This Internet site is primarily concerned with the factors affecting IQ in adults, age 21 and older. Studies discussed on this site have shown that the IQ of children is influenced by the families they grow up in, but that this influence fades almost completely for late teenagers and adults. The public discussion of IQ has been confused by ignoring this. Please keep in mind that heredity is not entirely predictable. In a child here are over a trillion possible combinations of chromosome strands (2 possibilities for each of the 46 strands that make up the child’s chromosomes), some random shifting of DNA, and random variations in DNA expression that take place before the egg is fertilized by the sperm.
This article will not get into the scientific names of different brain regions, except to note that crystalized intelligence (the ability to use acquired knowledge) resides mainly in the grey matter of the frontal left hemisphere, whereas fluid intelligence (the ability to solve new problems) resides throughout the brain in both hemispheres. Both make some contribution, however, to all of the IQ sub-tests.
The graphic below is taken from the 2008 publication cited in Source 1. VIQ means Verbal IQ, which is a principal component of “crystalized intelligence; and PIQ is Performance IQ which is “fluid intelligence” (see the definitions above).
Studies of brain volume date back to the 19th century. Brain volume,scientists found, correlates approximately 0.24 = 24% with IQ for men and about 0.26 = 26% for women (providing about 6% of total variance explained). (Source 2) This study also determined that, for the same body height, and for the same IQ, women’s brains were slightly smaller than men’s. More recent brain size measurements using MRI have found more than a 40% correlation with IQ, between races (caucasians, blacks and East Asians), and within families. The data has been vetted many times, and is also very closely correlated with physical characteristics of the body such as the size of the birth canal. The very exhaustive study of this will be found here: (Source 3 ).
The brains of identical twins (whose IQ test results correlate very highly) are scarcely distinguishable. For example, brain scans, published in 2001, of completely healthy Finnish identical adult twins found that in areas of the brain related to cognition, the brains of identical twins were over 90% indistinguishable. (Source 4). Measurable physical factors other than visible brain characteristics, such as blood oxygenation, have also been shown to influence IQ.
The authors of the journal article from which the graphic of brain changes with intelligence is taken, decided to test their findings using brain scans of a fresh group of university students. After examining each individual’s brain scans and noting his or her sex and calculating his or her brain volume, they were able to predict his or her IQ with an impressive correlation for the group of 0.71 or 71%, which is a variance explained of 50%. These results suggest a high heritability of intelligence. (There could be a caveat: It is possible that environmental effects may somewhat modify cortical thickness. We don’t know, and it seems unlikely that sufficiently large studies can be conducted to prove an effect or lack of one.). These brain studies do not rule out environmental, cultural, and chance influences on IQ test results, but do prove that heredity has a very important influence.
Next page: https://iqscience.net/heredity-dna-studies/
- http://www.jneurosci.org/content/28/41/10323.long, The Journal of Neuroscience, October 8, 2008 • 28(41):10323–10329 • 10323 Multiple Bases of Human Intelligence Revealed by Cortical Thickness and Neural Activation, Yu Yong Choi et al. ↩︎
- https://drive.google.com/file/d/0B3c4TxciNeJZWGNvQjBMNUJGVmc/view, Linde, Dunke, Madison, Sex differences in brain size and general intelligence (g) ↩︎
- https://pdf.sciencedirectassets.com/272071/1-s2.0-S0160289600X01110/1-s2.0-S016028960200137X/main.pdf. ↩︎
- http://www.ilf2000.de/Genetic%20Influences%20on%20Brain%20Structure-with%20Images.pdf, Nature Neuroscience, November 2001, Paul M. Thompson & al. (The study collected IQ data and three dimensional brain scans of 10 identical twins and 10 fraternal twins, in each case 5 males and 5 females). ↩︎