To be responsible for most crimes, defendants must act intentionally or recklessly. This mental state requirement acts to distinguish those who might accidentally do something (say someone trips and pushes someone down while falling) versus those who intentionally do something (say the person angrily shoves someone downs and injures them). The intentional act makes the person morally blameworthy for committing a crime. So what happens if the defendant doesn't remember what happened? Is amnesia a valid defense to a criminal act? For the most part, no. But memory loss might influence a criminal proceeding in other ways.
Amnesia refers generally to memory loss that can be caused by a variety of factors, such as physical or mental trauma, medical conditions, or medications. A person's memory, though, isn't the same as the mental state required to commit a crime. The inability to remember committing a crime doesn't necessarily mean the defendant didn't intend to and actually commit it.
A defendant's mental state at the time of the crime is what's important—amnesia occurring after the crime has no effect on conduct at the time of the offense. Accordingly, amnesia typically isn't a viable defense. (It might be, however, if, at the time of the alleged offense, it created the conditions for an insanity defense.)
Example: Amnesia caused by chronic alcoholism wasn't a defense to forgery. Even though the defendant couldn't recall his actions at the time he forged the signature on the check, his memory loss didn't affect his intent to commit the fraudulent act. (People v. Hibbler, 274 N.E.2d 101 (Ill. 1971).)
Example: The court upheld the defendant's homicide conviction because his failure to remember the shooting wasn't proof of his mental condition at the time of the act. His memory loss wasn't evidence that he didn't know right from wrong when the crime occurred. (Lester v. State, 370 S.W.2d 405 (Tenn. 1963).)
Though not typically a defense by itself, a judge can consider amnesia when determining whether someone is competent to stand trial. (For more on the competency standard, see Competency to Stand Trial.)
When considering whether amnesia compromises one's ability to stand trial, courts look at factors like the following:
Example: The defendant received a fair trial despite his claim of incompetency due to his inability to recall the events of the crime. There was no indication that he didn't understand the charges, that he wasn't able to understand the events on the day of the crime, or that he couldn't assist his attorney in defending against the charges. (Morris v. State, 214 S.W.3d 159 (Tex. App. 2007).)
While a claim of amnesia is rarely determinative of guilt or innocence, the judge or jury can consider it when deciding the penalty for the crime.
Example: Amnesia occurring after the crime wasn't admissible to prove the defendant's motives or conduct at the time of the murder. But if the judge or jury believed the defendant actually suffered from it, amnesia could be considered in determining the penalty. (Commonwealth ex. rel. Cummins v. Price, 218 A.2d 758 (Pa. 1966).)