Medical professionals and scientists have pointed to every reason under the sun to explain why we bite our fingernails, particularly when we're nervous. There's the idea it's a sign of inward aggression as a form of self-mutilation. There's the theory nail-biting stems from faulty psychosexual development (thanks, Freud).

It's hard to pinpoint exactly what causes some people to bite their nails while others avoid the compulsion. But scientists can agree on one thing: It's definitely a habit, motivated by our reward-seeking brain and its desire to automate tasks that we don't need to think about.

Habits form when three elements are in place: 1) a trigger, 2) an action, and 3) a reward. Together, these three things form what psychologists refer to as a "habit loop."

To get in that loop, at some point in time, you engaged in a behavior that resulted in a certain reward — like biting off a hangnail that was causing you pain, thus eliminating the source of pain. (Some people pair habits with stress relief, although the mechanism behind that is less clear.)

Your brain is essentially lazy, according to the Discovery News video below. Anything that your brain can stop thinking about and automate, it will. This is what happens when a habit forms: The action involved — like nail-biting — shifts to the unconscious part of your brain.

As any nail-biter will tell you, this also makes it a particular hard behavior to stop, even though it can be costly: A habitual nail biter may need $4,000 worth of additional dental care over a lifetime, the Academy of General Dentistry estimated.

Nail-biting is neither new nor uncommon, which accounts for the wide range of theories as to why people chew on their nails. Stoic philosopher Sidonius was the first person to mention nail-biting, when he wrote thousands of years ago in ancient Greece about a man named Cleanthes who apparently bit his nails while studying.

Now, as many as 45 percent of all young adults of both genders engage in nail-biting enough for it to be considered a habit.

You can learn more about current scientific thinking on nail-biting by viewing the full Discovery News episode below: