> The current study further confirmed the finding from our earlier work that the effects of individual genetic variants on educational attainment are extremely small. The average effect size across the 1,271 genetic variants was just 1.8 weeks of schooling per allele; even the SNPs with the strongest associations only predicted around 3 weeks of additional schooling per allele. Taken together, these 1,271 SNPs accounted for just 3.9% of the variation across individuals in years of education completed. > Here is another way to think about this result. Imagine that we used the results for these 1,271 genetic variants (not the ~1 million SNPs across entire genome we discuss in FAQ 2.3) to predict the educational attainment for a new group of people (separate from our discovery sample). We could then compare each individual’s predicted educational attainment to their actual educational attainment. If we did so, our results suggest that we would find that the predictions and actual outcomes correlate only very modestly (at about r = 0.20). That, in turn, means that if someone were predicted to complete an above average number of years of schooling (i.e., to be in the top half of educational attainment), that person would have about a 58% chance of actually being in the top half of educational attainment. Fifty-eight percent is better than chance (i.e., 50%), suggesting that a prediction based on these 1,271 SNPs has more power to predict educational attainment than a coin flip—but only a bit more power. By contrast, a prediction based on a polygenic score that combines ~1 million SNPs that we studied (see FAQs 1.5 & 2.3) has more predictive power: r = 0.33, corresponding to 11% of the variation across individuals. > The contrast between the 3.9% of the variation predicted by the 1,271 SNPs and the 20% known to be explained by common SNPs (see FAQ 1.7) implies that there are many other SNPs that have not yet been identified. Even larger sample sizes will be needed to identify them.
They only included people of European ancestry to eliminate sources of noise and cofounding factors. The effect size is very small, yet it does exist.
This seems to prove that intelligence influenced by genetics but is affected by many thousands of genetic variations. There is no "smart" gene, just lots and lots of randomness.