Behavioral Genetics in the Postgenomic Era, published by the American Psychological Association, 2003. Editors: Plomin, Defries, Graig & McGuffin "Willerman, Schultz, Rutledge, and Bigler correlated brain size as measured through MRI within a sample of 40 unrelated participants. They found a correlation of 0.51, which was higher in men (0.65) than in women (0.35). In a follow-up study, Willerman, Schultz, Rutledge and Bigler suggested that, in men, a relatively larger left hemisphere better predicted verbal IQ than it predicted performance IQ, whereas in women the opposite was true. Since then, several studies have provided confirmative evidence that brain volume and IQ correlate around 0.40. "In a large MRI study including 111 twin pairs and 34 additional siblings, the heritability of volumes of several brain structures was investigated. Heritability estimates for intracranial volume, total brain volume, gray-matter volume, white-matter volume, and cerebellar volume were all between 80% and 90%. Genetic intercorrelations between these measures were all very high, indicating that a largely overlapping set of genes is responsible for individual differences in each of these measures…. This indicates that about half of the genetic influences on either cerebral brain volume or IQ is due to genetic factors influencing both. Put differently, 80% of the phenotypic [observed human differences] correlation is explained by genetic mediation." "This suggests that genes important for brain size, reaction times, inspection time, and theta coherence may also be important for intelligence, which fits very well in the myelination hypothesis as formulated by Miller. According to this hypothesis, generally, the relation between speed and intelligence can be explained if part of the interindividual variance in intelligence can be ascribed to interindividual variance in the degree of myelination of cortico-cortical connections. If true, this could explain why more intelligent brains show faster nerve conduction, faster reaction times, and faster inspection times. And, all other things equal, thicker myelin sheaths will result in larger brain volume, thus explaining the positive relation between brain size and IQ." ----------------------------------------------- The Scientific Study of General Intelligence: Tribute to Arthur R. Jensen. Edited by Helmuth Nyborg, 2003 "In Chapter 2, Britt Anderson takes a closer look at the brain size-g relationship. He first discusses the low and variable relationships found in earlier studies using such rough measures as external head circumference, and then presents the outcome of studies using more exacting in vivo neuro-imaging techniques. The overall conclusion is that anatomical and metabolic imaging techniques using magnetic resonance technology suggest a correlation in the order of 0.35 between brain size and IQ, a finding that is consistent across multiple experimental groups. Another important conclusion is that the majority of individual variation in intelligence is not explained by variation in brain volume. A third conclusion is, that we still do not know whether specific brain regions or compartments are the principal basis for the size-IQ correlation, and this sets the stage for further experiments exploiting the many new capabilities of magnetic resonance imaging and other brain image techniques." He also notes that others have concluded that, "between 80~90 percent of brain volume is heritable, and that the genetic correlation between brain volume and IQ is 0.48." "The causes of genetically based racial differences in intelligence should be sought in their evolutionary history. Differences in IQ must have developed together with differences in skin color, morphology and resistance to diseases as adaptations to the environments in which the races evolved. We can reconstruct the broad outline of how this occurred. Modern humans evolved in Central East Africa about a quarter of a million years ago. Their brain size was the same as that of living blacks and it can be assumed that their intelligence was the same, represented by an IQ of 69." "The morphological basis of the increase in intelligence in the Caucasoids and Mongoloids was an enlargement of brain size the evidence for which is set out by Rushton." "Myopia (near-sightedness) is positively correlated with IQ. The relationship appears to be pleiotropic, that is, a gene affecting one of the traits also has some effect on the other. Further, there are significant racial and ethnic differences in the frequency of myopia, with the highest rates found in East Asians, the lowest rates among Africans and Europeans intermediate. Among Europeans, Jews have the highest rate of myopia, about twice that of gentiles and about on a par with that of Asians. Miller suggested that myopia is caused by extra myelinization in the eye and is similarly linked to brain size." "Ulric Neisser, Chair of the recent American Psychological Association's Task Force Report on The Bell Curve, acknowledged that, with respect to 'racial differences in the mean measured sizes of skulls and brains (with East Asians having the largest, followed by Whites and then Blacks) ... there is indeed a small overall trend.'"