Social psychological experiments have demonstrated that individuals' failure to assist others in emergencies is not due to apathy or indifference, but rather to the presence of other people. This is explained by both bystander effect and diffusion of responsibility. In 1968 and a series of experiments that followed, John Darley and Bibb Latané demonstrated that an individual's choice to help or intervene when there is an emergency depends on the number of bystanders. Group size significantly influenced the likelihood of helping behavior in a staged emergency: 85% of participants responded with intervention when alone, 62% of participants took action when with one other person, and only 31% did when there were four other bystanders. Other studies have replicated the phenomenon, including reports from real emergencies such as calling an ambulance for overdose patients and offering CPR after cardiac arrest.