IT came as a surprise; it usually does. One minute he stood there, in the prime of beard, with a mustache, too. The next minute, not even an upper-lip hair was left standing.

''Men in this country are judged less on their appearance than women are,'' said Ann Weber, a social psychologist and an associate professor of psychology at the University of North Carolina at Asheville. ''That is why men are thought to owe no one an explanation for changing their appearance, while women are thought to owe everyone an explanation.''

Men are particularly silent on the subject of beards. ''For men, shaving off a beard is one of the few self-centered acts possible in terms of looks,'' said Bonnie Jacobson, the director of the New York Institute for Psychological Change in Manhattan.

Dr. Weber explained: ''Since a man's appearance has little to do with his social value, whether to wear a beard or not is a matter of how he feels about the face staring back at him in the mirror.''