Imperfect self-defense is a common law doctrine recognized by some jurisdictions whereby a defendant may mitigate punishment or sentencing imposed for a crime involving the use of deadly force by claiming, as a partial affirmative defense, the honest but unreasonable belief that the actions were necessary to counter an attack.
If the defendant's self-defense was imperfect, the self-defense may only reduce the defendant's liability. Imperfect self-defense is self-defense that was arguably necessary but somehow unreasonable. For example, if a person had a good faith belief that deadly force was necessary to repel an attack, but that belief was unreasonable, the defendant would have a claim of imperfect self-defense. In some jurisdictions, the successful invocation of such a defense reduces a murder charge to manslaughter. Most jurisdictions do not recognize imperfect self-defense.
- —West's Encyclopedia of American Law: Self-Defense
Examples in common law - United States
The doctrine of imperfect self-defense recognizes a defendant’s honest but unreasonable belief that deadly force is needed. An appellate court in Kansas held that "Imperfect self defense is an intentional killing committed with an unreasonable but honest belief that circumstances justified deadly force."
Another court, in Maryland, held that:
When evidence is presented showing the defendant’s subjective belief that the use of force was necessary to prevent imminent death or serious bodily harm, the defendant is entitled to a proper instruction on imperfect self defense....The theory underlying the doctrine is that when a defendant uses deadly force with an honest but unreasonable belief that it is necessary to defend himself, the element of malice, necessary for a murder conviction, is lacking.
- —State v. Faulkner, 483 A.2d 759,769 (Md. 1984) 
Michigan also recognizes imperfect self-defense as a qualified defense that can mitigate second-degree murder to voluntary manslaughter. However, the doctrine can only be used where the defendant would have had a right to self-defense but for the fact that the defendant was the initial aggressor.
- Answers.com web site entry on "self defense", quoting West's Encyclopedia of American Law. Accessed December 21, 2007.
- Supreme Court of California (August 29, 1996, Decided), State v. Humphrey, 921 P.2d 1; 1996 Cal. LEXIS 4222
- Janet Grumer, Note, Loyola Law Review, Summer 2003, p. 1575, found at Loyola Law Review article. Accessed December 21, 2007.
- State v. Jones, 8 P.3d 1282, 1287 (Kan. Ct. App. 2000).
- State v. Faulkner, 483 A.2d 759, 769 (Md. 1984)
- Michigan Jury Instructions on line. Accessed December 21, 2007.
- People v. Deason, 148 Mich. App. 27, 31, 384 N.W.2d 72 (1985)