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When comparing different species brain size does present a correlation with intelligence. For example the ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50. However within the human species modern studies using MRI have shown that brain size shows substantial and consistent correlation ( r = .35 to .43 in various studies) with IQ among adults of the same sex  Some scientists prefer to look at more qualitative variables such to relate the size of measurable regions of known function. For example relating the size of the primary visual cortex to its corresponding functions, that of visual performance..
The brain is a metabolically expensive organ, and consumes about 25% of the body's metabolic energy. Because of this fact, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains. Skull size correlates with brain size, but is not necessarily indicative.
Brain size is a rudimentary indicator of the intelligence of a brain, and many other factors affect the intelligence of a brain. Higher ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Brain size in vertebrates may relate to social rather than mechanical skill. Cortical size relates directly to a pairbonding life style and among primates cerebral cortex size varies directly with the demands of living in a large complex social network.
Here is a list of some species, along with their rough average brain sizes:
- Homo erectus: 980 cm³
- Homo habilis: 750 cm³
- Homo floresiensis: 380 cm³
- Homo neanderthalensis: 1200-1750 cm³ skull capacity (10% greater than modern human average)
- Homo sapiens:1350-1400 cm³
A study on twins (Thompson et al., 2001) showed that frontal gray matter volume was correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002).
In a study of the head growth of 633 term-born children from the Avon Longitudinal Study of Parents and Children cohort, it was shown that prenatal growth and growth during infancy were associated with subsequent IQ. The study’s conclusion was that the brain volume a child achieves by the age of 1 year helps determine later intelligence. Growth in brain volume after infancy may not compensate for poorer earlier growth.
- Association/sensation ratio
- Brain to body mass ratio
- Brain weight
- Cerebral atrophy
- Cephalic index
- Cranial capacity
- Neuroscience and intelligence
- ↑ Brain size does not predict cognitive abilities within families
- ↑ Brain size and intelligence
- ↑ Dunbar RI, Shultz S (2007-09-07). Evolution in the social brain. “Science” 317: 1344-1347.
- ↑ Catharine R. Gale, PhD, Finbar J. O'Callaghan, PhD, Maria Bredow, MBChB, Christopher N. Martyn, DPhil and the Avon Longitudinal Study of Parents and Children Study Team. The Influence of Head Growth in Fetal Life, Infancy, and Childhood on Intelligence at the Ages of 4 and 8 Years. PEDIATRICS Vol. 118 No. 4 October 2006, pp. 1486-1492.