Mastication is a fancy word for chewing. Most of us chew food. Many of us chew gum. Some of us chew our nails or a pen. Unless you consume a liquid diet, chewing our food is the first stage of digestion and it is pretty necessary in order to achieve maximum nutrient absorption from one's diet. However food is not the only thing we chew, and digestion is not the only benefit that comes from chewing.
There are numerous health benefits associated with chewing gum, not only in the area of dental health (assuming that you’re munching on the sugar-free variety). The motion of mastication itself is known to activate the hippocampus of our brains, the part of our mind that affects mood, cognition, learning ability and memory.
There is a debate surrounding whether or not nail biting presents a health-risk, as obviously consuming the bacteria that accumulates under the nail is a potential risk. On the other hand, some suggest that nail biting from a young age and constant exposure to these bacteria can act to strengthen the immune system. I cannot find any studies that have looked at if nail biters’ chewing also activates the hippocampus, though it would seem as though this is feasible.
In terms of nail biting, there are also a number of theories and on-going debates surrounding the reasons why so many people, 30-60% of children and 10% of adults, indulge in the habit. One of the first given explanations of why we abscent-mindedly chew derives from Freudian psychoanalysis. The basics of it come down to if an infant was breast fed excessively or insufficiently, they may become an orally-fixated individual, with nail biting being one of the symptoms of this psychological state. However, total lack of evidence has effectively debunked this claim
Another predominant assumption surrounding those who bite and chew on that which is not food is that their habits act as a manifestation of a mental health issue, such as anxiety and/or OCD. Though again, these are highly debatable explanations. Despite the American Psychiatric Association categorising nail chewing as an OCD behaviour in 2012, some say that it does not fit the criteria for such a classification, as while those with OCD are motivated by a perceived negative consequence, nail biters appear to be motivated by pleasure and a sense of satisfaction.
The most prevalent nail-biting explanation theory at the moment seems to emphasise that nail biting is really not so big a deal. It is simply a way that we use our hands and mouths, two body parts that mediate huge parts of our sensory experiences, when we are bored, in thought, hungry or distracted. As such, chewing gum is often suggested as a way that can help kick the habit of nail biting, as a way to occupy the mouth in mastication.