Hoyt v. Florida

The first use of the temporary insanity plea occurred in 1859 when Daniel Sickles, leader of the Democratic party, used it as his defense in the murder of Philip Barton Key, who had been having an affair with Sickles's wife, Teresa. Sickles's defense team tried to appeal to the "unwritten law" to make Key's murder look like justifiable homicide.

Sickles found out about the affair between his wife and Key on 24 February 1859. Sickles had Teresa sign a detailed confession and consulted two of his political cronies for advice. Unaware that Sickles knew everything, Key tried to signal Teresa from Lafayette Park, which was across from the Sickles' house, on 27 February. Enraged, Sickles stormed out of the house and gunned Key down amongst the Sunday afternoon strollers in the park.

The high publicity surrounding the event made it difficult to locate jurors who did not sympathize with Sickles. One of Sickles's advisors, former Secretary of the Treasury Robert J. Walker, testified that Sickles was in "an agony of despair, the most terrible thing I ever saw in my life . . . I feared if it continued he would become permanently insane."

The judge instructed the jurors that, in the eyes of the law, any delay between becoming aware of an adultery and the slaying of the adulterer by an enraged husband made the killing deliberate murder, or at the very least manslaughter. On 26 April 1859, the jury returned a not guilty verdict.