DeFeo's trial began on October 14, 1975. He and his defense lawyer, William Weber, mounted an affirmative defense of insanity, with DeFeo claiming that he killed his family in self-defense because he heard their voices plotting against him. The insanity plea was supported by the psychiatrist for the defense, Daniel Schwartz. The psychiatrist for the prosecution, Dr. Harold Zolan, maintained that, although DeFeo was a user of heroin and LSD, he had antisocial personality disorder and was aware of his actions at the time of the crime.