Obviously a major portion of nurture is non-brain parts of the body. Neural and vascular health, lipids, things that can help the brain function correctly. All the rest is context: what good is any literature to a successful hunter-gatherer society? This is why Flynn Effect maxes out at a 4-6 point IQ boost; thus nurture can be low as about 7% relevant, with heredity taking the lion's share for determing IQ. Overemphasis on nurture is utterly dismal thinking rooted in moronic feelings of guilt about human inequality.
How, then, do you explain the difference between hunter-gatherer homo sapiens and homo sapiens who have civilisation? We are not genetically different from our stone-age ancestors whose technical knowledge maxed out with the use of spears, rocks and sticks, but we have sent members of our tribe to other celestial bodies. If the genes are the same, then necessarily this means the advancement is due to environmental factors.
We are unique as a species. Other animals and even primates simply have cognitive capities that fit with their habitats and which allow them to function successfully in them, but nothing more. This is the standard understanding of evolution of some trait: it evolves to fit the environment which is static. Human beings, on ther other hand, modify their habitats to scaffold their cognitive abilities
, lifting them to new heights. Rich developmental environments, technology, calculators, language, written language (to store longer thoughts and to pass down knowledge so that it deosn't need to be discovered again from scratch), peer-review, thinking tools like induction and deduction... are all cultural scaffolding on cognition.
Now I'm not sure about the role of 'nature vs nurture' in individuals living at one time in rich cognitive environments... but i just want to bring up the important point that environment potentially means a great deal with it comes to the mental capacities of human beings.
I Know there are extensive studies been done with identical twins who have been seperated since birth (i.e. genes are the same but environment is variable). I can't remember the specifics but the general concensus was that genetics was more than a 50 per cent factor.