One of the primary factors lies in the education of the parents. I took a speech class that stated - in homes with parents who had maybe a high school education the parents usually used approximately 400 words. In middle class some college homes 1200 were spoken and those with higher education - post college I believe the information was near 2000 words were heard regularly in the home.
It has been a few years since I took the clas but the information is close to what I was told.
Positive interaction with parents and neighbors makes for a more confident child as well.
Mntnapoet