Social psychologists Darley and Latané, along with their colleagues, have done many studies to investigate helping behavior. This description will provide a simplified version of one of their experiments. The full reference is:

Darley, J. M., & Latané, B. (1968). Bystander intervention in emergencies: Diffusion of responsibility. Journal of Personality and Social Psychology, 8, 377-383.

The experimenters’ hypothesis was that being in a group of people causes an individual to be less likely to help somebody. The researchers arrange a scenario in which an individual participant has to interact with a “target” person. (The target is actually a partner in the experiment, called a confederate.) The interaction is occurring over an intercom system, because each person is seated alone in a small room. The participant cannot see anybody else in the experiment. Partway through the experiment, the target says he is having a seizure and calls out for help. The purpose of the experiment is to see if the participant helps the target, even when the participant is one of several in a group interaction.

The independent variable in the experiment is size of group a participant is in. The treatment condition is a group of four other people. The participant believes that four other people are in other rooms, all communicating with the target through the intercom system. The control condition is no others—the participant is the only one communicating with the target person.

The dependent variable is helping the target. The researchers record whether the participant helps the target, and measure how quickly the participant responds.

The findings were clear. Over several trials (with a different participant each time), people in the group setting were less likely to help, and when they did help it took them longer. The data are shown in the table below. 

Darley and Latané were able to conclude that being in a group caused an individual to be less helpful.