Neuroscience and intelligence
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[edit] Brain size
The correlation between brain size and IQ seems to hold for comparisons between and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found no such family-related connection (Schoenemann et al. 2000).
The average brain is the size of a clenched fist, and is a metabolically expensive organ, which 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.
A recent University of Chicago study linked 2 sets of genetic variation (alleles) to brain size, race and spurts in human evolution. In particular, these genetic variations—arguably responsible for greater intelligence—were relatively common in Europe and Asia, but markedly less common in sub-Saharan Africa. ["Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans", Bruce T. Lahn et al. , SCIENCE www.sciencemag.org, 9 September 2005, Vol 309]. The variations can be established as appearing 8-10,000 years BC, spreading to 70% and 30% of humans respectively, and are associated with agriculture and written language.
Here is a list of some species, along with their rough average brain sizes:
- Homo erectus: 980 cm³
- Homo habilis: 750 cm³
- Homo floriensis: 380 cm³
- Homo neanderthalensis: 1200-1750 cm³ skull capacity (10% greater than modern human average)
- Homo sapien:1350-1400 cm³
[edit] Height
Surprisingly, epidemiological studies have shown that intelligence is positively correlated with body height in human populations. Similar associations have been found in early and late childhood and adulthood in both developed and developing countries, and associations persisted after controlling for social class and parental education. The reasons for this association between height and intelligence remain unclear, but possible explanations include that height may be a biomarker of nutritional status or general mental and physical health during development, that common genetic factors may influence both height and intelligence, or that both height and intelligence may be affected by adverse early environmental exposures. Alternatively, it may be explained by differences in brain size, which has a positive relationship with height. A large recent twin pair study of the height-intelligence relationship showed that both shared environment (59%) and shared genetics (35%) are responsible for significant portions of the observed correlation between intelligence and height.
[edit] Neuroanatomy
A study on twins (Thompson et al. 2001) showed that frontal gray matter volume was correlated with g, a measure of generalized intelligence, and is 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). Note that none of the MRI studies have studied racial differences, with the exception of the University of Chicago study above.
[edit] Neurobiology
Other neurological paramaters have been associated with IQ. Haier et al (1995) found a correlation of -0.58 between glucose metabolic rate "GMR" (an indicator of energy use) and IQ. This suggested that intelligence is associated with more efficient brains. Others found a positive correlation between IQ and GMR (DeLeon et al 1983; Chase et al 1984). It seems like difference in results comes from different cognitive tasks (complicated vs. simple) that were performed by examinees (Fidelman, 1993).
[edit] See also
[edit] References
- Race and intelligence (References)
- Washington State University
- Neuroscience for Kids
- Haier, R. J., Chueh, D., Touchette, P., Lott, I., Buchsbaum, M., Macmillan, D., et al. (1995). "Brain size and cerebral glucose metabolic rate in nonspecific mental retardation and Down syndrome". Intelligence 20: 191–210.
- Michael A. McDaniel, Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence, Intelligence, Volume 33, Issue 4, July-August 2005, Pages 337-346. [1]
