When John Wilkes Booth shot and killed President Abraham Lincoln on April 14, 1865, he did so with clarity, determination and purpose. Booth was clear about his motives. He was dedicated to the cause and willing to accept the consequences of his actions. More importantly, he was of sound mind and body.

In the more than 150 years since Lincoln’s assassination, few assassinations or assassination attempts have been executed with such planning, precision and purpose.

While Booth was hoping to execute a coup d’état for the Confederacy, other assassinations and assassination attempts seem – through a historical lens – much more random and significantly more bizarre. A common theme emerged over the course of time: assassinations or assassination attempts against a United States president are often blamed on the assailant’s mental defect rather than his or her political motivations.

In 1881, defense lawyers for Charles Guiteau, the man who fatally shot President James Garfield, unsuccessfully argued that their client was clinically insane.

In fact, Guiteau’s trial was one of the first high-profile cases in the United States where the insanity defense was considered. Guiteau vehemently insisted that while he had been legally insane at the time of the shooting, he was not really medically insane, causing infighting between him and his court-appointed attorneys.

During the trial, Dr. Edward Charles Spitzka, a leading alienist — the term then used for psychiatrists because the insane were thought to be “alien” to their peers — testified that it was clear “Guiteau is not only now insane, but that he was never anything else.” [Charles Rosenberg, 1968. The Trial of the Assassin Guiteau: Psychiatry and the Law in the Gilded Age. Chicago: University of Chicago Press.]

The prosecution vehemently dismissed the insanity defense, and Guiteau was found guilty and executed.

Two decades later, Leon Czlgosz, a 25-year-old anarchist, shot and fatally wounded President William McKinley in a crowded setting on the grounds of the Pan-American Exposition in Buffalo.

After firing his gun twice at point blank range, Czolgosz was immediately attacked and disarmed by the large crowd that surrounded the president. He was indicted on first-degree murder, and though he reportedly spoke freely with his guards, he refused to speak with his court-appointed lawyers or with the psychiatrist who was ordered to evaluate his mental competence.

As a result, his attorneys argued at the trial that Czolgosz could not be found guilty for the murder of the president because he was insane at the time of the shooting.

The jury was sympathetic to the defense’s case that Czolgosz was insane because “no sane man would have shot and killed the president in such a public and blatant manner in which he knew he would be caught.” But the insanity defense ultimately failed and Czolgosz was later executed in the electric chair.

And then there is the case of Lee Harvey Oswald, an obviously troubled man who allegedly served as the sole assassin of President John F. Kennedy.

When he was 13 years old, Oswald was briefly placed at the Youth House in New York City because of truancy issues and the fact that his mother was having an increasingly harder time controlling her son’s violent outbursts. Lee then, as he was later in life, was described as an anti-social loner. In his psychiatric assessment of Oswald, Dr. Renatus Hartogs found no profound mental defect in the 13-year-old Oswald, but diagnosed him as suffering from a “personality pattern disturbance with schizoid features and passive – aggressive tendencies.”

After his attempted defection to Moscow failed, Lee made a superficial suicide attempt by slicing his wrists. He was held in a Russian psychiatric institution and later allowed to remain in the USSR.

By all accounts, Oswald was a loner in search of belonging and meaning and often bragged about working as a spy for the Central Intelligence Agency. If anyone could be easily discredited, Lee Harvey Oswald fit that pattern perfectly.

In 1974, Samuel Byck hijacked a commercial airliner at Baltimore Washington Airport with the intent of having it flown into the White House and killing President Richard Nixon. Two years earlier, Byck began to suffer from severe bouts of depression after his wife divorced him and after experiencing many job failures. Due to his depression, he admitted himself to a psychiatric ward where he stayed for two months. He was shot by police before the plane ever left the ground.

A year later, Lynette “Squeaky” Fromme, an ardent follower of Charles Manson and member of the “Manson Family,” attempted to assassinate President Gerald Ford in Sacramento, California. She pointed an unloaded Colt .45 caliber pistol at the president. Earlier that year, she unsuccessfully tried to contact Led Zeppelin guitarist Jimmy Page. She told the band’s manager that she wanted to warn the musician about “bad energy.”

In 1979, Raymond Lee Harvey, an unemployed American drifter, was arrested by the Secret Service after being found carrying a starter pistol with blank rounds, ten minutes before President Jimmy Carter was to give a speech at the Civic Center Mall in Los Angeles on May 5.

Although Harvey had a history of mental illness, police were forced to investigate his claim that he was part of a four-man operation to assassinate the president.

On March 30, 1981, John Hinckley stood outside the Hilton Washington Hotel and fired several shots from a small-caliber handgun at President Ronald Reagan and members of his entourage as they were leaving the hotel.

Hinckley was immediately subdued and arrested at the scene. Later, he claimed to have wanted to kill the president to impress actress Jodie Foster. He was deemed mentally ill and was confined to a psychiatric institution.

On September 12, 1994, Frank Eugene Corder flew a stolen single-engine Cessna airplane onto the White House lawn and crashed into a tree in attempt to kill President Clinton. A truck driver from Maryland, Corder reportedly had alcohol problems. He was killed in the crash.

A few weeks later, on October 29, 1994, Francisco Duran fired at least 29 shots with a semi-automatic rifle at the White House from a fence overlooking the north lawn, thinking that President Clinton was standing outside. Nearby tourists tackled Duran before he could injure anyone. Found with a suicide note in his pocket, Duran was sentenced to 40 years in prison.

Oscar Ramiro Ortega-Hernandez, a man who believed he was Jesus and that President Barack Obama was the Antichrist, hit the White House with several rounds fired from a semi-automatic rifle in 2011.

Random, or the foundation of deniability?