Each has been supported by many studies. My own personal experience in Thailand supports the environmental point of view, especially in the following case.
A family I knew in Udon Thani was very education-minded. Grandma had worked abroad and her husband was a neighbourhood headman. Living with them were three grandchildren (two girls and a boy) plus the mother.
The domestic educational regime was quite strict. Large posters of the Thai and Roman alphabets and basic mathematics hung on the walls in the living room. School homework took priority over playtime. After school the grandparents would help the kids with homework as much as they could (alphabet, maths, writing), as well as trying to inspire them to learn while emphasising the importance of a good education. The boy quit school at the end of Mattayom 3, and somehow got away with it. The arrival of a home computer plus Internet was a great help for the kids. The girls finished Mattayom 6 and their future looks bright.
Of course, this “case study” does not include data on genetics. But it does illustrate the importance of domestic support, attention, inspiration and a vivid educational atmosphere at home.