The Bystander Effect
The bystander effect describes the phenomenon in which people are unlikely to help individuals in immediate need if and when they are part of a large group. The bystander effect gained national attention in 1964 after the murder of a young woman, Kitty Genovese, was witnessed, but uninterrupted, by 38 New Yorkers. The incident raised questions among social psychologists about how individuals and groups deal with emergencies and responsibility.
Diffusion of responsibility
The best known quality of the bystander effect is the diffusion of responsibility. Researchers John Darley and Bibb Latané conducted a series of experiments, which revealed that people are less likely to intervene in potential emergencies, a mugging for example, as their group gets larger. Individuals assume that another group member will take responsibility and help, but as groups get larger and add more potential helpers, each person feels less and less individual responsibility and help is less and less likely to come.
Group size is only the start
Many factors influence the magnitude of the bystander effect. If bystanders are certain, as opposed to unsure, that a person is in danger, they are more likely to help, if the incident occurs in a familiar environment people are more likely to help, groups of friends are more likely to intervene than groups of strangers and people are more likely to help individuals that they believe to be similar to themselves.
The Bystander Effect; Why Bystanders sometimes fail to help
The Bystander Effect: Reactions and Causes
Breakdown of Bystander Effect Video from Khan Academy
Darley, J. M., & Latane, B. (1968). Bystander Intervention In Emergencies: Diffusion Of Responsibility. Journal of Personality and Social Psychology, 8(4, Pt.1), 377-383. doi:10.1037/h0025589
Erratum. (2004). Personality and Social Psychology Bulletin Pers Soc Psychol Bull, 30(7), 938-938. doi:10.1177/0146167204266604
Rutkowski, Gregory K, Gruder, Charles L, Romer Daniel. (1983) Group Cohesiveness, Social Norms, and Bystander Intervention. Vol 44(3) 545-552