This is called “cultural deprivation,” and it is certainly a known problem. I worked with retarded adults years ago in Pennsylvania, and we had some residents of the group home who probably had no organic problems but came from isolated families who lived out in the woods (in the Appalachians) and were severely culturally deprived to the point that they functioned and tested as retarded. Everybody was either illiterate or nearly so, the people who brought them up had a limited vocabulary (and many lived on a combination of welfare and hunting, so they didn’t even work with other people), and on top of that they usually had a horrible diet which led both to prenatal problems, possible organic brain damage, and later physical problems.
It’s hard to understand why television (blacks watch more TV than any other group) has not had a broadening effect on the culturally deprived, but I guess these isolated “communities” are so closed in on themselves and exert such negative pressure that there’s really nothing strong enough to oppose it.
Prenatal alcohol exposure (fetal alcohol sydrome) also results in the same symptoms. I worked extensively with foster children in “disadvantaged communities” and this was the elephant in the room. The effects are devastating and generational—especially when comorbid with welfare.