That's why they're called "bystanders"

As reported by Jamie Hall, in the Edmonton Journal, January 18, 2008:

EDMONTON - Bystanders are less likely to step in and help someone whose life is in danger if they're part of a crowd, according to a University of Alberta professor asked to comment about the recent actions of two Good Samaritans.

One man, an off-duty restaurant employee at Joey Tomatoes, was shot in the leg when he and a manager intervened in a fight; another, 46-year-old Randy Lintner, was killed when he tried to stop a fight outside a bar.

The men acted in separate incidents over the past few days, but in both cases several people were present.

"The likelihood that someone will intervene in dangerous situations diminishes as the number of bystanders increases; psychologists call it the bystander effect, " said Mitchell McInnes.

He...wrote his doctoral dissertation on Good Samaritans while at Cambridge University several years ago.

The bigger the crowd, the easier it is for people to diffuse responsibility, and shift or avoid blame. "People kind of freeze in those situations; they're inhibited by uncertainty, and by the fear of acting inappropriately," McInnes said.

Fawn Hagen thinks the recent spate of violence will deter others from taking similar actions. The restaurant employee who was shot is her sister's boyfriend. She identified him only as "Justin."

"He chose to help a man who was being beaten. His reward? A gunshot to the leg," Hagen wrote in a letter to the Journal.

"If this isn't a deterrent for the Good Samaritans on our city, I don't know what is."

Fear of personal harm, or legal liability, are among the factors that make them hesitate or stop altogether, McInnes said.

The latter has prompted a number of provinces--Alberta was among the first, in 1969--to enact legislation to protect Good Samaritans.

Essentially, the legislation protects Good Samaritans from legal liability in the event of damages or injury "unless they're grossly negligent or do something wildly wrong," McInnes said.