Why do people bite their nails?

Excessive nail biting is a surprisingly widespread human activity.It goes back millennia: the ancient Greek philosopher Cleanthes, for instance, was said to be addicted to biting his nails. In the modern era, no one has any good data on how many of us share the affliction (technically called onychophagia), but small-scale studies indicate about 20 percent or so of adults bite regularly — which would suggest millions of Americans do it.