Edwin happens upon a crime in progress but does nothing to assist the victims or even summon the police. later, he says he was afraid for his personal safety. this type of explanation, which may explain the inaction of witnesses in the case of kitty genovese, is best described by the principle of

The correct answer is the bystander effect. 

The bystander effect refers to a phenomenon in which people refrain from intervening or helping victims in need when other witnesses are present. The larger number of bystanders or witnesses present, the less likely it is that someone will intervene and help the victim. The bystander effect occurs due to diffusion of responsibility. This means that individuals do not help victims because they see other bystanders are equally responsible for helping the victim.