A.M. Rosenthal, the renowned former executive editor of The Times, who was then the metropolitan editor — and who had gotten the tip that led to the story — wrote a short book called “Thirty-Eight Witnesses.” In it, Rosenthal asked the question that haunted the country in the aftermath of the murder: Why?

Why didn’t anyone do anything?

Rosenthal could only guess at the answer because there had been no research on what is now known as “pro-social behavior.” But after the story gripped the country, two young social scientists — Bibb Latané, then at Columbia University, and John Darley, who taught at New York University — conducted a series of experiments on the behavior of bystanders.

Their startling conclusion, which is now known as the bystander effect, is that the more people who witness a crime, the less likely any one of them will come to the aid of the victim. Partly this is because when people see others not doing anything, they become confused, not sure if it really is an emergency — “a collective ignorance” says Latané. Another reason, though, is something called the diffusion of responsibility. “You think to yourself, there are all these other people here. This isn’t entirely my problem,” says Latané.

Go back to the beginning of this column. The crucial detail in 2007, when viewed through the prism of behavioral science, is that the subway platform was nearly empty. Autrey acted heroically — even leaving his two young children unattended to do so — because there was no one else who could help. On Monday, the 49th Street subway platform was full of people, each possibly thinking that someone else was closer, someone else was stronger, someone else should be responsible for the heroic act. As a result, no one acted.

“I wouldn’t do the wrong thing,” one man waiting for a subway train told The Times on Tuesday. That’s what we all want to think. It’s why we are so quick to condemn those who do nothing at such moments.

But let’s be honest: We don’t really know how we’d act until the moment is upon us. Sadly, the science says we’re more likely to do nothing than respond like Wesley Autrey.