Ideally, the two people greeting each other consider not only the level of intimacy they share, but also their milieu. If, to take an extreme example, they find themselves in an area overrun with Ebola virus, they will want to follow others’ example and adopt the elbow bump.

Or, more commonly, they may want to take a cue from Carl Sprague, a film production designer in Stockbridge, Mass. Mr. Sprague views the world’s populations in ascending gradations of intimacy: ‘London. New York. Los Angeles. Paris. Bohemian Paris. Suburban Paris lycée girls.’