A narrator (Earl Mann) explains that in modern society, natural selection has become indifferent toward intelligence, so that in a society in which intelligence is systematically debased, stupid people easily out-breed the intelligent, creating, over the course of five centuries, an irredeemably dysfunctional society. Demographic superiority favors those least likely to advance society.
Consequently, the children of the educated élites are drowned in a sea of sexually promiscuous, illiterate, alcoholic, degenerate peers.