The bystander effect is the idea in social psychology that the more people are around in a situation the less likely it is that any one person will stop and help. This is because we all assume that "someone else will sort it".
This is the opposite of feeling obligated to help with a situation because you are alone with a person or in a small group. In this scenario people feel they can’t avoid responsibility so step up whether they really want to help or not.
It can be useful to think of the bystander effect being the "Someone Else’s Problem field" from Hitchhikers Guide to the Galaxy. If it’s strong enough to hide the Heart of Gold at Lords then it’s strong enough to ignore the little old lady who fell outside the shop in the city centre.