The number of people believed to have witnessed that nightmare was strikingly similar to that of the video-recorded rape in Chicago. A seminal New York Times article said that 38 of Ms. Genovese’s neighbors had watched as the killer went after her and had heard her cry for help. But not one of them called the police.

Later investigations showed that version to have been a gross exaggeration. True, some people ignored the mortally wounded woman’s pleas. But only a few, it turned out, had a clear sense of what was happening, or glimpsed the attacks as they occurred. Many thought the street screams had come from drunks or perhaps quarreling lovers. And two people did in fact phone the police, though not in time to save Ms. Genovese.

(Mr. Moseley, captured five days later during a burglary, confessed to that homicide and to killing two other Queens women. He died last year at 81, having spent his last 52 years in prison.)

But the story of 38 people coldly ignoring a murder beneath their windows had a life of its own. It became emblematic of big-city apathy. The terms “bystander effect” and “Kitty Genovese syndrome” entered the language.

As Retro Report notes, two social psychologists in New York, John M. Darley and Bibb Latané, conducted experiments that led them to posit that Ms. Genovese might have survived had there been fewer witnesses. Numbers can inhibit action, they concluded. “You think that if there are many people who are witness to something that other people certainly already have done something — why should it be me?” Dr. Latané said.

A 2015 article in The Wisconsin Law Review cited studies showing that most instances of school bullying are witnessed by other students and that in nearly one-third of reported sexual assaults, third parties are present.

But for some people it doesn’t take a crowd to do nothing. An infamous case was the 1997 murder of Sherrice Iverson, a 7-year-old girl who was dragged by a young man, Jeremy Strohmeyer, into a casino restroom in Nevada. There, he sexually assaulted and choked her and snapped her neck. A friend of his, David Cash Jr., was at the scene. He saw the evil in progress, but walked away – a moral barrenness reflected in his later comments to The Los Angeles Times: “I’m not going to get upset over somebody else’s life. I just worry about myself first. I’m not going to lose sleep over somebody else’s problems.”

In the age of social media and instant communication, the potential rises for a Kitty Genovese syndrome on steroids. Chicago again provides an example. On Dec. 31, the authorities there say, four young people kidnapped and tortured a mentally disabled teenager, streaming their brutality on Facebook Live. One assailant was so devoid of empathy for the victim that she whined on camera about not having much of a digital audience: “Ain’t nobody watching.”

In Columbus, Ohio, last year, an 18-year-old woman witnessed her teenage friend being raped. Instead of stepping in to help the victim, or at least calling the police, she streamed a live video of the assault on the Periscope app. “She got caught up in the ‘likes,’” a prosecutor said. In a plea bargain, she was found guilty of obstruction of justice and sentenced to nine months in prison.

There is reason to wonder if certain deplorable acts would have occurred in the absence of technology. In Los Angeles, a 14-year-old boy was seriously injured in December when he was sucker-punched by another teenager. That assault was recorded by another teenager who then posted the video on Snapchat.

While such episodes are not common, they inspire “there ought to be a law” demands. Fewer than a dozen states, though, have “duty to rescue” or “duty to report” requirements. Such statutes are distinct from better-known Good Samaritan laws, which exist nationwide and offer protection against liability to those who act when they see people in peril. They do not mandate a duty to intervene, however, and state laws that do so tend not to be stringently enforced.

There is an inherent ambiguity in some situations. As with the Genovese murder, people watching events unfold in a forum like Facebook Live may not be sure what they are seeing or hearing: Is that a real crime or a simulation?

Still, it doesn’t take much imagination to figure out what the philosopher John Stuart Mill might have thought of all this.

“Bad men,” he said in 1867, “need nothing more to compass their ends than that good men should look on and do nothing.”

CLYDE HABERMAN, a regular contributor to Retro Report, has been a reporter, columnist and editorial writer for The New York Times, where he spent nearly 13 years based in Tokyo, Rome and Jerusalem. Subscribe to our newsletter here and follow us on Twitter @RetroReport.

This article first appeared in The New York Times.