What factors affect the g factor? Is / are there any scientific studies on factors which affect the general intelligence (g) factor?

I did a Google search, but could not find any academic papers on the topic.
So you're basically asking what are the causes of interpersonal differences in intelligence as typically measured by standardised psychometric tests. This question forms the basis for a large proportion of the literature on intelligence. For example see a journal like Intelligence.

For a review you could read  Intelligence: Knowns and Unknowns

A few quick observations include that 


Genetics is a major cause
Environmental causes are often less well understood
Head start and related programs designed to increase intelligence typically lead to limited long term gains
Severe environmental factors will negatively affect intelligence