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?

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Real-Time Visualization of Joint Cavitation (PLoS One)
"Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. Unfortunately, bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon. Here we present direct evidence from real-time magnetic resonance imaging that the mechanism of joint cracking is related to cavity formation rather than bubble collapse."

Nail Biting: Mental Disorder Or Just A Bad Habit? (NPR)
"Do you bite your nails? For 30 years, I did. We nail biters can be "pathological groomers" - people for whom normal grooming behaviors, like skin picking or hair pulling, have become virtually uncontrollable. But psychiatry is changing the way it thinks about pathological grooming, and these changes will be reflected in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders."

Onychophagia (Nail biting), anxiety, and malocclusion (Indian Journal of Dental Research)
"Nail biting is a stress removing habit adopted by many children and adults. People usually do it when they are nervous, stressed, hungry, or bored. All of these situations are having a common phenomenon between them is anxiety. Onychophagia is also a sign of other emotional or mental disorders."