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the most important determinative factor in terms of how good someone is at something is the amount they practice (and how good the practice is). this is heavily affected by the environment, of course, but i think it's also affected by what the person wants; it's harder to practice something you don't enjoy doing. i think these desires are mediated both by the environment and by internal factors in a way that's difficult to distinguish. it's like favorite colors and tastes.
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'talent' also does exist, but it's maybe 5-10% of the result. there are some people who are just good in a way that i think is not explainable even by practice, like ramanujan or terry tao or euler or what have you. but this is very much the exception rather than the rule.
