Things have happened in different parts of the world, but it depends on the situation that you are talking about. SHTF could be many things. Personally I believe that something simple like running out of fuel could cause a breakdown of society, as portrayed in the movie "Tooth and Nail", where cannibalism ensues shortly after the world gas reserves are depleted. But in reality, when this situation occurred in Cuba, the public banded together to grow food and help each other. Wiki page: http://en.wikipedia.org/wiki/The_Power_ ... d_Peak_Oil
I think it's hard to say exactly what would happen in certain situations, and I haven't really found any university studies on the topic. It's safe to say that mass panics would obviously follow nearly any disaster. During the 9/11 attacks, there was a lot of panic even all the way in my tiny Indiana town. Cars lined up buying gas, people were walking down the street smoking pot in the open, totally convinced that the world was ending. Lol, it didn't , and as soon as the one local cop was done directing traffic at the gas station, those kids went straight to jail, haha. But I think that situations would differ depending who (and where) you are. Some people freak out and begin looting, while others calmly wait indoors for things to settle down. Who Knows? Some people panic when the electricity goes out, while other people feel a sense of relief. It's hard to say. I would love to see a real scientific study, but I have not come across one yet.