To be responsible for a crime, the defendant must have had the mental state to commit it. Accidentally striking someone, for example, typically doesn't constitute an assault or battery. (See Unconsciousness as a Defense to Criminal Charges.)
Amnesia is loss of memory caused by psychological or physical trauma. It's not the same as not having the mental status required for 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 NE2d 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 SW2d 405 (Tenn. 1963).).
Though not typically a defense by itself, amnesia is a factor courts can consider 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 looks 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, or 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 A2d 758 (Pa. 1966).)