Possible shift in PA against self-defense
Prof. Volokh has a post on the subject. English common law treated self-defense as a true affirmative defense: the burden is on the defender to prove it, and by a preponderence of the evidence (i.e., do more than raise a reasonable doubt). American law almost unanimously treated it differently: the defender must raise some proof of self-defense, and at that point the prosecution has the burden of disproving it beyond a reasonable doubt. Now it appears that the PA Supreme Court may shift to the English rule.