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