this entire thing is academic anyway since we both seem to admit that jury nullification can't be prevented.
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Nullification could be prevented by allowing judicial review of jury verdicts. It could also be specifically banned by law. Of course, that would fly in the face of the expressed intent of the Founding Fathers.
And the standard for rendering a JNOV is high enough that it pretty much never happens, and when it does, it happens in civil suits. Basically, if the jury came to a decision, based on the evidence, that a reasonable person could come to in the capacity of a trier of fact, their verdict will not be disturbed. When you factor in the reasonable doubt standard that the prosecution already has to overcome, it becomes clear that a reasonable jury could find someone not guilty in pretty much any criminal case.
Even in civil suits, where the standard of proof is a mere preponderance, JNOV is used very sparingly, and is usually accompanied by a new trial, not a simple reversal. The odds of a JNOV being granted in that second trial, if a second jury comes to the same conclusion, is so small as to be negligible.