I don’t know how the jurors would be instructed. Perhaps they would be told that they must consider the expert’s opinion, or maybe they would be told that they could ignore the expert’s opinion; or, that the field is questionable and so the handwriting should be ignored. So many ways it could play out.
It wouldn’t surprise me at all if a jury identified Mrs Ramsey as the author, if left to their own devices. But, what would that mean? That a group of people made a decision on something that they weren’t qualified to make a decision on? I don’t think that this sort of thing should ever be allowed.
If jurors are permitted to make these kinds of decisions on their own, then I hope some method of ensuring impartiality would be devised. Show them samples from 10 different people and let them choose which one (if any) they think was written by Mrs Ramsey (or, any known sample). Let them use the same 10 samples and have them match one (or, none) to the ransom note.
If it was up to me to decide, I would probably tell jurors to ignore the handwriting. I would tell prosecutors/defence to make their case for/against authorship through some other means. IMO, this is how most jurors would be thinking, anyway.
Anti-K, you and I probably agree on more than you think. Especially that last paragraph.