> There is nothing in the movie that suggests that the decline in average intelligence is a result of cultural factors or education.
Have you skipped start of the movie or did we watch different editions? They literally open the story with redneck family breeding and spreading their redneck ways through family ties while “smart” family waits for the perfect moment.
I think that the movie makes it really, really obvious that the intellectual degradation goes beyond just culture. The people are presented as being borderline mentally disabled.
I rewatched the intro, and it is true that they label the character IQs. That said, I still don't think that the particular mechanism is the most important part