One of the first scholars to address this question was the German sociologist Norbert Elias in his classic 1939 work, The Civilizing Process. Elias argued that from the 13th to 18th century, Europeans developed the notion that they were more "civilized" than other peoples (including their forebears), largely because they had developed a set of customs that emphasized bodily propriety and impulse management.

These customs ranged from the emergence of table manners to the practice of confining sex to the bedroom (and urination to the bathroom), and tended to be focused on the principles of self-restraint and a concern for reducing a sense of disgust in others. This culture of "civilization" originated among court circles in Italy in the Renaissance, and gradually spread to other countries and classes over the ensuing several centuries.

One of the chapters of Elias' book is titled "On Blowing One's Nose", which provides multiple examples from etiquette guides on the proper method of nose-blowing. During the Middle Ages, people tended to wipe their noses on their sleeves or blow their nose into their hands (when at the table, they would eat with their right hands and blow snot into their left). But beginning in the 13th century, the handkerchief became increasingly common - typically among members of the nobility - as a method of concealing the discharge of mucus.

The humanist scholar Erasmus wrote in De civilitate morum puerilium (1530), a moral guidebook to the education of children:

To blow your nose on your hat or clothing is rustic, and to do so with the arm or elbow befits a tradesman; nor is it much more polite to use the hand, if you immediately smear the snot on your garment. It is proper to wipe the nostrils with a handkerchief, and to do this while turning away, if honorable people are present.

By the 18th century, the use of the handkerchief had become even more widespread, and one French etiquette guide also cautioned against nose picking in 1729:

It is very impolite to keep poking your finger into your nostrils, and still more insupportable to put what you have pulled from your nose into your mouth.

Elias noted that this was deemed to be habit common in children, but which needed to be discouraged. He also noted that the admonitions against changed over the course of the 18th century: originally it was considered to be gauche because of its social impropriety, but by the end of the century, it was also discouraged because it promoted poor hygiene - allegedly nose-picking was more likely to spread disease.

As Elias pointed out, this medical/hygienic rationale was adopted rather late in the game - in point of fact, the admonition against nose-picking essentially derived from a social concern over appearing "civilized", and hygienic concerns were added later to justify a set of manners that now seemed uncontestable - because that's what "civilized" people did.