the tendency for any given bystander to be less likely to give aid if other bystanders are present
For example: There was a much publicized case in 1970 in NYC where a man stabbed a woman, dragging her from one block to another for about two hours. till he finally killed her. At least 38 people watched part or all of this protracted murder. In other words, they were bystanders and not one of them called the police. When asked later why not, they nearly all said that as there were so many people watching they assumed that somebody had already called the police.