The answer is that, bluntly, environment matters vastly less than we are led to believe by people invested (even for morally noble reasons!) in maintaining the prestige of education and redistributionist policies. It is not only impossible to make an average person a genius, but also very hard to destroy a genius through adverse conditions short of actual murder. Pretty much all of the variance is explained by natural talent. American rhetoric about expenditure per pupil, middle class advantages and SAT prep seems so petty and myopic in comparison to those stories because it objectively is.