As with voluntary manslaughter Massachusetts statutory law does not define involuntary manslaughter. Rather, Massachusetts common law, as pronounced by the courts, provides the definition for involuntary manslaughter:
One can commit involuntary manslaughter through:
(1) an unintentional killing occasioned by an act which constitutes such a disregard of the probable harmful consequences to another as to be wanton or reckless; or
(2) an unintentional killing resulting from a battery.
The first theory under which a person may face conviction for involuntary manslaughter requires an unintentional, yet unlawful killing resulting from the wanton or reckless conduct of the defendant. This theory of involuntary manslaughter is sometimes called "Welansky manslaughter," after the 1944 case in which the owner of a nightclub was convicted of involuntary manslaughter when a fire in his club caused the death of over 400 patrons. That case also established that wanton or reckless conduct includes both affirmative acts and failures to act where a duty to act exists. Such acts or omissions must embody a disregard for the probable harmful consequences to another. The conduct must involve a high degree of likelihood that substantial harm will result to another. The law requires that the defendant have knowledge of the circumstances and the intent to do the act that caused the death, and also requires that the circumstances presented a danger of serious harm such that a reasonable man would have recognized the nature and degree of danger. Wanton and reckless conduct is distinct from negligence or gross negligence for which, in the common law of Massachusetts, there is no criminal liability.
The second theory on which a defendant may face conviction for involuntary manslaughter requires that the defendant commit a battery, not amounting to a felony, which causes death. A person who uses a level of force against another that is likely to cause harm and which produces death is guilty of involuntary manslaughter. The law requires that the prosecution establish that the defendant knew, or should have known that his conduct created a high degree of likelihood that substantial harm would result to another. This means that the same standards of proof apply to both voluntary and involuntary manslaughter.
The punishment for both voluntary and involuntary manslaughter, as set by statute, is the same. The maximum sentence for an involuntary manslaughter conviction is imprisonment for twenty years, except in circumstances where the voluntary manslaughter involves explosives or infernal machines, in which cases the maximum punishment is life imprisonment.