The neural basis of the bystander effect — The influence of group size on neural activity when witnessing an emergency

• Influence of group size when observing an emergency was measured with fMRI.
• Decrease in activity in the pre- and postcentral gyri and the medial frontal gyrus.
• Increase in activity in regions underlying visual perception and attention.
• Group size already influences preparation for action during an emergency.

Naturalistic observation and experimental studies in humans and other primates show that observing an individual in need automatically triggers helping behavior. The aim of the present study is to clarify the neurofunctional basis of social influences on individual helping behavior. We investigate whether when participants witness an emergency, while performing an unrelated color-naming task in an fMRI scanner, the number of bystanders present at the emergency influences neural activity in regions related to action preparation. The results show a decrease in activity with the increase in group size in the left pre- and postcentral gyri and left medial frontal gyrus. In contrast, regions related to visual perception and attention show an increase in activity. These results demonstrate the neural mechanisms of social influence on automatic action preparation that is at the core of helping behavior when witnessing an emergency.