The bystander effect occurs when the presence of others discourages an individual from intervening or taking action.
The larger the group, the greater the probability of experiencing the bystander effect. The size of the group amplifies the bystander effect.
Social psychologists credited with first describing the bystander effect, Latané and Darley attributed the bystander effect to:
Social psychologists Latané and Darley popularized the concept following the infamous 1964 Kitty Genovese murder in NYC. Genovese was stabbed to death outside her apartment while bystanders who observed the crime did not step in to assist or call the police.
In Genovese's case, each onlooker concluded from their neighbors' inaction that their own personal help was not needed.
Credit: Khan Academy
[Content Warning: Details about violent crime around minute 4]