Insanity defense in the context of 1953 British Royal Commission on Capital Punishment report

On May 31, 2014, two twelve-year-olds lured their friend, who was also twelve, into a wooded area of Davids Park in Waukesha, Wisconsin, where they attempted to kill and sacrifice her to the Slender Man, a fictional supernatural being originating in creepypasta. Their target, Payton Isabella Leutner, suffered 19 stab wounds, but survived after being rescued by a cyclist.

The perpetrators, Anissa E. Weier (born November 10, 2001) and Morgan E. Geyser (born May 16, 2002), were motivated by their belief in the Slender Man, and believed killing Leutner would appease him and prove his existence. Weier and Geyser were tried as adults in 2017. Both were found not guilty by reason of insanity, and committed indefinitely to mental institutions. Weier was sentenced to 25 years to life, and later released in 2021 to live with her father. Geyser was sentenced to 40 years to life and was later released in 2025 to live in a group home, to be kept under supervision until 2058.

Insanity, madness, lunacy, and craziness are behaviors caused by certain abnormal mental or behavioral patterns. Insanity can manifest as violations of societal norms, including a person or persons becoming a danger to themselves or to other people. Conceptually, mental insanity also is associated with the biological phenomenon of contagion (that mental illness is infectious) as in the case of copycat suicides. In contemporary usage, the term insanity is an informal, un-scientific term denoting "mental instability"; thus, the term insanity defense is the legal definition of mental instability. In medicine, the general term psychosis is used to include the presence of delusions and/or hallucinations in a patient; and psychiatric illness is "psychopathology", not mental insanity.

In English, the word "sane" derives from the Latin adjective sanus, meaning "healthy". Juvenal's phrase mens sana in corpore sano is often translated to mean a "healthy mind in a healthy body". From this perspective, insanity can be considered as poor health of the mind, not necessarily of the brain as an organ (although that can affect mental health), but rather refers to defective function of mental processes such as reasoning. Another Latin phrase related to our current concept of sanity is compos mentis ("sound of mind"), and a euphemistic term for insanity is non compos mentis. In law, mens rea means having had criminal intent, or a guilty mind, when the act (actus reus) was committed.

Vincent Louis Gigante (/dʒɪˈɡænti/ jig-AN-tee, Italian: [dʒiˈɡante]; March 29, 1928 – December 19, 2005), also known as "Chin", was an American mobster who was boss of the Genovese crime family in New York City from 1981 to 2005. Gigante started out as a professional boxer who fought in 25 matches between 1944 and 1947. He then started working as a Mafia enforcer for what was then the Luciano crime family, forerunner of the Genovese family. Gigante was one of five brothers. Three of them, Mario, Pasquale, and Ralph, followed him into the Mafia. Only one brother, Louis, stayed out of the crime family, instead becoming a Catholic priest. Gigante was the shooter in the failed assassination of longtime Luciano boss Frank Costello in 1957. In 1959, he was sentenced to seven years in prison for drug trafficking, and after sharing a prison cell with Costello's rival, Vito Genovese, Gigante became a caporegime overseeing his own crew of Genovese soldiers and associates based in Greenwich Village.

Gigante quickly rose to power during the 1960s and 1970s. In 1981 he became the family's boss, while Anthony "Fat Tony" Salerno served as front boss during the first half of the 1980s. He also ordered the failed murder attempt of Gambino crime family boss John Gotti in 1986. With the arrest and conviction of Gotti and various Gambino family members in 1992, Gigante was recognized as the most powerful crime boss in the United States. For about 30 years, Gigante feigned insanity in an effort to throw law enforcement off his trail.

Arkham Asylum (/ˈɑːrkəm/) is a fictional psychiatric hospital appearing in American comic books published by DC Comics, commonly in stories featuring Batman. It first appeared in Batman #258 (October 1974), written by Dennis O'Neil with art by Irv Novick. Located in Gotham City, the asylum houses patients who are criminally insane, as well as select prisoners with unusual medical requirements that are beyond a conventional prison's ability to accommodate. Its high-profile patients are often members of Batman's rogues gallery.

Stanford White (November 9, 1853 – June 25, 1906) was an American architect and a partner in the architectural firm McKim, Mead & White, one of the most significant Beaux-Arts firms at the turn of the 20th century. White designed many houses for the wealthy, in addition to numerous civic, institutional and religious buildings. His temporary Washington Square Arch was so popular that he was commissioned to design a permanent one. White's design principles embodied the "American Renaissance".

In 1906, White was murdered during a musical performance at the rooftop theatre of Madison Square Garden. His killer, Harry Kendall Thaw, was a wealthy but mentally unstable heir of a coal and railroad fortune who had become obsessed by White's alleged drugging and rape of, and subsequent relationship with, the woman who was to become Thaw's wife, Evelyn Nesbit, which had started when she was aged 16 and White was 47. At the time of White's killing, Nesbit was a famous fashion model. With the public nature of the killing and elements of a sex scandal among the wealthy, the resulting trial of Thaw was dubbed the "Trial of the Century" by contemporary reporters. Thaw was ultimately found not guilty by reason of insanity.

On July 2, 1881, at 9:30 AM, James A. Garfield, the 20th president of the United States, was shot at the Baltimore and Potomac Railroad Station in Washington, D.C., less than four months into his term as president. The shooter was Charles J. Guiteau, a disappointed and delusional office seeker, who had distributed copies of a speech he wrote aimed at promoting Garfield in the 1880 United States presidential election. Guiteau believed his campaigning had been vital to Garfield's eventual victory, and that Garfield owed him a diplomatic post in Europe for his assistance. After months of failed attempts to solicit such a reward from the Garfield administration, he purchased a revolver and began stalking Garfield with the goal of assassinating him.

Guiteau shot Garfield twice from behind; one bullet grazed Garfield; the other entered his back. Garfield was carried back to the White House, where he underwent medical treatment for over two months. His condition fluctuated, though generally worsened over time as he began to suffer from sepsis and infection. His treatment in part consisted of doctors trying in vain to find the bullet still lodged in his body; by doing so, they likely aggravated his existing wounds and introduced new sources of infection, decreasing his chances of survival. Garfield was later transported by train to a mansion in Elberon, New Jersey, where he died at 10:30 p.m. on September 19 and was succeeded by his vice president, Chester A. Arthur. Garfield was the second American president to be assassinated, following Abraham Lincoln in 1865. Guiteau's trial was widely publicized, and his legal team's attempts to use the insanity defense failed. Guiteau was sentenced to death and executed by hanging.

An affirmative defense to a civil lawsuit or criminal charge is a fact or set of facts other than those alleged by the plaintiff or prosecutor which, if proven by the defendant, defeats or mitigates the legal consequences of the defendant's otherwise unlawful conduct. In civil lawsuits, affirmative defenses include the statute of limitations, the statute of frauds, waiver, and other affirmative defenses such as, in the United States, those listed in Rule 8 (c) of the Federal Rules of Civil Procedure. In criminal prosecutions, examples of affirmative defenses are self defense, insanity, entrapment and the statute of limitations.

A fact pattern or fact situation is a summary of the key facts of a particular legal case, presented without any associated discussion of their legal consequences.

For example, at common law, "Murder is the killing of another human being with malice aforethought and without justification or excuse." The elements of the crime are killing (actus reus) and malice aforethought i.e. intentional action (mens rea). Possible defenses include legal justification (e.g. self-defense) or excuse (e.g. no mens rea due to legal insanity).