Latané and Darley found that a lone bystander helped about 85% of the time and one among five helped about 31%. Why does adding witnesses make each individual less likely to act, when the need is unchanged?

C. Because responsibility divides and inaction reads as reassurance.

Two mechanisms run at once: diffusion divides the duty, and pluralistic ignorance works on interpretation, everyone reads everyone else's calm as evidence it is not an emergency. They make different predictions, since diffusion works over an intercom and the other needs visibility. Point at one person to defeat the first; say loudly that this is an emergency to defeat the second.