Interesting that you should mention population density as an issue and relate it to caged mice. When I was in college I remember reading a study done that was about that very issue. The person that did the study used mice and studied their behavior as the population in cages grew. When few mice were in the cage they were very nice and courteous to each other; very helpful friendly community. The researcher kept adding mice, and as the population grew more dense they became crankier until they reached the point of killing each other. The researcher made sure there was plenty of food, water, and good sanitary conditions kept in the cages- it really seemed as though just having too many mice kept in an area where they could not have personal space, etc. seemed to make them become violent. The researcher questioned whether it could be related to humans and population density and he seemed to think it could.
I am sure it is not the whole answer, but do think it is a part of the puzzle.
I remember reading about that mice/rat study too where you increase the population density, they turn violent and so on.
The same type of experiment was don with rabbits. In a nut shell, overcrowing of rabbits resulted in the same type of societal issues tha we see in the metropolitan areas around the country. including gangs, rape, mental disorders, murder.