I gotta say the idea that you have to always be suspicious in the "bigger cities" is bunk. Bad things can happen (I've been mugged before -
I must have a heck of an imagination to come up with those totally fictional scenarios all on my own. And besides, you WEREN'T suspicious and you got mugged. Just like I said. Luckily you were able to fend for yourself - many cannot.
I'm not sure I understand about the suspicious part. My point is that if you're able to do something, you should. I could certainly defend myself, but if I saw an old lady in the same situation, I'd think it was my duty to try and do something. Not everybody SHOULD do this, of course.
FWIW, the law in France, for example, says that if by your inaction you deliberately caused something bad to happen, you're responsible. Mugging or other violence is not a good example here... A better example: you'd be liable if you were walking down the street and saw a woman lying in the road bleeding to death and did nothing to help her. Going back to the original sub-topic, if you see somebody obviously drunk trying to start a car, it's your -duty- to stop them, and if you don't do anything (as opposed to trying and then he kicks you in the nuts and drives off) I think you should be liable for anything that he does as a result of you not trying to prevent him from doing something obviously stupid. There's of course a lot of grey area here and this shouldn't apply all the time. Extenuating circumstances and all that.
There's a super interesting podcast from WNYC's Radiolab about Morality and why we make the decisions we do (http://www.radiolab.org/2007/aug/13/
) I haven't finished it yet, but they cover some of this stuff, like what makes people react the way they do in different situations.