2. IQ derives almost all of its validity because it is a good proxy for psychometric g. Arthur Jensen reports (P. 91, The g Factor) a g loading of about 0.88 for most IQ tests. Jensen suggests that the word intelligence not be used in scientific discussions, because it lacks a scientific definition, and that we should instead focus on g, since it is unambiguously defined as the product of a hierarchical factor analysis. It happens that all categories of test items correlate with one another to at least some degree. The ultimate relationship between the various categories of cognitive activity is reflected as g, which is common to all mental abilities.
3. Psychometric g is substantially heritable (rising from 70% in young adults, to over 80% in older adults). There are some identified genes; one example is Igf2r, which is associated with high intelligence. Although there are various physiological candidates that may account for some degree of what we see as g, two are especially salient: nerve conduction velocity (NCV) and myelination. (see the recent New Scientist article on this:
"By comparing brain maps of identical twins, which share the same genes, with fraternal twins, which share about half their genes, the team calculate that myelin integrity is genetically determined in many brain areas important for intelligence. This includes the corpus callosum, which integrates signals from the left and right sides of the body, and the parietal lobes, responsible for visual and spatial reasoning and logic (see above). Myelin quality in these areas was also correlated with scores on tests of abstract reasoning and overall intelligence (The Journal of Neuroscience, vol 29, p 2212)." http://www.newscientist.com/article/mg201
4. It is also linked to cortical thickness:
"In healthy adults, greater intelligence is associated with larger intracranial gray matter and to a lesser extent with white matter. Variations in prefrontal and posterior temporal cortical thickness are particularly linked with intellectual ability." Cerebral Cortex 2007 17(9):2163-2171
6. The differences are seen in children's brain development:
"The researchers found that the relationship between cortex thickness and IQ varied with age, particularly in the prefrontal cortex, seat of abstract reasoning, planning, and other "executive" functions. The smartest 7-year-olds tended to start out with a relatively thinner cortex that thickened rapidly, peaking by age 11 or 12 before thinning. In their peers with average IQ, an initially thicker cortex peaked by age 8, with gradual thinning thereafter. Those in the high range showed an intermediate trajectory (see below). While the cortex was thinning in all groups by the teen years, the superior group showed the highest rates of change."
7. Note that the general intelligence factor or 'g' is strongly predictive of academic performance. See the recent paper headed by Ian Deary:
"Deary took the analysis a step further however and did a little latent variable modeling. As the IQ test had three components/subtests (verbal, nonverbal, quantitative), he correlated a latent g factor with a latent academic factor using the following subtests: English, English Literature, Math, Science, Geography, French (n=12519). The correlation between the latent factors was .81. That is: 66% of the variance in latent (general) academic achievement can be explained by latent cognitive ability---measured 5 years previously.
Take home message: While general cognitive ability and academic achievement are not isomorphic, the former is necessary for the latter, while the converse is not necessarily true. Spearman suggested this more than a century ago, and, to quote the last sentence in Deary's work,
These data establish the validity of g for this important life outcome.
Ian J. Deary, Steve Strand, Pauline Smith and Cres Fernandes: Intelligence and educational achievement
9. This is the case internationally:
"International cognitive ability and achievement comparisons stem from different research traditions. But analyses at the interindividual data level show that they share a common positive manifold. Correlations of national ability means are even higher to very high (within student assessment studies, r = .60-.98; between different student assessment studies [PISA-sum with TIMSS-sum] r = .82-.83; student assessment sum with intelligence tests, r = .85-.86). Results of factor analyses indicate a strong g-factor of differences between nations (variance explained by the first unrotated factor: 94-95%)."
The g-factor of international cognitive ability comparisons: the homogeneity of results in PISA, TIMSS, PIRLS and IQ-tests across nations
Heiner Rindermann *
10. At a macro level this is very important to economic performance:
"In longitudinal analyses with various samples of nations, education and cognitive abilities appear to be more important as developmental factors for GDP than economic freedom. Education and intelligence are also more relevant to economic welfare than vice versa, but at the national level the influence of economic wealth on cognitive development is still substantial."
"Relevance of education and intelligence at the national level for the economic welfare of people"
Intelligence Volume 36, Issue 2, March-April 2008, Pages 127-142
11. The fact that there are some differences is not surprising when you consider new evidence of recent evolutionary change (see 'The 10,000 Year Explosion: How Civilization Accelerated Human Evolution' for a summary). For example, you see new versions of SLC6A4, a serotonin transporter, in Europeans and Asians. There's a new version of a gene (DBA1) that shapes the development of the layers of the cerebral cortex in east Asia. Over time more of this will be understood.