I do not agree with the rule, and I have seen much more socially cooperative behavior in disaster situations in various cultures. However, if it goes on long enough people will change their behavior, and if they feel that scarcity is bad enough, and other people may be cheating, they will turn violent. Think "Maslow's hierarchy", when you are at the bottom and trying to survive you will do things you would not do otherwise. When people cooperate in emergency situations they have a social influence to do so, when they loot and riot they have a social influence for that also. Also think "relative deprivation". Their perception of their situation, and their perception of the social environment will influence how they act.

Human behavior can be fairly predictable, but it can also surprise you just when you think you have it figured out.