The movie gives the view that dumber people get more kids compared to smart people. So the world will, after a point, contain more dumb people than smart people because they simply breed more.
the movie posits the dumb people reproduce faster because they start sooner (i.e. football guy gets cheerleader pregnant when 16, smart girl waits to have a career before getting pregnant) and since getting to reproductive maturity doesn't need special skills it still works out in favor of dumb people (i.e. by the time smart couple has 1 kid dumb couple has >=1 and their descendents have >=1).