Does nail biting satisfy an oral fixation or a finger sensation?

My guess is its an oral fixation. If this is the case, why do nail biters not like to chew gum? (I only have 2 data points on this!)

Is there something else they could chew/bit/suck on that would satisfy the nervous tension? Is it the texture of the nail and repeated biting/eating that provides the stress relief? Or is it the pain of cutting close to the nail bed? Or is it the removing of an imperfection from the edge of the nail (which ultimately perpetuates a cycle of unhealthy/imperfect nail edges).

My nervous habit of choice has never been nail biting (a habit which has already shown up in my 5-year-old housemate), but for me it is hair texture obsession. I get some kind of trance-like calming/focus sensation from running my nails down a hair shaft searching for imperfections. I’ve done this from at least age 11 I think. When I find an imperfection I break the hair at that point and inspect it.

Looking for parallels, I’m wondering if the hair pulling is stimulating for the scalp, which helps disperse electrical mental energy around the skull. Just as nail biting might direct mental energy toward the front of the skull, perhaps activating the frontal lobes? Or perhaps both nervous habits are simply triggered by a lack of motion/exercise in athletic creatures, a way of activating the hands to deal with excess energy stores.