Portraits in Banknote Design

Why do so many banknote designs include portraits?

I don't think there is a definitive answer to this question, but I think I can posit some reasons. (The old 10 Deutsche Mark note, featuring Carl Friedrich Gauss, is shown.)

Tradition. Since antiquity, the portraits of rulers have appeared on money. This tradition transitioned from coins to banknotes. And, the type of subject portrayed expanded from rulers to leading people in a nation's history.

Identity. Using the portrait of a historical personality helps to identify the nation-state issuing the banknote and to portray/celebrate that nation in a certain light.

Design. A portrait creates an instant focal point in a banknote design. People viewing the note immediately focus on the eyes of the subject being portrayed. The individual portrayed also supports the narrative of the note.

Counterfeit Deterrence. A human portrait is very difficult to engrave and thus copy for counterfeiting purposes. Also, the human ability to recognize the subtleties of the human face, makes it easier for a viewer to recognize an error in a portrait. This makes things more difficult for the counterfeiter. The counterfeited portrait will be quickly recognizable as such if it is not perfect.

Look for portraits on your own currency, and how they work on the banknote.