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The conclusion is it's a common behavior. Interestingly, they study found 12% of the survey population picked the nose purely for pleasure. Other interesting facts (from another study on nose-picking) include 65% use the index finger to pick the nose, 20% the pinky, and 16% the thumb.
Although nose-picking may be considered an issue with cosmetic or etiquette consequences, medical problems may arise from rhinotillexomania including epistaxis, septal perforations, cellulitis, etc.
At the very least, cut the fingernails short to minimize potential medical problems from chronic nose-picking.
A Preliminary Survey of Rhinotillexomania in an Adolescent Sample. Journal of Clinical Psychiatry, vol. 62, no. 6, June 2001, pp. 426-31.
Rhinotillexomania: psychiatric disorder or habit? J Clin Psychiatry. 1995 Feb;56(2):56-9.