Since the beginning of the 21st Century, new technology has been available to analyze DNA – our genetic code. The technology uses specialized chips to analyze DNA sequences, and extremely powerful computers and specialized computer software to calculate results. This is called Genome-wide Association Studies (GWAS), which is part of the specialized scientific field that studies “genomics”.
To test the role of heredity, scientists use IQ tests given to vast numbers of people and also catalogue the variations in their DNA. Certain variations (+ or -) correlate with differences in IQ. These are called SNPs. These variations appear to affect all fluid intelligence and/or all crystalized IQ sub-tests (Source 1).
One must be clear what these studies show. They do show that heredity accounts for much of intelligence. They do not rule out any causes other than heridity. Because DNA varies widely in the population, and these studies use actual IQ tests to derive data, they cannot rule out causes other than DNA affecting those tests.
Until about 2015 it has only been possible to analyze the DNA that varied in more than 1% of the population, and only those 20,000 groups of DNA that coded for proteins. Improved technology and the DNA availability from many more individuals now permits the analysis of many rarer DNA variations, and many more base-pairs of DNA. (Source 2 and other sources).
Though almost all variants are extremely rare in the population at large, within families, including aunts, uncles, cousins, etc., some alterations occur more often, and are larger. Thus, even though the number of members of a family typically are not large, the effect of the alterations can be detected. By this method one study, published in 2018, has found 7,000 additional DNA alterations, accounting for about 23% of total IQ test variance, bringing the total explained to 35% to 50%. Most of these alterations occur in the general population only in 1 out of 100 to 1 out of 1,000 individuals (Source 3). The technology of this study still was insufficient to capture all genetic variation, as only 700,000 possible DNA changes were examined out of a possible 6.4 billion base pairs in each individual. . (Source 4)
Is imputation reliable?
All the recent studies use imputation, which increases the heredity found by up to 50%. Imputation assumes that the many thousands of unmeasurable DNA locations have together an influence in a similar ratio as to what can be measured. The math used is very, very complicated, and relies upon computers. It brings the total amount of explanation of heredity up from 35 – 40% to over 50%. Imputation implies that perhaps 200 million of a person’s 6.2 billion base pairs of DNA are involved in modifying IQ. The lay person has no way to confirm the mathmatical models used. Analyzing the modeling assumptions is beyond my capability, but based upon what I have learned about such models in general, I am skeptical.
The presence of a small group of powerful alleles correlates highly with the IQ of certain countries, but random groups of alleles, used for control, also correlated highly, sometimes positively and sometimes negatively, though somewhat less so. (Source 5) ) Perhaps this is not surprising, because the alleles were derived because they correlated with population IQ tests. Since the increase in correlation over random was not great, environment could also play a major role.
Below I have highlighted some of the data in some GWAS studies.
Allele Frequency and Impact , 24,090 individuals - data from source 41
| size of impact | Amount of IQ explained (estimate from graph) | Number of alleles |
|---|---|---|
| .4 - .5 | 8 | 872,346 |
| .3 - .4 | 7 | 930,841 |
| .2 - .3 | 6 | 1,061,603 |
| .1 - .2 | 1 | 1,413,929 |
| .01 - .1 | 5 | 3,320,146 |
| .001 - .01 | 27 | 3,898,626 |
| Total of IQ explained | 54 | 11,497,491 |
Though there may thousands of DNA pairs affecting intelligence in the world, there are many fewer in one individual’s genome. That these are not distributed randomly on chromosomes means larger differences occur in IQ between individuals. The data in the chart below is taken from the following reference. (Reference 6) The variation of intelligence DNA between chromosomes — with only a few having most of the impact— explains in large part why there is so much variation in IQ between siblings, and why you cannot say, as classic studies of heredity often did, that two siblings will half the same amount of heredity.
“Genome-wide association studies establish that human intelligence is highly heritable and polygenic” Source:40A https://pmc.ncbi.nlm.nih.gov/articles/PMC3182557/
| Chromosome number | Fluid | Crystalized |
|---|---|---|
| 1 | 0.01 | 0.02 |
| 2 | 0 | 0.04 |
| 3 | 0.09 | 0.09 |
| 4 | 0.04 | 0.056 |
| 5 | 0.095 | 0.03 |
| 6 | 0.013 | 0 |
| 7 | 0.02 | 0.005 |
| 8 | 0.03 | 0 |
| 9 | 0.042 | 0.018 |
| 10 | 0.062 | 0.03 |
| 11 | 0.044 | 0.032 |
| 12 | 0.01 | 0 |
| 13 | 0 | 0.06 |
| 14 | 0 | 0.08 |
| 15 | 0.055 | 0.02 |
| 16 | 0 | 0.03 |
| 17 | 0 | 0 |
| 18 | 0 | 0.05 |
| 19 | 0.03 | 0.03 |
| 20 | 0.01 | 0 |
| 21 | 0 | 0.02 |
| 22 | 0.013 | 0 |
The results of these studies, of course, are for mainly from relatively homogenous groups in northern Europe. It does not tell us anything about the amount of variation within a family or about the DNA of a different ethnic groups. What it does show is that for these northern European groups DNA can explain, on average for the group, depending upon how you trust the calculations about 40% to 55% of the variations of IQ.
Next page: https://iqscience.net/heredity-twin-studies/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3690183/ DNA Evidence for Strong Genome-Wide Pleiotropy of Cognitive and Learning Abilities, Maciej Trzaskowski et al., Behavioral Genetics, 2013 ↩︎
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4270739/#box1 Genetics and IQ differences: five special findings, R. Plomin & IJ Deary, Molecular Psychiatry, 2015 ↩︎
- https://www.nature.com/articles/s41380-017-0005-1Genomic analysis of family data reveals additional genetic effects on intelligence and personality, W. David Hill et al., Molecular Psychiatry, 2018 ↩︎
- See the discussion of chance effects below. The amount of hormones in an individual is highly variable. ↩︎
- https://www.sciencedirect.com/science/article/abs/pii/S0160289615001087 A review of intelligence GWAS hits: Their relationship to country IQ and the issue of spatial autocorrelation ↩︎
- Mol Psychiatry. 2011 Aug 9;16(10):996–1005. doi: 10.1038/mp.2011.85 Mol Psychiatry. 2011 Aug 9;16(10):996–1005. doi: 10.1038/mp.2011.85 ↩︎