25 The relation between P300 onset and IQ has not been consistent, however, and depends In part on the intelligence test utilized. Brain size and intelligence Some investigators have examined the relationship between brain size and intelligence.26 For humans, the statistical relationship is modest but significant. Obviously, the finding is only correlational: greater brain size may cause greater intelligence, greater intelligence may cause greater brain size, or both may be
dependent on some third factor. Moreover, how efficiently the brain is Inhibitors,research,lifescience,medical used is probably more important than its size. For example, on average, men have larger brains than women, but women have better connections, through the corpus callosum, between the two hemispheres. So it is not clear which sex would have, on average, an advantage—probably neither.27
The relationship between brain size and intelligence does not hold across species.28 Rather, there seems to be a relationship between intelligence and brain size Inhibitors,research,lifescience,medical relative to the rough general size of the organism (level of encephalization). Genetic and heritability studies of intelligence Although numerous attempts have been made to identify genes that are IOX2 datasheet critical to intelligence,29,30 no single gene has been conclusively identified, and it looks as Inhibitors,research,lifescience,medical though there will be no “gene” for intelligence to be found.31 So far. Investigators have conducted at least six genome-wide scans for genes contributing to intelligence and other aspects of cognition. The data from these scans vary, but there are definitely some partial overlaps. In particular, the data suggest genes related to Intelligence in regions on chromosomes 2q (In four of six of the investigations), 6p (for five of the six Inhibitors,research,lifescience,medical investigations), and 14q (for three
of the six investigations).32 Inhibitors,research,lifescience,medical The overlap in investigations in identifying these regions suggests the existence of genes that might account for at least some of the variation in IQ. In addition, particular genes including APOE, COMT, and BDNF may play a part in the origins of intelligence (see ref 20). IQ QTL is a research project attempting to identity quantitative trait loci (QTLs) responsible for genetic variation in intelligence.33 The investigators have sought to identify QTLs linked to intelligence. But positive findings have gener ally failed to replicate, or generated weak signals Cytidine deaminase that have not yet been convincingly replicated in independent samples.34 Deary and his colleagues have found that “there is still almost no replicated evidence concerning the individual genes, which have variants that contribute to intelligence differences.”35 Recently, Davies and colleagues, in a study involving 3511 unrelated adults and almost 550 000 single-nucleotide polymorphisms (SNPs), have found that genetic bases of intelligence are very widely distributed across genes rather than localized.