depends. if it was a child or woman who was getting hurt by someone else, I'd definitely step in. if it was a man getting mauled by other men, I'm not so sure - I'd be scared that it could be a gang-related situation and either way, what can a pint-sized girl do but call the police? so I'd probably just do that and maybe try to take some pics of the attackers from a safe distance.
I was once in a situation where an old woman got a heart attack on a bus stop and me and one other stranger were the only people who didn't completely ignore her but tried to help
there was a ton of people around and they just stood around ignoring the situation completely.
I can think of at least a dozen situations when I really wished I was a guy so I could step into the fight and help the person being attacked, because no other people cared. it's terribly disturbing to see grown men not react when an innocent person is getting hurt. then again, I received help from complete strangers many, many times myself, so I guess there are some good people around.
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not a bug - a feature.