Involuntary manslaughter is an accidental killing that is either
- the result of criminal negligence or
- caused during the commission of either a crime or a felony that does not trigger the felony murder rule.
The first type of involuntary manslaughter (caused by a negligent act) can apply even if the negligent act that caused the death was not illegal. See Commonwealth v. Welansky, 55 N.E.2d 902 (Mass. 1944).
However, the negligence that is required to meet the standard of criminal negligence is greater than that required for civil negligence. A defendant will only be guilty of involuntary manslaughter if the act that resulted in the victim’s death had a high and unreasonable risk of death. For example:
Bart is totally blind in one eye and legally blind in the other. Nevertheless, his father, Homer, buys him a new motorcycle for his twenty first birthday. Despite the fact that he can’t see very well, Bart takes the new motorcycle out for a spin. Unfortunately, Millhouse is crossing the street as Bart speeds down the road and Bart hits and kills Millhouse. In this situation, Bart has killed Millhouse unintentionally. However, because Bart acted in a fashion that presented a high and unreasonable risk of death to others, his actions were criminally negligent and, therefore, Bart will be convicted of involuntary manslaughter.
Some states further break down the levels of homicide by separating a homicide committed in a reckless manner (a more serious offense) from those committed in a merely "criminally negligent" manner (a less serious offense). The states that do so often call reckless homicide "manslaughter" and negligent homicide "criminally negligent homicide."
In addition to an unintentional killing committed in a criminally negligent or reckless manner, an unintentional killing caused during the commission of an unlawful act or a felony that does not trigger the felony murder rule is also involuntary manslaughter. In contrast with the "felony-murder" rule, this rule is sometimes known as the "misdemeanor-manslaughter" rule. See Commonwealth v. Mink, 123 Mass. 422 (1877). For example:
Springfield has recently made it a misdemeanor to ride a skateboard on public sidewalks. Ignoring the law, Bart takes his skateboard out for a spin. Bart is moving at a considerable speed when Mr. Burns steps out of his limousine onto the sidewalk. Bart hits Mr. Burns who falls and hits the back of his head on the sidewalk. Mr. Burns dies of his injuries a few days later. In this situation, Mr. Burns’ death was obviously unintentional. However, because Bart committed this unintentional killing while committing a misdemeanor, he can be convicted of involuntary manslaughter.
Finally, please note that many modern statutes have created a new category of manslaughter called vehicular manslaughter which applies to deaths caused through the negligent operation of a motor vehicle or through the unlawful operation of a motor vehicle. Even in states that do not have such statutes, deaths that result from drunk driving are usually prosecuted under the crime of involuntary manslaughter (although sometimes driving while very drunk can rise to the level of depraved indifference murder).