The Oxford English Dictionary defines a bystander as one standing by, one who is present without taking part in what is occurring. One may immediately think of the phrase innocent bystander. In this association what is occurring is a crime. In a crime a bystander is neither perpetrator nor victim and thus innocent of all active involvement. The bystander is present only as passive observer or witness.

Although all bystanders to crime initially find themselves passively observing, some abandon passivity to intervene. They actively seek to help the victim and, in so doing, move from bystander to rescuer. In contrast, other bystanders, remaining passive throughout, have come to be called nonresponsive bystanders.

Although there is a range of possible bystander behavior between all-out rescue and complete nonresponsiveness, many bystanders to crime do remain entirely nonresponsive. Why do so many people so...