size over the course of the last centuries, estimated to have been around 0.5% per decade.¹⁰

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. Males have been found to have on average greater cerebral, cerebellar and cerebral cortical lobar volumes, except possibly left parietal.¹⁰ 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 hippocampus. When covaried for intracranial volume, height, and weight, the balance of studies 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.⁸

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 hippocampus (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 await replication.¹¹¹²

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.¹³ Structural neuroplasticity (increased grey 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.¹⁴¹⁵

Studies have tended to indicate (in terms of group averages) small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and the narrow measure known as the IQ test. 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 itself only shows a partial relationship to the general concept of intelligence and real-world performance.¹⁶¹⁷ In addition, brain volumes do not correlate strongly with other and more specific cognitive measures ⁹ In men, IQ correlates more with gray matter volume in the frontal lobe and parietal lobe, whereas in women it correlates with gray matter volume in the frontal lobe and Broca's area, which is involved in language.⁸

There is variation in child development in the size of different brain structures between individuals and genders.¹⁸ Total cerebral and grey matter volumes peak during the ages from 10–20 years (early 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.¹⁹ Young girls have on average relative larger hippocampal volume, whereas the amygdala is larger in boys.⁸

Significant dynamic changes in brain structure take place through adulthood and ageing, 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 hippocampus and parietal lobes.⁸ Men show a steeper decline in global grey 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.²⁰

Some anatomical trends are correlated in our evolutionary path with brain size; basicranium becomes more flexed with increasing brain size relative to basicranial length²¹ Due to human migration spacial differentiation of brain size is more diffused than earlier in time. In paleolithic past largest cranial capacity, in vivo filed by brains, is observed in Neanderthal specimens. An average Neandertahl had bigger brains than worldwide average person today. Microcefalin common allele was introgested, as dated by -D lineage mutational variance around 35 kya. Neanderthal population is major candidate among ancestral genetic pools where this gene evolved to be spread and shared by majority of Humans.

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References

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