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.