Kaminski, J.A. et al.:  Epigenetic variance in dopamine D2 receptor: a marker of IQ malleability? Translational Psychiatry (2018)8:169 DOI 10.1038/s41398-018-0222-7

Excerpt: “Genetic variance is known to explain a substantial part of variability in cognitive capacity. The largest available study describes that polygenic scores (i.e., those common genetic variants that are most strongly associated with test performance in previous studies) explain up to 4.8% of the variance of general intelligence quotient IQ (gIQ). A more recent larger but not yet peer reviewed study, shows up to 5.4% of variance explained. On the other hand, environmental factors have a significant impact on general cognitive capacity, as indicated by the strong rise in average IQ performance following the decades after World War II.  According to Flynn et al. the change ranged from 5 to 25 IQ points (eg. 0.3 to 1.7 standard deviation (SD)) within one generation. This change appears to be too strong to be explained by genetic changes. While various environmental factors…..”

https://www.nature.com/articles/s41398-018-0222-7.pdf