When someone is in distress on the street or in a market — somewhere with other people are around — sometimes no one responds.

After the video showing Chinese toddler Wang Yue went viral in October 2011, people tried to understand how 18 people could have passed the injured girl without helping.

A woman who walked by while holding her own daughter's hand said: "If someone was helping at that time, I would have done the same."

Psychologists who study human behavior in similarly tragic situations would find nothing extraordinary in that remark.

It is not unusual in a crowded place because people are waiting for someone else to do something. It's what psychologists call the bystander effect.

Experiments in 1968 found that "as the number of bystanders goes up, the probability of any individual bystander intervening goes down," Joachim Krueger of Brown University in Rhode Island wrote in an article published on cnn.com. Recent experiments by Krueger found that the effect still exists but has declined in magnitude over the past half-century.

Krueger's research leads him to conclude that the apathy of witnesses towards the Chinese toddler struck by a van in a market, is not, "a signature of the local culture."

As proof, he notes that public outrage that followed the video's release.

And Krueger also points to similar cases in other countries.

The attack and murder of Kitty Genovese in New York led to those first bystander effect experiments.

"Thirty-Eight Who Saw Murder Didn't Call the Police," was the headline in the New York Times on March 27, 1964, although accounts much later challenged that version.

In 1993, at a shopping centre near Liverpool, England, two-year-old James Bulger was kidnapped and later killed by two boys. While the three boys walked around Liverpool, 38 people saw them, with Bulger in distress, but most did not intervene.

When two people did challenge the older boys, they were convinced by the boys that Bulger was a younger brother. The outraged British tabloids dubbed the witnesses the "Liverpool 38."

In April 2010, a case in New York brought together the themes of the Good Samaritan and the bystander effect.

Hugo Alfredo Tale-Yax came to the aid of woman being threatened by a man with a knife. The man stabbed Tale-Yax, who eventually lay bleeding on the sidewalk for 80 minutes until an ambulance arrived. By then he was dead.

Surveillance video shows 25 people walking by, doing nothing to help. One person even took a picture with his cellphone. Another man rolled Tale-Yax over, exposing his bloody stab wounds, but then he walked away