Psychology
The more witnesses to an emergency, the less likely any one of them helps
In 1968 John Darley and Bibb Latané staged fake emergencies and found bystanders grew less likely to intervene as the group grew, since each assumes someone else will act. The effect was inspired by reports that dozens of witnesses did nothing during the murder of Kitty Genovese, though that specific account was later shown to be exaggerated — the underlying experimental finding has replicated widely since.