That said, there's at least one more reason citizen's arrests are rare, and it's because they're a legal nightmare. "Police have something called qualified immunity," DeCarlo explains. "So they’re authorized to use force, they're operating within the confines of the law. In most instances, it’s going to be harder for a citizen to justify using force. They're probably going to get sued."
And you might not only get sued for assault. Say you decide to frisk a suspicious citizen while "arresting" them -- because you've already made it this far, right? A cop would have certain freedoms to do so under Terry v. Ohio. But you? You have none of that. You're just a vigilante on the sidewalk, who's suddenly decided he's T.J. Hooker.
On top of all that, you have to step back and consider what you're doing: intervening in a situation where you have little to no authority and (probably) no means to defend yourself, aside from whatever karate kick you picked up from your third-grade dojo visits. The person who's committing a crime might have a weapon or, at the very least, less to lose. "I wanna say I made hundreds of arrests on the job, and it’s not easy," DeCarlo says. "People are going to fight you, they might be intoxicated or high on a substance. That’s why, sometimes, police have to use force. So when a private citizen walks up to you and says, 'You’re under arrest,' how are you going to back that up?"
Obviously, you don't wanna be Kitty Genovese's neighbors, but reporting crime to the police rather than making a citizen's arrest is still usually the better call -- even if you're technically allowed to do so. After all, just look at what happened to Billy Costigan.
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Kristin Hunt is a staff writer for Thrillist, and has legitimately been curious about this since The Departed came out. Follow her to the citizens envelope at @kristin_hunt.