Much has been made of Donald Trump's sniffing during the Presidential Debate with Hillary Clinton. Of course a person can sniff due to allergies or an upper respiratory tract infection - and certainly there are other medical causes for this behavior.
If we can reasonably rule out medical reasons for sniffing (and Mr. Trump has gone on record denying these causes since the debate), from a nonverbal communication perspective, when a person sniffs excessively the most common cause is:
In addition, when the sniffing is clustered with other specific nonverbal signals, it can also be associated with:
2. Deception/Exaggeration (deliberate exaggeration in the context of a debate is a deception)
It's important to point out that anxiety, deception and contempt - all have other nonverbal manifestations - and of course a person can have these emotions without sniffing.
During the first Presidential Debate, Donald Trump was certainly exhibiting other signals of anxiety (hard swallowing, excessive drinking of water, excessive holding on to lectern, fear expressions, tight tongue jutting, etc.) but some of his sniffing displays were also associated with deception (and correlated with independent fact checkers) as well as contempt.
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