Heredity versus Environment

Some arguments that favor the hereditability of intelligence:

Some arguments that favor the cultivation of intelligence

More about the Flynn Effect

    Testing of older British civil servants and corporate employees by John Raven in 1948, coupled with follow-up tests administered by Dr. Raven's son in 1992, has shown that Raven Test subjects born in 1877 who scored at or above the 90th percentile on the Raven Progressive Matrices Test matched the performances of  individuals born in 1967 who scored in the 5th percentile! Now the 5th percentile represents a Raven-derived IQ of 75, while the 90th percentile indicates a Raven-derived IQ of 121. Someone born in 1877 with a Raven-derived IQ of 121 would fall at the 5th percentile (IQ = 79) if they had taken the Raven test in 1992! To say it another way, someone born in 1877 would have a Raven-derived IQ of 65 today, while someone born in 1967, if they could travel back in time to 1902, would have a Raven-derived IQ of 153. (I'm emphasizing "Raven-derived" because the cohort born in 1877 would had vocabularies, arithmetic capabilities and general information close to those of today's citizenry.)
    The point is that the environmental changes attributable to the Flynn Effect are huge, and do represent an elevation of those with IQ's of 65 to an average level of 100 with respect to those pattern recognition aptitudes which the Raven Test measures.

Bottom line:

    My guess is that there is a hereditary component to human intelligence, but that environmental influences, when they involve "total immersion", can produce very large effects in cognitive powers.

(Gauss learned to read before he was three, and Sarah Sidis observed of her son, William Sidis, that, "Before he was two he would go gravely to the book case and pick out any book that a visitor asked for. This so amused and pleased them, that he soon took pleasure in opening the books and reading from them to his father and guests, and by the time he was three he read well.")