In Japan, the custom of covering one's mouth has existed for over a thousand years, dating back to the Heian period. Court ladies of the Heian era would hide their mouths behind folding fans. It is also said that aristocratic men of the time would conceal their mouths with a shaku (a flat ceremonial baton). In Japan, covering the mouth can serve to conceal one's identity. Ninja attire and samurai hoods, for example, also hide the mouth.
In contrast, in the West, it is more common to hide the eyes. At masquerade balls, the mouth is visible while the eyes are covered by a mask. Similarly, American comic book heroes often hide their eyes but leave their mouths uncovered—Batman and Captain America, for instance. This reflects a cultural difference.