Airymouse - on a complete side-note, 'sere' in that line from Macbeth just means 'dry' or 'dryness'. You're thinking of a weird & possibly corrupt line in Hamlet, "the clown shall make those laugh whose lungs are tickled o' the sere". The 'hair trigger' explanation is ingenious, but that meaning of the word 'sere' isn't in the OED. If Shakespeare wrote 'sere', I suspect he meant 'dryness' again - i.e. the clown will make everyone laugh, even people who don't want to laugh because they've got a dry cough.

Fred:

The ballad is a social construct, like folksong itself in fact. That doesn't deny its usefulness from a scholarly point of view, and it doesn't stop the hairs from lifting off the back of my head every tme I hear one.

Good way of putting it. Child was working by feel & rule of thumb, but using that method he managed to identify something we can still identify now.