Neuroscience and intelligence

Brain size
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 realtes 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.

Neuroanatomy
Many different sources of information have converged on the view that the frontal lobes are critical for fluid intelligence. Patients with damage to the frontal lobe are impaired on fluid intelligence tests (Duncan et al 1995). The volume of frontal grey (Thompson et al 2001) and white matter (Schoenemann et al 2005) have also been associated with general intelligence. In addition, recent neuroimaging studies have limited this association to the lateral prefrontal cortex. Duncan and colleagues (2000) showed using Positron Emission Tomography that problem-solving tasks that correlated more highly with IQ also activate the lateral prefrontal cortex. More recently, Gray and colleagues (2003) used functional magnetic resonance imaging (fMRI) to show that those individuals that were more adept at resisting distraction on a demanding working memory task had both a higher IQ and increased prefrontal activity. For an extensive review of this topic, see Gray and Thompson (2004).

In 2004, Richard Haier, professor of psychology in the Department of Pediatrics and colleagues at University of California, Irvine and the University of New Mexico used MRI to obtain structural images of the brain in 47 normal adults who also took standard IQ tests. The study demonstrated that general human intelligence appears to be based on the volume and location of gray matter tissue in the brain. Regional distribution of gray matter in humans is highly heritable. The study also demonstrated that, of the brain's gray matter, only about 6 percent appeared to be related to IQ.

A study involving 307 children (age between six to nineteen) measuring the size of brain structures using magnetic resonance imaging (MRI) and measuring verbal and non-verbal abilities has been conducted (Shaw et al 2006). The study has indicated that there is a relationship between IQ and the structure of the cortex—the characteristic change being the group with the superior IQ scores starts with thinner cortex in the early age then becomes thicker than average by the late teens.

Genetics
A recent University of Chicago/HHMI study showed that the amount of genetic diversity at two separate gene loci (microcephalin and ASPM) indicated a signature of positive selection during the primate lineage leading to humans. Both of these genes are expressed in developing brain and are thought to regulate brain size. Certain mutations at both of these genes cause human primary microcephaly ("small brain"), a hereditary disorder causing reduced size of the cerebral cortex. The University of Chicago/HHMI studies, however, caused a minor media stir when they published further papers, speculating that a common allelic signature found at these two gene loci in Euroasian populations but not in sub-Saharan African populations, indicated the evolution at these loci continued to occur into the recent past. They further speculated that these alleles were associated with distinct anthropological milestones such as the beginning of cave-painting (~37,000 years before present) and cities (~10,000 years before present) in the forementioned populations. They also speculated that existing racial differences might also be explained by the distribution of these alleles, even though it has yet to be shown that the amino acid substitutions in question confer specific differences in IQ. ["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]. Nonetheless, a greater part of the controversy stems from the studies being used to justify diminished educational opportunities and status quo policies affecting specific racial groups as well as for justifying commeasurate educational resource allocation to prevent further "wastage". Others point to a clear recipe for biased science given that it is natural for power groups to fund and promote studies that cement that power. (See Links Between Brain Genes, Evolution, and Cognition Challenged, Science 314:1872, by Michael Balter.)

However recent evidence suggests that these two alleles have no influence on brain size or intelligence.

Controversy
Lahn suggested that these mutations coincided with upper paleolithic technology and the development of writing. He also suggested that these mutations conferred a survival or reproductive benefit, perhaps a cognitive one and had been strongly selected sweeping through most of the world except Sub- Saharan Africa.

Websites promoting white racialism quickly seized on the evolutionary findings. One magazine called the discovery "the moment the antiracists and egalitarians have dreaded". the National Review Online, wrote that the research implied that "our cherished national dream of a well-mixed and harmonious meritocracy may be unattainable."

Lahn's study began to attract considerable controversy. Many scientists criticized Lahn stating that he overinterpreted and sensationalized  his findings. One of the co-authors Sarah Tishkoff, distanced herself from the study saying that she was bothered how the paper drew a link between the genetic changes and the rise of civilization. She felt that it was too early to reach any conclusions about why the changes spread and says it is "very simplistic" to imagine that a single gene could have a major effect on complex cultural traits. Richard Lewontin stated that the two papers were egregious examples of going well beyond the data to try to make a splash. Lahn would later concede that there was no real evidence natural selection had acted on cognition or intelligence through these genes.

Height
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.

Health
Several environmental factors related to health can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood-brain barrier is less effective. Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.

Improvements in nutrition, and in public policy in general, have been implicated in worldwide secular IQ increases (the Flynn effect).

Neurobiology
Other neurological parameters 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).