Bystander effect
Etymology
Studied by psychologists Bibb Latané and John Darley after the 1964 murder of Kitty Genovese in New York, widely reported at the time as witnessed by dozens who did nothing. Later journalism showed that story was substantially exaggerated — some witnesses did call for help — but the laboratory effect Latané and Darley documented is real and repeatedly replicated. The mechanism they named, "diffusion of responsibility," is the core: the more onlookers, the less any one feels bound to act.
noun · /by-STAND-er ih-FEKT/
The tendency for individuals to be less likely to help someone in trouble when other people are present, because responsibility feels spread across the group.
What it names
It explains why a person can collapse in a crowded place and be ignored — everyone assumes someone else will act, so no one does.
Example
She’d read about the bystander effect, so instead of shouting to the crowd she pointed at one man and told him to call 911.
Related idea
Diffusion of responsibility is the mechanism inside the bystander effect — the sense that obligation is shared, and therefore shouldered by no one.