In late September, on a crowded commuter train in San Francisco, a man shot and killed 20-year-old student Justin Valdez. As security footage shows, before the gunman fired, he waved around his .45 caliber pistol and at one point even pointed it across the aisle. Yet no one on the crowded train noticed because they were so focused on their smartphones and tablets. "These weren't concealed movements—the gun is very clear," District Attorney George Gascon later told the Associated Press. "These people are in very close proximity with him, and nobody sees this. They're just so engrossed, texting and reading and whatnot. They're completely oblivious of their surroundings."

Another recent attack, on a blind man walking down the street in broad daylight in Philadelphia, garnered attention because security footage later revealed that many passersby ignored the assault and never called 911. Commenting to a local radio station, Philadelphia's chief of police Charles Ramsay said that this lack of response was becoming "more and more common" and noted that people are more likely to use their cellphones to record assaults than to call the police.

Indeed, YouTube features hundreds of such videos—outbreaks of violence on sidewalks, in shopping malls and at restaurants. Many of these brawls, such as the one that broke out between two women during a victory parade for the New York Giants in 2012, feature crowds of people gathered around, cameras aloft and filming the spectacle.

Our use of technology has fundamentally changed not just our awareness in public spaces but our sense of duty to others. Engaged with the glowing screens in front of us rather than with the people around us, we often honestly don't notice what is going on. Adding to the problem is the ease with which we can record and send images, which encourages those of us who are paying attention to document emergencies rather than deal with them. The fascination with capturing images of violence is nothing new, as anyone who has perused Weegee's photographs of bloody crime scenes from the early 20th century can attest. But the ubiquity of camera-enabled cellphones has shifted the boundaries of acceptable behavior in these situations. We are all Weegee now.

But if everyone is filming an emergency, who is responsible for intervening in it? Consider an event from December 2012, when a man was pushed onto the subway tracks in New York City. Struggling unsuccessfully to heave himself onto the platform, he turned, in his final seconds, to see the train barreling down on him. We know this because a freelance photographer who happened to be on the platform took a picture of the awful episode and sold it to the New York Post, which ran it on the front page the next day, prompting public outrage about profiting from a man's death. The photographer noted that others on the platform closer to the man made no effort to rescue him and quickly pulled out their phones to capture images of his dead body.