They're chilling words. They're the words of former Queensland coroner Michael Barnes, used to describe the horrific last moments of two young nurses.

Wendy Evans and Lorraine Wilson were raped and murdered in 1974 on the Toowoomba range. On the night of their deaths, up to 10 people had seen the women trying to fight off men on the steep road leading up to the city.

One witness heard the women's frantic screams. "Help us, oh God, help us!". The man was too scared to stop. Another said her husband told her not to get involved because it "was probably just a domestic".

Nearly two years later, the women's' bodies were found, hog-tied, their belongings scattered around them.

It seems to defy all standards of decent human behaviour that so many people could have done nothing.

Yet, as a 29-year-old Sydney woman known only as "Aida" discovered this week, a crowd of people can be watching and you can still be very much on your own. At 4.30pm on Monday, on busy Railway Street in Liverpool, Aida fought off a man who tried to drag her into a car. There were three other men inside the car. There were "dozens of people around".

Not one of the people came to Aida's assistance, despite her screams.

Twitter, of course, stepped in with opinions. "Shootings, bashings and now abductions all caused by a lack of morality and values," the Christian Democratic Party said. "A real loss of empathy and humanity," tweeted the Reverend Fred Nile. David Oldfield announced that the attempted abduction indicated "the good Samaritan is long gone". Other tweeters chided Liverpool for its heartlessness, suggested that bystanders were too cowardly, too busy recording the event on their phones or taking selfies, or too concerned about getting bashed up or sued.

It's easy to have a reflex response to such an incident with all its horrible echoes of the Anita Cobby case. Someone is in trouble. You help them. If you don't, you are morally flawed and/or a coward. But psychologists offer an alternative view. They call it the "bystander effect".

Research has found that in any situation where someone needs help, as the number of bystanders grows, the probability that any one bystander will help decreases and the time taken to help increases.

"I really don't think it's a lack of caring; it's not knowing what to do," Dr Helen Paterson, a senior lecturer in forensic psychology at the University of Sydney, says.

Experts accept that the phenomenon of a "bystander effect" — or "Genovese syndrome" — is a reason why people can stand idle, even as a victim is in grave danger. Kitty Genovese was stabbed to death in New York in 1964. She is said to have screamed for help but neighbours did nothing.

Barnes, now the NSW Coroner, referred to the "bystander effect" in his findings after a second inquest into the deaths of Wendy Evans and Lorraine Wilson in Toowoomba.

"There's a diffusion of responsibility that can take place," Paterson says. "If people can see that there are others around who are witnessing the incident, everybody thinks, 'well perhaps I'm not the best person to help them, that person there looks stronger, or maybe that person there is more capable'."

Paterson also uses the expression "pluralistic ignorance" to describe such situations. "Oftentimes these situations are unclear, ambiguous, we don't really know how to respond, so we look to other people and how they're responding and think, 'oh well, they're probably not doing something because there's no reason to do something, it's not a real emergency', and they don't realise these people are doing exactly the same and looking at your inaction."

And perhaps there are other things going on in people's heads. Perhaps they're thinking about the story of Brendan Keilar, who was shot dead in Melbourne in 2007 when he tried to stop Hells Angels bikie Christopher Hudson from assaulting a woman. Or perhaps they're remembering young Daniel Christie, who died on Saturday after a New Year's Eve assault. Mr Christie had, apparently, tried to be a good Samaritan in the face of an assailant's attack on someone else.

Correction: An earlier version of this story said Wendy Evans and Lorraine Wilson were raped and murdered in 1972. It was 1974.