I don't really intend to risk my life going up against a knife-wielding crazy person, just so you'd be able to keep your phone. I would probably call the police

I don't really intend to risk my life going up against a knife-wielding crazy person, just so you'd be able to keep your phone. I would probably call the police though, which was also my reaction last a bigger fight happened near me. For me to risk my life for you, I would have to know you somewhat well, and then I would very likely intervene. It's not only the bystander effect, but also the quite rational question "do I want to risk my own life for the sake of this woman I don't know, in this situation I know very little about?" Once, my friend intervened in two drunk girls fighting each other. He was calm, but instead they just turned against him. It's simply a game of pros and cons, and I can very much follow the community thing that Anderkent mentions in another thread.