I believe both play a factor in determining the cognitive ability of the individual. Some people learn things a lot easier than others.
It's a proven fact people's IQs just don't change that much and physical exercise can be much more helpful than mental exercises. However, that's only the case with all other factors being equal. Poison a kid with lead paint and guess what, environment wins.
It's tricky. There is certainly merit in both sides, but it seems that the extreme intellectual abilities of geniuses is not so much a product of nurture. I have also read that behavioral problems in some students is a reaction to their frustration in not getting the cognitive stimulation they need to keep focused. It's quite complex it seems, but very interesting, nonetheless.
It would seem to be a mix. The thing that I wonder about is if the deliberate changing of thought patterns and behavior by an an individual get passed somehow into their genes. My guess is that, yes, they do, not by traditional natural selection, but by some yet to be identified process. So, the environmental conditions of the past get pushed into the present by way of genetics, often at very clearly defined points.
Nature, but it needs the right nurture to thrive.
face it hon - either way or both, YOU are fuckked....