That was hyper-interesting. You should ask Bruce Lipton! Curiously no mention of any particular causes. There are really only 5 races (African, Caucasian, Asian, Inuit, and Aboriginal) and largely most people's DNA is 99.999% flawless, and 98% same as Bonobo Chimps. We blame our DNA but we should look within instead. That's why you never see Bonobos in Zoo's - their sexual habits are also the same as us but worse, basically too crass and disgusting for children to be exposed to compared to Pan Troglodytes chimps. Since intentionally giving embryos, fetuses, and babies genetic defects is unethical the reason we have not discovered it: Brain injury be a clue. Cellular membranes are the real brains of the cell (not the nucleus), but are very hard to study compared to DNA. I don't know if there is an equivalent to PCR DNA amplification but for cell membranes not the DNA. My point, intelligence could be a epigenetic (changes to DNA expression that is profound, rooted in situ DNA expression than inheritance, very hard to detect if it is or isn't passed down to offspring like how we imagine genetics to work: I suggest intelligence be a form of intra-cellular La Markian evolution that takes place inside our bodies mostly from -9 months to +2 years; but also can be triggered by thoughts, meditation, activity, food etc. this genetic morphing may or may not be passed down to our children (because we all have this potential, for the most part) variability spectrum go right the way through all the races. The brain uses 20% of our energy so a way to spin it up and down very fast (much faster than Darwinian style) in response to the environment - basically how many BBQs you got to eat (seriously, cooked food also be the key). There is some genetic basis for some variations in human optical processing and the retinal ganglion preprocessing circuits that I suggest could be involved or useful as indicators of the epigenetic changes in the prefrontal cortex etc; the ability to thread a needle, spot mouse poo, discern subtle visual details, colour, texture, motion de-blur etc.  I forget but there is a trade off to it maybe related to left/right brain spectrum (symbolic sequential linguistic of the left hemisphere versus the visual synthetic spatial simultaneous processing modes of the right hemisphere) or maybe short term memory. For sure, if you need corrective lenses this can make it hard to read which is easy to see how it might affect ones education. Then again - Stephen Hawking - 'splain that guy!