旁观者效应（Bystander effect）是一个社会心理学术语，在紧急情况下，个体在有人在场时，出手帮助的可能性降低，援助的几率与旁观者人数成反比。换句话说，旁观者数量越多，他们当中任何一人进行援助的可能性越低。

1964年的Kitty Genovese被谋杀案是旁观者效应最著名的事例，也是社会心理学在这一领域进行研究的最初动机。

但时隔43年后，Kitty Genovese案的报导（Kitty Genovese遇害时多达38人袖手旁观，任凶手三次行凶导致Kitty Genovese死亡）皆疑似为《纽约时报》记者查证不足下的杜撰，与警方数据有甚大的差异。

The bystander effect, or bystander apathy, is a social psychological phenomenon that refers to cases in which individuals do not offer any means of help to a victim when other people are present. The probability of help is inversely related to the number of bystanders. In other words, the greater the number of bystanders, the less likely it is that any one of them will help. Several variables help to explain why the bystander effect occurs. These variables include: ambiguity, cohesiveness and diffusion of responsibility.