Thank you for the opportunity to help you with your question!
One of the subjects you can use to evaluate is whether nail biting is pathological grooming
In pathological grooming,nail biters go may-wire.Instead of being triggered by say hangnail,the pathological biter is triggered by driving,reading or feeling stressed out."After a while the behavior goes triggered"."It just becomes an automatic behavior that has no relationship with external stimuli" says Mathews.
Please let me know if you need any clarification. I'm always happy to answer your questions.