This is an affirmative defense in which it is claimed that the defendant in a criminal trial is or was unable to understand the nature or unlawfulness of their actions due to a mental defect or disorder, and thus not responsible for the consequences of those actions. "Insanity" here is a legal term, not a medical one, and the court decides whether it applies—though it will take the advice of medical professionals into account.
While the use of insanity as a defense has been known since ancient times, it was codified in English law by the M'Naghten Rules of 1843, and the United States adopted similar rules for its use shortly thereafter. Most countries in the modern world have some variation of the insanity defense available to criminal defendants. Some jurisdictions instead have "Guilty But Insane," where insanity cannot be a complete defense, but can be a mitigating factor.
In Real Life, the insanity plea is rare and difficult to succeed with. It is also a risky gamble, since it is an affirmative defense that requires the defendant to concede culpability in the crime in question and places the burden of proof on the defense to prove without a doubt their client was insane. Perhaps one percent of criminal cases even attempt an insanity defense, and only about a quarter of those are accepted, primarily if the defendant already has a history of mental illness. Furthermore, it requires expert testimony from a reputable psychiatric authority that the defendant was insane at the time they committed the crime, not just insane in general. Naturally, since the insanity defense is dramatic, it is used much more often in fiction.
A variant is "Temporary Insanity," in which the defendant is claimed to have been suffering from an "irresistible impulse" during the crime, but is now sane. Thus, they can be released immediately, rather than being incarcerated for psychiatric treatment. This defense was first used in the United States by U.S. Congressman Daniel Sickles of New York in 1859, after murdering his wife's lover. It was most prevalent as a defense during the 1940s and '50s.
Along similar lines, the "Extreme Emotional Disturbance" defense argues mitigating factors compromised the defendant's ability to think rationally. For instance, a man who shot his wife after catching her in bed with her lover could argue his emotional state at the time makes him guilty of the lesser crime of Manslaughter and not 2nd degree Murder.
Since someone who has been declared "Not Guilty By Reason of Insanity" is incarcerated for psychiatric treatment until they are deemed to be no longer a threat to themselves or others, this can result in a longer loss of freedom than a normal jail sentence would have caused. Because of this, a defendant has the right to insist that this defense not be used in their case. This has not stopped some defendants — both in fiction and in Real Life — going for this defence under the mistaken belief that a plea of 'insanity' means a cushier time than a regular jail sentence. Also, in some jurisdictions, such as the District of Columbia, a person found not guilty of a sex crime by reason of insanity must register as a sex offender, as though that person had been convicted.
A "Catch-22" effect can be noticed with many juries—if the defendant is capable of understanding that an insanity plea would be a good idea, then perhaps they're not legally insane. (As insane people typically don't know they're insane - note emphasis on typically, since some actually do know there's something wrong with them.) This is in fact closely related to the original Catch-22, the dilemma discussed in that book that a serviceman who requests a discharge for insanity must be sane enough to think to do so.
Another use of this trope common in fiction is for a floundering attorney to switch to an insanity defense as a last resort, ignoring the real-life procedural steps needed for this action. It almost never works, just emphasizing how doomed the client is.
A related concept is fitness to stand trial. This is often confused with the insanity defense, particularly since they often apply to the same people, but fitness to stand trial says nothing about their mental state when they did the crime. It's about whether their current mental state allows them to understand the charges against them and conduct an appropriate defense. For example, a defendant who was clearly responsible for the crime he committed, but while awaiting trial, suffers brain damage from a suicide attempt and becomes unfit to stand trial. Rather than going to jail, he's sent to a care facility. Should he ever recover enough to be declared fit, he will stand trial.
Another concept related to insanity, although rarely seen in media, is diminished capacity. Diminished capacity is a claim that the defendant was unable to form the mental state (intent, recklessness, negligence, etc.) required for their action to be a crime. Because diminished capacity is not an affirmative defense, but rather the negation of an element of the crime, the burden of proof remains on the prosecution. Additionally, a successful diminished capacity defense results in a verdict of "not guilty", rather than "not guilty by reason of insanity", so the defendant is not incarcerated for psychiatric treatment (unless the state begins civil commitment proceedings). As a result, diminished capacity tends to be preferred to the insanity defense in jurisdictions in which it is allowed (although the two can be, and often are, combined).
In the United States neither psychopathology or a personality disorder is generally accepted as grounds for an insanity plea, and most states will simply throw the perpetrator in jail like any other criminal if that is their mental illness, since though such persons may have the adequate Lack of Empathy that you can say they don't appreciate the moral reasons for why their crimes were wrong, in general they still understood that they were breaking the law, and having pathological justifications for doing so isn't good enough. However, other countries such as the United Kingdom do accept these as valid grounds, and many notorious British Serial Killers are locked up in mental institutions whereas in America they would be in jail, probably waiting for the Death Penalty. Such British prisoners, however, are usually not expected to ever be released and most of their appeals fail. In fiction, many cases of the Insanity Defence are often based on personality disorders, even if they are American.
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Anime and Manga
In Tokyo Babylon, Subaru encounters a woman who is trying to summon a dog spirit to kill the man who murdered her young daughter, since he had been deemed too insane to stand trial. She plans on using the insanity defense for herself afterwards, figuring that a judge would see the "attempted" ritual as proof that she had lost her mind before killing the man herself.
Most of the inhabitants of Arkham Asylum for the Criminally Insane have had a successful Insanity Defense entered on their behalf. Many of them are also, not coincidentally, crazy. However most of them are not legally insane by any reasonable interpretation. A pathological obsession with hats, cats, birds, or riddles does not mean someone isn't aware of what they're doing. The same goes for being an eco-terrorist, wanting to eat people, or most of the various eccentricities Batvillains develop while pursuing a criminal career.
During the Golden Age and The Interregnum, the Joker was not considered insane enough to qualify for this defense. When he tried it, Batman easily proved that despite Joker's eccentricity, he was fully capable of understanding the illegality and consequences of his actions. It was only when the Joker became a homicidal maniac in the 1970s that Arkham Asylum became a necessity for him.
In The Joker: Devil's Advocate , a new D.A. decides to go for broke and push for the death penalty after a series of killings with the Joker's MO. After a "Trial of the Century" with accompanying media circus, he is found guilty and sentenced to death. However, he claims to have no knowledge of the murders. Batman, who has never known the Joker to deny any of his crimes, investigates and finds out that this time he really is innocent. Joker is returned to Arkham when this is revealed..
The nearly abusive use of this defense among Batman villains in particular has caused more than one rant from real legal professionals. It concluded that only Two-Face and sometimes The Joker are "legally" insane, though it's an old and incomplete list. For those who don't feel like reading the linked article in full, what the article concludes is that sometimes the Joker does appear to have a valid insanity defense for his actions (wrongfulness test: sometimes he literally doesn't think what he's doing is wrong; at other times he knows it's wrong but does it anyway For The Lulz, which is not a valid defense), and Two-Face pretty much always appears to have one (irresistable influence test: he knows it's wrong, but can't stop himself because the coin came up scarred). It specifically lists those two as examples of Arkham inmates who probably do belong there; it then gives a (short) list of others that don't: the Scarecrow, the Mad Hatter, the Penguin, Poison Ivy, Tweedledum and Tweedledee. They don't exhibit any behavior compatible with the legal definition of insanity ... mental illness, maybe; they're certainly eccentric ... but they fail both prongs of the insanity definition: wrongfulness and irresistable impulse. The author explicitly mentions that he's not trying to give an exhaustive list either way, just examples so that readers can understand what the actual criteria for being judged legally insane are. It also concedes that the audience doesn't know that Gotham uses the same test the real-world US legal system does (which varies from state to state, but they're all more or less based on the Model Penal Code), so maybe under Gotham's definition voluntarily being seen in public in green spandex is all it takes.
Not all the people listed are sent to Arkham. The Penguin is normally just sent to a normal prison (Blackgate Penitentiary). It was also a plot point that most of the Mooks you fight in Batman: Arkham Asylum had to be sent to the Asylum after a "mysterious" fire.
The general consensus is that the insanity defense is almost guaranteed to work in the Gotham court system regardless of how much sense it makes legally but since it means going to Arkham, you'd need to be crazy to try it. In Arkham Asylum: Living Hell, one white-collar criminal, unfamiliar with Gotham, made this mistake and found himself in Arkham instead of the cushy rehab center he expected to be sent to. Things didn't go well for him.
Also, not everyone who gets sent to Arkham is sent there due to being considered insane. Mr. Freeze is (usually) sane, but he's still there because it's the only imprisonment facility with the technology to keep a cell refrigerated enough for him to survive in it. Granted, this probably counts as an abuse of his human rights because it keeps him from interacting with sane people, but he hates everyone anyway so he won't complain.
When Hal Jordan, Green Lantern, fought an opponent codenamed the Aerialist, it became obvious that the Aerialist was under a severe delusion making him believe his criminal actions were entirely legal and correct. Hal referenced the M'Naghten rules when wrapping up the case, his opinion being that the Aerialist would qualify for an Insanity Defense.
In Anatomy of a Murder, the defense's entire argument rests entirely on the "irresistible impulse" insanity defense.
Aversion in Nuts, where Barbra Streisand demands they declare her sane. The film notes (accurately) that if you are declared insane, you can be kept locked up for life, even for a crime that would only entail a few years in prison.
One Flew Over The Cuckoo's Nest deconstructs the trope; playing insane got McMurphy transferred into a mental institution, which he knows about and thinks that's better than prison. He is sorely mistaken.
Training Day: Mentioned by one of the three wise men when he recaps to Alonzo how an off-screen criminal recently got off this way by pulling a stunt in court that made him seem mentally unsound.
Depending on which version you watch, William Friedkin's Rampage is either the opposite, or agrees.
Inception inverts this; Mal had herself declared sane by three different psychiatrists, so as to prevent Cobb from being able to explain the nature of her madness, as part of her plan.
Rowdy Rathore treats this as a "Get Out Of Jail Free" card without any mention of psychiatric treatment; justified in that the Big Bad has clearly bought the judge and no one in the story believes otherwise.
In Miss Congeniality, Grace sarcastically suggests that Cathy Morningside shoot for an insanity plea after rigging the Miss United States crown to blow up and kill the contest winner, purely out of spite for being fired from running the pageant, and then having the audacity to claim the whole thing was a plan to make the world "a more beautiful place".
Side Effects has the protagonist take various antidepressants before taking a new experimental drug called Ablixa, which appears to be working until she murders her husband while sleepwalking. At the trial, her current psychiatrist (Jude Law) argues that the drug is to blame, and the defense ends up offering the insanity plea. The trope is slightly subverted in that it's specifically mentioned how low the percentage of success for this type of defense is. The woman is, instead, put into a mental institution until her psychiatrist deems her cured. Subverted in that she knew precisely what she was doing and planned the whole thing with her former psychiatrist in order to kill her husband and cash in on the pharmaceutical company's stock plunge. At the end, Jude Law's character figures it out and puts her back into the mental hospital.
In the 1962 film version of Chushigura, Lord Asano is allowed the chance to claim that he was mentally deranged when he attacked Lord Kira, and thus not responsible for his actions. Asano refuses to take this easy way out, as it would be dishonorable to lie to save himself.
After the trial starts, an accused murderer's attorney realizes that his client has Multiple Personality Disorder (i.e. split personality) and that one of the personalities committed the crime. He tries to change his plea from "Not Guilty" to an Insanity Defense. After a dramatic courtroom scene the judge agrees and says she'll send the client to a mental hospital for treatment. At the end it's revealed that the client was faking insanity to get away with the murder. The concept remains in The Film of the Book. The killer, for all his cunning, seems not to be aware of the "institutionalization is usually longer than prison" trope.
On the other hand, in the sequels he manages to fool his doctors into thinking he's cured and get out-where he kills again.
In A Time to Kill the defense lawyer protagonist argued that the rape of his client's daughter combined with the likelihood of them getting off lightly drove the defendant temporarily insane, causing him to kill both rapists with an assault rifle. This plea is somewhat undermined when the prosecution points out that the psychiatrist he brings in to support his argument was convicted of statutory rape and pressures the defendant into angrily declaring that he still thinks the rapists deserved to die, but in the end he's found not guilty. The defense attorney asked the jury to place themselves in the defendant's place with a thought experiment in his closing argument. The jury realized just how much the rape had to shatter the defendant and voted to acquit. Notably, this is one case where the actual results of on insanity plea would likely match the fictional. As it was a temporary phenomenon brought on by a unique strain, he would probably not be institutionalized.
One Flew Over The Cuckoo's Nest - McMurphy CLAIMS to haven gotten himself transferred to the institution to serve out the rest of his sentence in cushy surroundings, and is more than a little alarmed when he realizes that 'the rest of his sentence' is no longer the few months he thought it was, but when the doctors decide that he's no longer a threat to himself or others — which, considering he's pissed off the evil Nurse Ratchet, could mean an indefinite stay.
Comes up in The Krytos Trap. Tycho Celchu is accused of murder, and there's a lot of evidence that he did it. A few years ago, he'd been kidnapped by Ysanne Isard, Manchurian Agent maker extraordinaire. People who think he did it are divided between thinking that he'd been brainwashed and thinking that he was a garden-variety traitor; there's evidence for both. His lawyer tells Tycho that if the Tribunal decides he was brainwashed, he'll be declared not guilty by reason of diminished sapience and put into a hospital to be treated, and released when he's cured. That sounds nightmarish to Tycho, but the Tribunal's nightmare is that he'll only be there for a week or two before treatment ends and he'll be released. This would make the justice system seem impotent; the nonhuman public thinks he's a traitor and isn't very sure about their government as is. The human public is starting to believe that he's actually innocent, since there's proof of that too, and is being offered up to placate the nonhumans. It's sticky. In the end he's declared completely innocent when the victim turns up alive.
In Red Dragon and Silence of the Lambs, Dr. Hannibal Lecter was committed to a mental hospital after being convicted of killing and cannibalizing several people and the attempted murder of an FBI agent. The hospital's director Dr. Chilton describes him as a "pure psychopath". Lecter's case wouldn't fit the legal definition of insanity since, based on the discussions he had with Starling and others, it's clear that he was in full possession of his (admittedly impressive) mental faculties and capable of understanding the consequences of what he was doing. In Hannibal Starling explains that Lecter did not actually plead insanity, but rather that it was the jury that found him insane, the reasoning of the courts being that Lecter was such a respectable, successful, and intelligent individual that the only possible explanation for his crimes was that he was stark raving mad. In the Film of the Book the headline reads "Lecter Given Nine Life Sentences" or something like that, but he is still put in a mental institution for treatment, which can often occur even when the defendant is found guilty but still has a disorder.
"...The landlord shouted to them to leave him alone, for he had already told them that he was mad, and as a madman he would not be accountable even if he killed them all..."
Parodied in Soul Music, when a murderer who has already been executed tries to convince Death that "the balance of my mind was disturbed". His logic? "I really wanted to kill him. You can't tell me that's a balanced state of mind."
In the third book of The Hunger Games, Katniss was declared this after she killed President Coin for sending Prim and other children to their deaths.
In The Adventures of Tom Sawyer, Muff Potter's lawyer said to the jury he wanted to introduce this defense in account of his client's drunkenness before making Tom and Huck testify.
In After The Golden Age, the supervillain Simon Sito, also known as The Destructor, uses the insanity defense at his trial. The protagonist initially believes it to be a ploy and works with the prosecution to prove that Sito is sane; later in the book, however, she contemplates how Sito has never shown any interest in money, power, or sex, seeming to be solely obsessed with destruction for its own sake, and starts to consider the possibility that he really is insane.
Live Action TV
The Made-for-TV MovieThe Burning Bed is a good example of a well-supported "temporary insanity" plea.
The show frequently addresses this, as many defendants plead 'not guilty by reason of mental disease or defect' (which is how they phrase it) and court/prosecution-appointed psychiatrists are often featured for this reason; rarely are the defendants actually found to be legally incompetent to the degree that is required by law for such a plea to be accepted without trial, and even fewer actually have this plea accepted by a jury. The Catch-22 bind is often addressed in episodes where the situation is inverted; the defendant clearly is mentally incompetent, but insists they are sane.
One episode has the unusual spectacle of a guy using an insanity defense while defending himself. His claim is that he was insane when he committed the crimes, but now is normal thanks to medication, so it's not as ludicrous as it first seems. He is a paranoid schizophrenic, leading to him becoming homeless and murdering people due to his delusions. Before, however, he earned a law degree, and is practicing (while defending himself) for the first time.
One inversion occurred when an obviously insane defendant who murdered three people refused to allow his attorney to plead insanity, even though he was obviously guilty and eligible for the death penalty if convicted.
Another subversion was what appeared to be an extremely deranged person pleading it, appearing to be actually insane (voices in his head, erratic behavior, violent reaction to loud noises, etc.) only for the court psychiatrist to see that his symptoms were too good, figuring out that he was only pretending to be insane.
Then there was an interesting case. The defendant actually WAS hearing voices in his head. However, it wasn't insanity causing the voices, it was a brain tumor, and the defendant had such a short life expectancy that putting him on trial would be rather pointless. His doctor was charged for criminal incompetence, with the promise that once the defendant died the doctor would also be charged with homicide (since the tumor was operable when the doctor discovered it).
Law & Order: Special Victims Unit: Sexsomnia, watching too much TV (Oz), borderline mental retardation (as noted below), schizophrenia, sexual addiction, even a brain tumor have all been successfully proffered as rape defenses, and in nearly all of these cases, the defense is valid. As with the mothership, defense lawyers still habitually trot out the defense as a last resort when the client is caught red-handed only to get their asses handed to them when a last-minute fact torpedoes their whole case.
It's used to chilling effect in an episode where a mentally disabled man rapes an old woman (causing her to have a heart attack) because her position greatly resembles a porn flick he was shown, and he didn't understand the nature or consequences of what he was doing, thus he is acquitted. The prosecutor is infuriated that he 'got off easy' by not being put in prison—but then it cuts to the man being put by one of the detectives into a mental institution, still unsure of what's going on, and seeing the various mentally disturbed people talking to themselves or wandering around in an unpleasant gray facility. The final shot is of the horror on the man's face.
Subverted in episodes of both shows when insane defendants refused to plead insanity for different reasons, forcing the prosecutors to come up with a roundabout way to introduce the evidence of their insanity or compel them to reveal it by goading them into a courtroom outburst of some sort.
In one episode of Law & Order: Criminal Intent, the user of the insanity defense is a police psychologist. The detectives eventually prove that he'd killed a man in a misguided and misinformed attempt to impress a woman he was in love with, with the Insanity Defense as a contingency plan.
Parodied in Community. The Insanity Defense is used by Jeff to keep Britta from being expelled from Greendale, arguing that she deliberately sabotaged herself by cheating on a test in order to fail and get kicked out, thus validating her own low sense of self-worth. He then goes on to argue that since everyone at Greendale is crazy in one sense or another, it would be an act of gross hypocrisy to kick her out based on this. It actually works.
Used in Bones. Zach Addy was deemed insane in an off-screen trial, and went to a mental institution. Eventually, the audience finds out that he didn't actually commit the crime he was accused of, but believes that he may have committed a lesser crime and refuses to confess, because then he could be re-tried and might go to jail.
One episode of Criminal Minds has Hotch tell the killer himself "You were sick. You didn't know what you were doing", because the man is freaking out while in custody, having just realized that he is actually guilty of the crimes they're accusing him of. The last shot of the episode is the killer in a mental institution. There are a number of other episodes where you don't see the killer in an institution, but it's pretty obvious that the defense applies.
The Firm on The Practice would cut-and-paste the Insanity Defense around any sympathetic defendant who performed a Vigilante Execution to give the jury an excuse to acquit their client, since arguing an eye for an eye would be considered jury nullification. The chances of a jury buying it were generally low. The episode "Committed" showed the aftermath of one these: a serial killer who used a successful insanity defense some years ago hires Lindsey to help him get declared sane and released from the institution he is now in.
Attempted by Serial Killer Nate Haskell in CSI: Crime Scene Investigation, who argues that a combination of childhood abuse and possessing the MAOA gene, which has been linked to an increased propensity for violence, turned him into a sadistic psychopath. His Arch-Enemy Ray Langston shoots him down in court by announcing that he also had an abusive childhood and also possesses the gene, but he became a doctor rather than a murderer, and Haskell later admits in private that he made a conscious decision in his youth to blame everything on the two in the event he was caught. This would be a Crowning Moment Of Awesome for Langston, were it not for the sheer amount of Fridge Logic involved in the case- it is strongly implied that if Haskell were found insane, he would actually be let out of custody, despite his entire defense boiling down to "I'm a sadistic, murdering psychopath. And I'll probably do it again."
It's Always Sunny in Philadelphia: When Mac and Charlie learn that “Psycho Pete,” the former leader of their “freight-train gang” killed and ate his parents for Christmas dinner and got out of prison on an insanity plea.
JAG: In "Act of Terror", a Marine guard is charged for murder after he shot a terrorist suspect during a prisoner transfer, on live TV. Harm, as defense counsel, begins to explore the option of an insanity defense, but he’s taken off the case by the defendant, who’s hired a different attorney.
Cracked: Given that the show deals exclusively with cases involving The Mentally Ill in some capacity, many of its perpetrators would be judged not fit to stand trial, not guilty by reason of insanity, or to have suffered from diminished capacity. The episode "No Traveller Returns" is dedicated to establishing whether cannibal murderer Mandar Kush, who has spent eleven years in an institution, can be released into society. The answer is yes.
Used in an episode of Psych. Interestingly, the episode begins after the defense has been used and the sentence passed. Lassiter becomes enraged by the verdict and thinks the defendant was faking it, so he sends Shawn to investigate. The defendant was actually insane, but wasn't actually the murderer.
Well, they put him in jail again They tried to give him life Pete beat the case, he pleaded insane They gave him back his same ole knife
"Tabard of Pizarro", an episode of Bold Venture, had the guest star claim that he was framed for murdering his wife, and pled insanity so as to be free to eventually claim his revenge (the villain of the story tries to frame the protagonist for murder during the course of the episode, so it seems likely the guest star is telling the truth.) Unfortunately, it was many years before he was considered fit to be released. The eponymous tabard is a fake, created as physical therapy in the asylum, and used as bait to trap the villain. Oh, and the revenge thing didn't turn out so well, not being satisfying at all.
Addressed in at least two editions of Champions, thanks to Hero author Steven Long also being a practicing attorney. Game-world legal precedents have established that dressing up in a silly costume does not count as insanity, and neither does committing bizarre crimes. The villain Foxbat, who genuinely believes that he is a character in a comic book, is listed as an example of someone who could use the insanity defense.
The first Phoenix Wright: Ace Attorney game had an Amoral Attorney pressure his client into using an insanity defense. It worked, but the defendant's life was ruined as a result, when he could have been found Not Guilty because he hadn't committed the crime. So he killed the attorney fifteen years later.
In Knights of the Old Republic, if you let the lawyer appointed to you handle the trial (without telling him that you have evidence of a Sith plot) after you've broken into the Sith embassy in Manaan, then he'll try this as a last defense. It fails, and you're executed in a Nonstandard Game Over.
In Remember11, Keiko Inubushi avoided going to prison after murdering twelve people due to suffering from Dissociative Identity Disorder. Instead, she was sent to the SPHIA psychiatric hospital.
In Corpse Party, this is how Yoshikazu Yanagihori avoided going to jail for abducting four children and killing three of him. He actually was pretty insane at that point and he wasn't even the real killer, just an accomplice to the real killer, who was manipulating him, so a mental hospital was probably the best place for him. Unfortunately, security was not very good (it was 1973 Japan) and he ended up breaking out. He then broke into the school where the children were killed and hanged himself.
In the Futurama episode "Insane in the Mainframe", Simple Country Lawyer Hyperchicken uses the insanity defense in favor of Fry and Bender, and offers as proof the fact that "they done hired me as their lawyer." What then happens is that they're put in the mental institution, similar to what would have happened real life if they weren't found "Guilty but Mentally Ill". Note that this in a country where being poor has recently been classified as a mental illness.
Judy suggests using this excuse when Doug is framed for stealing Mr. Bone's trophy. Doug mentions that someone else actually tried that before but still got in trouble and had to go to the school counselor every day.
In Archer, Cheryl uses this tactic effectively to avoid jail time, instead being thrown into a mental hospital for ten months. She urges her friends to follow her lead in order to avoid getting arrested for trespassing.
He used this defense earlier in "The Dabba Don" when he defended Fred Flintstone from charges of violating the RICO Act (in other words, being The Don). He successfully argued that Fred suffered from Identity Amnesia and only THOUGHT he was a mafia boss. Then it turns out that despite all clues to the contrary, Fred never wasThe Don. Barney Rubble was.
Louis Riel's lawyers wanted him to plead insanity to be found not guilty of treason. People will never know if it would have worked, because Riel wanted to use his time in court as a final opportunity to make public the natives' problems. He was found guilty and hanged.
One notable case in British legal history (it led to the end the rule of "constructive malice", aka Felony Murder) was the case of Derek Bentley and Christopher Craig in 1952. Bentley, 19, was severely mentally disabled, although not insane. Craig was 16. When breaking into a warehouse one night, Craig was carrying a gun. Confronted by the police, Bentley (just arrested by one of them) allegedly said "Let him have it, Chris!". Craig shot dead another police officer. Craig was too young to be hanged (it was for 18-year olds and above only) and received "detention at Her Majesty's pleasure", paroled after 10 years. Bentley was hanged. In 1993, he was retroactively pardoned and his sentence quashed in 1998.
One of the better known cases in recent American history was the murder of Supervisor Harvey Milk and Mayor George Moscone in San Francisco. Former Supervisor Dan White's defense team was able to convince the jury that he was in such a mental state as to be incapable of premeditation, so he ended up being convicted of voluntary manslaughter. Among other things, they cited White's shift from health nut to junk food addict as evidence of his declining mental state. The references to junk food led to the coining of the term "Twinkie Defense". Although the term "Twinkie Defense" is often used nowadays to mean that eating the twinkies caused him to enter the state of diminished capacity. This argument was not actually used by the defense (and Twinkies were mentioned only in passing), but people think it was.
A man in Alberta beheaded a fellow passenger on a Greyhound bus. He was found not criminally responsible because he is insane. He was, however, deemed a significant risk—which means he will likely spend the rest of his life in a rubber room. A lot of people misunderstood what the verdict meant for the defendant, and there was some anger that he wasn't found guilty. Others, of course, pointed out that while he might not be in an actual prison, he'd spend a lot longer in a psychiatric institution—essentially, instead of "life with chance of parole in some number of years", he got "life with no real chance of parole".
A man strangled his wife in his sleep and was released as it was decided that his sleep disorder meant that he wasn't responsible for this act. New Scientist has included a disussion of how you can show whether or not someone has a sleep disorder (after staying awake for 25h a given tone woke up all the sleep disordered positive people and none of the healthy controls).
John Hinckley, Jr attempted to assassinate Ronald Reagan and is one of the few "successful" Insanity Defense stories. The verdict led to widespread dismay; as a result, the U.S. Congress and a number of states rewrote laws regarding the insanity defense. Idaho, Montana, and Utah have abolished the defense altogether. The dismay is ironic, as he has spent almost 30 years in a mental institution, probably more time than he would have spent in prison if convicted, certainly longer than many "sane" murderers spend in prison. Judging by what he did before the attempt and the way he defended himself during trial, he had been everything but sane. Unfortunately, the only way to be sure is to release him now and provide the logistics needed to trace back his obsession - Jodie Foster - if he could cling to it for 32 years and still not give up, he's too insane to count.
The US Supreme Court ruled that people who are insane can not be executed, on the grounds that someone who is to be executed must be capable of comprehending the gravity of their own death. Therefore, if medication can make someone sane, they can be forced to take it so they can be executed.
The lawyers of Serial Killer Herbert Mullin tried this. Somewhat bizarrely, it didn't work, as it was considered that his crimes were too well-planned to be the work of an insane person, even though his motive was that he had a delusion that he had to kill people lest a whole coast of the US be wiped out by natural disasters.
The Richard Chase trial was similar.
Jared Loughner is an interesting example of just how hard it is to pull this off in real life. Prior to killing six people, he'd been forced out of college and told he couldn't come back unless he could maintain a mental health clearance proving he wasn't a danger to himself and others. The sheer incoherence of his political and philosophical ramblings got him ejected from abovetopsecret.com, one of the more out-there conspiracy theory sites on the Internet, and countless witnesses on and off-line report him as having done everything from accusing his math teacher of "denying math" to spending thirty minutes in a bathroom and asking afterwards what year it was. He has since been medicated and pleaded guilty.
An interesting case is an ongoing thread of Jon Ronson's book The Psychopath Test, where he meets a man who pleaded insanity in the expectation that this would be a lighter sentence. Pleading insanity under those circumstances was seen by his doctors as evidence that he actually was insane - but not the kind of insane he claimed to be; it checked the item "Cunning / Manipulative" on the aforementioned Psychopath test, so he was held far longer than he would have been if he had pleaded guilty to the original offense. The book also covers a fairly chilling catch-22; if he expresses no remorse for his crime, he meets one criteria for psychopathy, but if he does express remorse, that could easily be viewed as yet another reason to check "Cunning / Manipulative". Ronson has a TED talk where he discusses this case.
The lawyers of the British child-killer Ian Brady tried this, basically relying on circular reasoning: "My client has done something so horrible he must be insane, therefore he is insane." It didn't work initially, though he has since been transferred to Broadmoor Psychiatric Hospital - which, for someone like him, basically means the most secure detention facility in the UK. "Hospital" is not Broadmoor.
James Holmes, the perpetrator of the 2012 Aurora Colorado shootings, has apparently been called this by his defense attorney. Whether it will work is a matter of debate, because not only is the trial not over, Holmes also dyed his hair orange and was referring to himself as "the Joker." Then again, he propped open an emergency exit before his rampage so that he could sneak into the theater past the front, and had placed a large number of complicated, connected traps in his apartment that police took several days to disable.
This was the plea entered in the trial of Winston Moseley, murderer of Kitty Genovese. Although he did show signs of mental illness, the jury decided he did not fit the legal criteria; among other things, he said that he had deliberately counted on no one interfering with his attack, showing that he was aware of the nature of his actions.