Why do we bite our nails and crack our fingers?

It's all to do with "habit loops"...

Why Do We Bite Nails & Crack Fingers When We're Nervous?

Many of us have bad habits like biting our nails or cracking our knuckles. But why exactly do we do it?

People usually bite their nails when they feel anxious. Because feelings of anxiety are usually associated with feelings of loss of control, they bite their nails when they feel they want to be more in control.

Behind most bad habits, there is often a strong psychological drive: lots of bad habits have the goal of bringing the sense of control back to the person who is doing them.

In addition to this, controlled movements that are predictable help us reduce the anxiety we feel when we experience a vague situation. This is one of the reasons some people bite their nails when they're nervous.

There's no doubt that nail biting is a bad habit.
This and other unconscious habits are formed in a part of the brain called the basal ganglia.

A habit needs a trigger, an action and a reward, creating a 'habit loop," say experts.