New york city could have been next. That is what investigators are now saying was the next target for bombing suspect dzhokhar tsarnaev, who tonight is in fair condition after being transferred from a... See More
New york city could have been next. That is what investigators are now saying was the next target for bombing suspect dzhokhar tsarnaev, who tonight is in fair condition after being transferred from a boston hospital to a medical facility in a state prison. Of course, so many asking tonight, elizabeth, what if those brothers made it to new york to times square. The crowd chaos could have been boston all over again. So, what decision do you make in indiana stand, when it can make the differences between life and death? Here's matt gutman. Reporter: There is a moment when, in a crowd, split-second decisions have to be made. It happened dozens of times in boston last week. People kind of freeze and a we just started to react. Wait, where is everyone? There's got to be people hurt up there! That instant is really amazing, some of us are hardwired to run. And other people are hardwired to fight. I need to go help, people are in distress and I go. And then there's people that go, "oh my god, it's too much for me. I need to leave." Reporter: In an instant humans can cease being individuals, turning instead into a herd, blindly moving in a single direction. A swarm of screaming tweens tear through the gates just to catch a glimpse of the jonas brothers. A river of soccer fans streaming through the stadium gates at the world cup. And amidst the solemnity of a presidential funeral. Everybody wants to get close to the body of the man they so revere and so idolized but this is getting dangerous. Just this week in denver, 20,000 people gathered for a marijuana rally. But soon, the pot smoke turned to gun smoke when shots rang out. Instantly, mellow turns to mayhem. There were just so many people literally just running at people. Tons of people just massively running. Reporter: The crowd bolting, pushing, shoving and trampling to escape. It wasn't bullets but bargains that caused this swarm to charge towards a black friday doorbuster deal. In the shopping stampede, this man was trample death. . Carl jung said, you put 100 clever people into a group, they lose their intelligence. Reporter: It is at those times that we become frighteningly similar to a herd of cattle. What's fascinating about crowds is that we all tend to follow them. We don't tend to stand up and be individuals in them. We tend to want to blend in and be part of it. Reporter: Just look at the classic psychological experiment illustrated here. This woman sits quietly doing her work. Then smoke starts pouring in. She's visibly worried. Still, five minutes go by. The other people in the room, all in on the experiment, ignore the smoke. She's just sitting there. No one else is reacting. Reporter:10, 15, 20 more minutes pass. Still, she doesn't take any action, because the others don't. In comparison, a second subject, in the room alone, reacts immediately and leaves. That's how powerfully socialized we are. Reporter: Socialized to death in some cases. And that's what that experiment shows. Reporter: In january, nearly 2,000 people packed a nightclub in brazil, doubling its capacity. The band sparks up fireworks. Within seconds, fire engulfs the club. People try to flee. Unquestionably following the crowd to a single exit. A stampede. People start falling. Stacking up like cordwood. Blocking the entrance. More than 200 died. Those are the doors through which nearly 2,000 college students tried to squeeze through. As that fire raged, you can also see here, where good samaritans and firefighters used sledgehammers to break through into the bathroom and try to drag people out here to safety. There are those few, who in an instant, decide to run against the grain, surging towards the flash point. A life experience can force you to think about things differently and then do things differently. Reporter: Carlos "the cowboy" arredondo, whose son was killed in iraq, was a spectator at the marathon. We first noticed him, amidst the carnage at the finish line, of that hat. But what made carlos stand out was when most people sprinted away, he stayed. I just jumped the fence and i ran towards the other side to see if I can help in any way i can. Reporter: Psychologists tell us what distinguishes ordinary man from hero is a unique ability to ignore basic instinct. When I stepped into the street, you get a sense of tunnel vision. And I ran as fast as I could to the epicenter of the blast. Reporter: Michael chase was a few away from the second boston explosion. Yet, he ran in to help this young child. I didn't think twice about it I was looking just to help people. Reporter: For others, their call to action is in the palm of their hand. Images taken at the scene, often capturing the precise moment of impact. When the stage collapsed at a sugarland concert in indiana, david wood was barely an arms length away. He managed to shoot this video with his cell phone. It was chaos. Are you okay? I threw my arms in my wife's back and shoved her. I knew she was safe. It was just kind of instinct to crawl back in and help the ones that were screaming that were in need. We need a medic really bad. It's a little girl! Reporter: While some may criticize those who whip out their camera during tragedy, others say it could be a new tool. People don't necessarily have to risk getting hurt to be able to help. If I have a camera here and i am taking pictures, I am not jumping in as much. I'm more protected. And that's safer to a lot of people. It's a more passive way of being part of the action. Reporter: Just last week, cell phops like these helped authorities in boston identify the alleged bombers. But for mike, gumjumping in is the only option. I'd do it all over again. That's not something you think about before hand. It just something that you do.
This transcript has been automatically generated and may not be 100% accurate.