Irresistible impulse

In criminal law, irresistible impulse is a defense by excuse, in this case some sort of insanity, in which the defendant argues that they should not be held criminally liable for their actions that broke the law, because they could not control those actions.

The Penal Code of the U.S. state of California states (2002), "The defense of diminished capacity is hereby abolished ... there shall be no defense of ... diminished responsibility or irresistible impulse..."

The policeman at the elbow test is a test used by some courts to determine whether the defendant was insane when he committed a crime. It is a variant of the M'Naghten Rules that addresses the situation in which the defendant knew that what he was going to do was wrong, but had no ability to restrain himself from doing it. The test asks whether he would have done what he did even if a policeman was standing at his elbow, hence its name.

Irresistible impulse in English Law
In English Law the concept of "irresistible impulse" was developed in the 1960 case R v. Byrne. The appellant (described as a violent sexual psychopath) strangled then mutilated a young woman, it was alleged that Byrne suffered from violent and perverted sexual desires which he found impossible to control. Lord Parker C.J. broadened the definition of "abnormality of mind" to include those lacking "the ability to exercise will-power to control acts in accordance with [their] rational judgment".

"Irresistible impulse" can be pleaded only under the defense of diminished responsibility, not under the defense of insanity. Thus it operates only as a defense to murder, reducing the charge to manslaughter, and giving the judge discretion as to length of sentence and whether committal would be more appropriate than incarceration.