How much living space does a person need? What happens when his space needs are not met? Scientists are making experiments on rats to try to determine the efforts of over crowded conditions on man. Recent studies have shown that the behaviour of rats is greatly affected by space. If rats have enough living space, they eat well, sleep well and produce their young well. But if their living conditions become too crowded, their behaviour and even their health change obviously. They can't sleep and eat well, and signs of fear and worry become clear. The more crowded they are, the more they tend to bite each other and even kill each other. Thus, for rats, population and violence are directly related. Is this a natural law for human society as well? Is enough space not only satisfactory, but necessary for human survival? The writer is mainly talking about ______.