The Volokh Conspiracy
Mostly law professors | Sometimes contrarian | Often libertarian | Always independent
Under U.S. law, killing someone is "voluntary manslaughter" rather than murder if the killer was (1) actually provoked into a rage by the victim, and (2) the killer's reaction was seen as "objectively reasonable," in the sense that the law empathized with the killer's rage though not his conduct (which, after all, remains a crime, just a less serious crime). Classic examples of such objectively reasonable reaction have been when the victim had seriously assaulted the killer (but when the reaction is immediate revenge rather than self-defense), when the killer learns that the victim had seriously injured a member of the killer's family, and when the killer learns of the victim's adultery with the killer's spouse.
This defense has also sometimes been raised when the victim had made a nonviolent homosexual advance towards the killer. And sometimes it has been raised when the killer and the victim had sex, the killer thought the victim was a woman, and the killer then learned that the victim was actually biologically male.
The California Legislature has just passed AB2501, which provides,
(f) (1) For purposes of determining sudden quarrel or heat of passion [for purposes of the voluntary manslaughter test], the provocation was not objectively reasonable if it resulted from the discovery of, knowledge about, or potential disclosure of the victim's actual or perceived gender, gender identity, gender expression, or sexual orientation, including under circumstances in which the victim made an unwanted nonforcible romantic or sexual advance towards the defendant, or if the defendant and victim dated or had a romantic or sexual relationship. Nothing in this section shall preclude the jury from considering all relevant facts to determine whether the defendant was in fact provoked for purposes of establishing subjective provocation.
(2) For purposes of this subdivision, "gender" includes a person's gender identity and gender-related appearance and behavior regardless of whether that appearance or behavior is associated with the person's gender as determined at birth.
UPDATE: Note that, in California, the "objective" prong of the voluntary manslaughter test is, in the first instance, a question of law for the court. Some forms of provocation have been treated by California courts as insufficient to lead to a voluntary manslaughter conviction: in particular, "a voluntary manslaughter instruction is not warranted where the act that allegedly provoked the killing was no more than taunting words, a technical battery, or slight touching." See People v. Guttierez (Cal. 2009). The statute thus adds other sorts of conduct to the list of conduct that, as a matter of law, doesn't qualify as objectively reasonable provocation.
Some states, such as Illinois, go further and identify only a limited set of circumstances that, as a matter of law, can constitute reasonable provocation—for instance, "[t]he only categories of provocation recognized by this court are substantial physical injury or substantial physical assault, mutual quarrel or combat, illegal arrest, and adultery with the offender's spouse," People v. Garcia (Ill. 1995) (emphasis added). Finally, I believe that some other states leave the "objective reasonableness" prong entirely to the jury. (The subjective provocation prong is always a matter for the jury, because it's all about the historical facts of this particular case.)