If you had been in the Montreal Métro station when an inebriated man staggered into an oncoming train, smashing his head and falling onto the platform, what would you have done?

If you were one of at least 40 passersby, the answer is nothing.

Radil Hebrich, a 59-year-old homeless man, lay bleeding just inches from the edge of the station platform for 16 minutes before any help arrived and he died a few hours later, according to the recent coroner's report on the 2014 incident.

People's reluctance to help is an example of a disturbing phenomenon known as "the bystander effect." The psychological term describes why people in a crowd sometimes don't react or become involved when someone is in trouble.

Radil Hebrich, a 59-year-old homeless man, staggered into a moving train on the Montreal Metro. He lay inches from the edge of the station platform for 16 minutes before any help arrived, and died hours later. (Radio-Canada) "I know that students in my classes, and people around me, were very shocked to hear that people had not come to the assistance of Mr. Hebrich," says Theresa Bianco, who teaches psychology at Concordia University in Montreal.

"It’s a natural reaction to have, because fundamentally we’re good people and we know that helping one another is part of the social contract — if I help someone, at some point, I’m going to get help in return when I need it. So, we have this understanding that we have a shared responsibility to help one another, and we’re always taken aback when that doesn’t manifest itself."

Even so, the Hebrich case is far from isolated. In January, a homeless man in his 50s was found frozen to death in a glass bus shelter at one of Toronto’s busiest downtown corners, for example.

CBC The Sunday Edition hosted a discussion of the factors that contribute to the bystander effect. Joining Bianco were Kristen Monroe, chancellor’s professor at the University of California at Irvine; and Ervin Staub, founding director of the doctoral program in the Psychology of Peace and Violence at the University of Massachusetts at Amherst. Listen to "Understanding the bystander effect" on Feb. 22 starting at 9 a.m. on CBC Radio, or stream it here.