When a situation is unclear, people are less likely to intervene because they are unsure if their help is needed or appropriate. A notable experiment from 1968 demonstrated this effect vividly. In the study, participants were placed in a room where smoke began to seep in through a vent.
When participants were alone, they were more likely to report the smoke quickly. However, when other people in the room ignored the smoke, participants were much less likely to take action. The presence of passive individuals led participants to assume that the situation was not an emergency.