People sniff their hands twice as much after a handshake, according to a Weizmann Institute study.

Why do people shake hands? A new Weizmann Institute study suggests one of the reasons for this ancient custom may be to check out each other’s odors. Even if we are not consciously aware of this, handshaking may provide people with a socially acceptable way of communicating via the sense of smell.
eLife 3/3/15 (cc)