It’s not easy to address the issue of human overpopulation, so filmmaker Mike Freedman did something clever: He tells the story with mice. He uses the cautionary tale from a science experiment by research psychologist John B. Calhoun to show what happens when rodents are crowded together: It’s not a pretty sight. Mice stop weaning their young; they often attack their progeny and cease normal behaviors. The conclusions of the experiment–and film–amplify the warnings of many demographic experts.