“The Mouse Utopia experiment failed because they were packed in far too tight a space.”
That was part of what they were testing for with the experiment; there’s a good video on it here: <https://www.youtube.com/watch?v=NgGLFozNM2o>
The point of the experiment was to see what would happen to a group of mice if they were placed in a medium-sized enclosure that required constant social contact and given everything they needed to survive in large enough quantities that no matter how big the colony became, there wouldn’t be a need for fighting over food or water. John Calhoun believed that the experiments could show what would happen as cities continued to grow out of control following WW2. All it ended up becoming was an excuse for the “Society will destroy itself because it’s totally depraved” crowd to insist that science agreed with their claims that we’re all going to Hell if we don’t do what they say…
The problem with attempting to extrapolate the findings of this experiment to humans was apparent from the beginning; it was Calhoun that didn’t want to hear it (which is why he kept performing the experiments). But the truth is, humans aren’t mice, and it’s nearly impossible to say in a similar situation humans would end up performing the same actions.