John P..."necessary criteria for something being considered a folk song. "....it's not that I consider acoustic a necessary criterion, but it is one of many ways in which folk or traditional has been identified.
My approach is a bit different, since I KNOW we can never all agree about a strict linear 'definition' of a genré.

Let me try again, as briefly as I can. If we take bunches & bunches (hundreds?..thousands?) of songs (and varied versions & performances of those songs) and ask bunches & bunches of people to rate them as folk or not folk, we will eventually get a list of songs that MOST people consider to BE folk. Perhaps "The Fox" will be on on it...perhaps "Barbry Allen"....perhaps "Mr Tamborine Man" will not be..*shrug*..(yet)

Then, we look at those songs & versions & performances and ask ourselves, "Now, what is common among those songs that made almost everyone agree about them?"

We will get some sort of list of characteristics that seem to commonly appear..(tune patterns, age, acoustic presentation, style, subject matter,..etc.) The list is not an absolute, as the analysis is partly subjective, but you can GET a list of 8 to 30 pretty easily.

Now....you look at a song, you look at your list, and you ask yourself..."does this songs have a majority of those, just a few, or hardly any?" The point is, if it has most of the items on your list, it tends to qualify...(the singer used his own tune variation for "Barbry Allen", but left everything else..or he did it straight, but played it on a electric dulcimer)

I do NOT presume that anyone will keep some long printout in their pocket so that they can run thru some tedious checklist every time they hear a song..*grin*...it is 92.3147% an attitude.
This system will get you some songs that fall right in the middle with no clear consensus....but....it will sure allow you to EXCLUDE some, and it will help you to be aware of what is happening to various music and formats.

After awhile, it isn't even a concious thing....you just sort of do a processing thing like you'd do with some variation in a favorite food..."like Mom used to make" or "wierd spicy version that I don't think I like".

All this assumes you CARE...I do care, because I like some spices and not others....but, yes, I have heard 'folk' songs played on non-acoustic instruments that still retained most of the feeling of 'folk', and that I enjoyed....not as MUCH as if they were played acoustically, perhaps, but *shrug*. And a song could be played on a Martin D-18, by a famous 'folk' singer, with words and tune right out of Child...but at a frenzied pace that left me gasping...I might CALL it folk, and still not like it!

so...not so brief, huh? Well, anyway, everytime I do this, the exercise helps me refine my own attitude about it all, so it ain't wasted, even if very few read it...I can go back and use it as notes to my 27 volume analysis of **FOLK MUSIC** (*grin*)