A special corollary of the Exasperated Perp. You've got three or more perps, all friends, relatives, or otherwise closely connected with each other, and none of them are talking. You know one or more of them committed the crime, but you've tried all the known (legal) tricks to get them to confess with no result. Solution: put the perps in one room, and let them stew in each other's presence until one or more of them snap. Sometimes it leads to a confession, usually it leads to the perps being worked up enough to finally respond to one last go-around in the interrogation room. Naturally, the detectives and security are always hanging just outside to intervene in case the shouting match that usually ensues in these kinds of situations leads to a physical confrontation between perps.
Trope named after an experiment that states if you leave a population of rats to stew and propagate inside a small box, eventually the rats will turn on each other for food and room.
This trope is the exact opposite of the classic "Prisoner's Dilemma" where you keep the prisoners apart and offer each one a reduced sentence if he rats out the other(s). You make sure each one knows that his friends have received the same offer. It only takes one to break and the other(s) go down for life, but if I decide to talk first...
Occasionally it turns out that they were all playing More Hero than Thou and are all innocent but trying to protect the others, whom, like the police, they believe to be guilty.
- Played with in the Batman graphic novel The Long Halloween. A gang of Irish crooks are arrested for the murder of Harvey Dent. They all confess, and their stories line up so perfectly that Batman and Gordon are convinced they're lying. They want the crooks to spill the name of their employer, hopefully Carmine "The Roman" Falcone, so Gordon locks them all in the one big cell hoping that they'll get chatty. The catch is, one of the crooks is replaced with Harvey Dent, alive and in disguise, to try and prod the rest into talking.
- The Punisher: The "Force of Nature" story has Frank do this to a trio of mid-level criminals who cooked up a system for Hookers and Blow-seeking weekenders to hide their spending. As they were taking a trip out to sea, he sunk their boat, sabotaged the flares and rations, replaced the water with seawater, and replaced their liferaft with a tiny model which he'd bugged. They start looking for who might be responsible for their plight, admit their part in the deal, start killing each other for their share... and that's when Frank moves in.
- Done in the Lord Peter Wimsey mystery The Nine Tailors. Neither Wimsey nor the police can figure out which of two brothers murdered the victim, so they put the brothers alone in a room and secretly listen to what they say to each other. It turns out that neither of them did it, but both thought the other did, and so they had been unnecessarily covering for each other.
- Sped up brilliantly in Night Watch. Vimes finds he has three assassins in custody, two of whom are thoroughly professional, but the third is a show-offy twerp; Vimes classifies him as "the sort that gets a kick out of showing his dagger to women in bars". So, enjoying a ham and hoosegow sandwich, he drags the two others in back but leaves the door open so "Ferret" (whom Vimes has dubbed so for his weaselly demeanour) can hear a hiss, pop, and sudden screaming. Before he gets a chance to even grab the keys so Ferret can have his turn, the guy spills - and Vimes reveals that the other perps are safe, and the screaming was just him and the boys goofing around. Ferret notices his fellow crooks glaring at him, starts doing some mental arithmetic, then demands protective custody in return for ponying up everything else he knows.
- In Season 2 of 24, CTU agents put Bob Warner and Reza Nayieer in a room together in order to determine who transferred the funds to Syed Ali. Reza cracks and offers to show the CTU agents how the funds were transferred, but they find out that Reza's fiancée Marie - the last person CTU or the audience would suspect - transferred the funds, and she kills Reza and the agents investigating the funds.
- Done in Burn Notice, when the second rat is the main character, trying to get some information out of the baddie by pretending to be a previously-unknown colleague. British intelligence once actually did that by putting spies disguised as Luftwaffe pilots in the same room with prisoners. It worked quite well.
- The Commish. In this case, the guilty party was the one who fell asleep in the prison cell (apparently Truth in Television as it was mentioned in the non-fiction book Homicide: A Year On The Killing Streets. Based on the idea that the others don't know why they've been arrested, but the guilty party is resigned to the fact that he's been caught).
- The Expanse. The protagonists are on the receiving end of this in "Remember The Cant". First the Martians interrogate the Canterbury survivors separately, then put them together in the same room having planted the seeds of doubt about each other.
- There's a particularly hilarious variant in The Wire involving some particularly uninformed perps. A photocopier masquerading as a lie detector was involved. Even better, the sequence was based on a real incident.
- Doctor Who. Played for Laughs in "Robot of Sherwood". The Doctor, Robin Hood, and Clara Oswald are chained up in a dungeon so that the Sheriff can figure out which of the three is in charge. After the Doctor and Robin spend the next few minutes bickering, Clara tells them to "shut up" and suggest plans one at a time. Then the Sheriff's lackey comes in and fetches Clara. Clearly, she's the sensible one and thus must be in charge.
- Person of Interest. Reese cites the trope by name in "Judgement". He has two villains in the trunk of his car. The first man doesn't have any useful information. The second refuses to cooperate, so Reese cuts the bonds of the first man and says he can go free if he gets the information. Cue first man immediately starts beating up the second until he talks.
- Cold Case: The team does this in one case to figure out which of three suspects murdered a young girl back when the four were in middle school together. While the three quickly turn on each other, they don't say anything incriminating. They're still able to figure it out thanks to behavior. Since the stressed out Alpha Bitch instinctively started digging through another woman's purse looking for drugs, they realized she probably had leverage due to witnessing the murder. They're right.
- Another variation combines Rats in a Box and the Prisoner's Dilemma: one perp is taken away from the others, then immediately reinserted into the group solely to instill suspicion. For instance, the cops sit one of the perps in the interrogation room, then stand in total silence for some amount of time while the perp looks on in puzzlement. Then the cops return the perp to his accomplices, who ask what the cops wanted with him. The perp honestly replies "nothing", but now the suspicion that the cops offered him a deal is planted in everyone else's mind, and group solidarity quickly collapses.
- A variation has the above, then the guards acting suspiciously nice to the person they took away. Your comrades thinking you already took a deal is quite a bit of pressure to actually take it.
- The Nine Tailors variant listed above was used on captured German physicists after World War II: they were kept together in a room which was bugged and their conversations studied to help figure out how far Germany had really got in developing nuclear bombs.