Right now, all but either one or two states provide that, once the defense in a criminal case introduces any evidence of possible self-defense, the prosecution must disprove self-defense beyond a reasonable doubt. In this respect the absence of self-defense is treated similarly to any element of the crime. Just as the prosecution must prove the defendant is the one who attacked the victim, and the defendant had the requisite mental state — e.g., that this wasn’t an unfortunate accident — so the prosecution must prove the absence of self-defense (though there is one difference, which is that the defendant must introduce some evidence of possible self-defense for a self-defense instruction to be given).
This wasn’t always the rule. The English common law rule at the time of the Framing was that the defense must prove self-defense by a preponderance of the evidence, and Ohio still follows that rule. (Some but not all Louisiana courts also follow this rule, but only in non-homicide cases; Louisiana courts tracks the majority rule in homicide cases — read State v. Glover, 106 So. 3d 129, 137-38 (La. Ct. App. 2012) for more details.) The Supreme Court has held, in Martin v. Ohio (1987), that placing this burden on the accused is constitutional.
One way of thinking about the two alternatives is that the nearly unanimous rule takes the view that, “Better that 10 guilty killers go free than one person who killed in proper self-defense go to prison for a long time (or be executed).” The Ohio rule, on the other hand, takes the view that, “It’s slightly worse for one guilty killer to go free than for one person who killed in proper self-defense to go to prison for a long time (or be executed).” This is a very important difference between states, I suspect at least as important as the stand your ground / duty to retreat difference and probably more important, since the who-proves-and-by-how-much question potentially arises in any self-defense case, regardless of where the act takes place (in the home or outside) and of whether a safe avenue of retreat is available.
And it turns out that, last year, the Pennsylvania Supreme Court seemed to signal that it might move back to requiring the defendant to prove self-defense by a preponderance of the evidence. In Commonwealth v. Mouzon (Pa. 2012), Chief Justice Castille’s majority opinion suggested that the 1975 Commonwealth v. Cropper decision, which shifted Pennsylvania to the “prosecution must disprove self-defense beyond a reasonable doubt” rule, was mistaken.
The court noted that, “The overall principle that emerges from the High Court’s decisional law is that federal due process permits States to place a burden on the defendant to prove an affirmative defense by a preponderance of the evidence, so long as the defendant is not thereby required to negate an element of the offense” (that’s right, given Martin v. Ohio). The court declined to reconsider Cropper in this case, stating, “appeal was not allowed in this case to reconsider or adjust the rule arising from Cropper; we decide the appeal in conformity with existing law concerning self-defense, which provides that, where self-defense is properly joined, the Commonwealth has the burden to disprove that defense beyond a reasonable doubt.” But in footnote 10, either the Court or just the Chief Justice (depending on how one reads the “This author would note” sentence) writes,
Appellee’s failure to offer any evidence to support the subjective aspect of his claim of self-defense highlights the difficulties associated with assigning the Commonwealth the burden to disprove a defense where necessary facts are peculiarly within the knowledge and control of the defense. In any event, the fact that the burden has been assigned to the Commonwealth — erroneously, as addressed in Part I supra — not only to affirmatively prove the elements of the offenses charged, but also to disprove self-defense where it is at issue, does not remove the necessity that there be some actual evidence to support the elements of the defense when proffered.
This author would note that this case illustrates the wisdom of the common law rule placing the burden upon the defendant to prove self-defense. Although the defense ultimately is subject to objective evaluation, the core is the defendant’s “reasonable belief.” That is a matter known peculiarly to the defendant, and there is no logical reason such an actor-sensitive defense should be permitted to arise from counsel’s speculative inferences from the testimony of others.
The two concurring Justices argued that the majority shouldn’t have opined on the subject, given that it was outside the scope of the questions presented to the court, suggested that the majority was mistaken on the merits of the matter, and specifically said that all or part of footnote 10 was just “the Chief Justice’s personal views offered on the issue.”
Since then, prosecutors have naturally taken up the court’s invitation, and have repeatedly argued that (for instance), “[w]ith the Supreme Court’s negative treatment of the Cropper decision, it is clear that the Commonwealth has no burden of proof for self defense cases.” I expect that it will be only a matter of time before the issue comes up again before the Pennsylvania Supreme Court — at least unless the legislature is prompted to codify the still-current “prosecution must disprove self-defense beyond a reasonable doubt” rule, which it is of course free to do (or to enact a “defense must prove self-defense by a preponderance of the evidence,” or some other variation of the burden and quantum of proof, which it is also free to do).
Interestingly, the Arizona Legislature apparently intervened on such a question twice: In 1997, when it switched from the near unanimous rule to the Ohio rule, and then in 2006, when it switched back.