Most people are aware that murder and manslaughter are distinct crimes. They might even know that each has subcategories—murder is either in the first or second degree, while manslaughter is either voluntary or involuntary. (For a detailed explanation of each crime, see Homicide: Murder and Manslaughter.)
But, even for those who understand the theoretical differences between these forms of homicide, distinguishing between them is notoriously difficult. It's also tremendously important, as the sentences for the crimes vary sharply. Perhaps the toughest distinction to conceptualize is between unintentional second degree murder and involuntary manslaughter.
When someone kills another accidentally, the crime can be second degree murder or involuntary manslaughter, or it might be no crime at all (such as when a driver makes a minor mistake that results in a fatal crash). The determination rests entirely on an evaluation of how careless the defendant was—a difficult, subjective task. And that's assuming the decision maker knows the facts. In the case of George Zimmerman, for example, the story was too cloudy for the jury to confidently determine what happened.
Unintentional second degree murder goes by several theatrical names, including:
"Reckless murder" is perhaps the best moniker, since it conveys the essence of the crime: that someone kills through extreme recklessness. Under California law, for example, reckless murder occurs when the defendant is aware of, but consciously disregards, a risk that takes another person's life.
Involuntary manslaughter, on the other hand, generally requires that the defendant act with "criminal negligence"—reckless behavior that a reasonable person would have avoided. It doesn't require that the defendant appreciate the risk before acting. The problem, of course, is determining what was in a defendant's mind at the time of a reckless act. Courts and juries are inevitably left to rely on what the circumstances reveal about state of mind.
Even when the facts are known, it can be a fool's errand to guess whether a crime constituted reckless murder or involuntary manslaughter. Consider a few noteworthy cases that involved the dichotomy between the crimes.
Michael Jackson's Death. Conrad Murray was Michael Jackson's private, round-the-clock doctor when the King of Pop died in 2009. Prosecutors accused Murray of administering a powerful sedative (propofol) to his patient without customary safeguards. The propofol caused Jackson's death. A jury convicted Murray of involuntary manslaughter for what it found to be grossly negligent medical treatment. Some called for a second degree murder charge, but prosecutors decided against it, perhaps believing Murray wasn't morally blameworthy enough. But some might argue that the doctor disregarded a known risk to Jackson's life.
San Francisco Dog Mauling Case. In 2001, two Presa Canarios killed Dianne Whipple in a San Francisco apartment building. The dogs, in the care of co-owner Marjorie Knoller at the time of the mauling, were tremendously dangerous. The prosecution introduced evidence that Knoller knew how vicious they were. A jury found that her taking the dogs out with insufficient precautions showed a "conscious disregard for human life." The case was so close that the trial judge reduced the conviction to involuntary manslaughter. (The murder conviction was eventually reinstated after appeal.) The jury also convicted the dog's other owner, Knoller's husband, of involuntary manslaughter even though he wasn't present during the attack.
BART Cop Shooting. In the early morning hours of January 1, 2009, Bay Area Rapid Transit police officer Johannes Mehserle shot and killed 22-year-old civilian Oscar Grant. A witness video recorded the Caucasian Mehserle shooting the African American Grant. The footage created a national uproar. A jury ultimately convicted Mehserle of involuntary manslaughter, and the judge sentenced him to the minimum two-year prison term. Some believed that the killing, even if not intentional, was outrageous enough for second degree murder.