The intro of the movie makes it pretty clear that their premise is "IQ is heritable, and stupid people have more kids." It's not a coincidence that the "high IQ" couple is portrayed as wealthy and the "low IQ" people are shown as poor.
Thank you. It's astonishing to me how few people remember the text of this movie -- it is so clearly saying dumb people are breeding, smart people are not.
https://www.youtube.com/watch?v=6TCsR_oSP2Q