People may sniff their hands for "chemosignals"

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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.

Not only do people often sniff their own hands, but they do so for a much longer time after shaking someone else’s hand, the study has found. As reported today in the journal eLife, the number of seconds the subjects spent sniffing their own right hand more than doubled after an  experimenter greeted them with a handshake...

Next, to explore the potential role of handshakes in communicating odors, the scientists used covert cameras to film some 280 volunteers before and after they were greeted by an experimenter, who either shook their hand or didn’t. The researchers found that after shaking hands with an experimenter of the same gender, subjects more than doubled the time they later spent sniffing their own right hand (the shaking one). In contrast, after shaking hands with an experimenter of the opposite gender, subjects increased the sniffing of their own left hand (the non-shaking one). “The sense of smell plays a particularly important role in interactions within gender, not only across gender as commonly assumed,” Frumin says.

At least we're more subtle than dogs.  More information here.