A recent study published in the "Emotion" scientific journal shows that smiling fighters are more likely to lose.
The smile is perhaps the most widely studied facial expression of emotion, and it is definitely a sign of physical dominance.
"We reason, on the basis of prior research, that prior to a physical confrontation, smiles are a nonverbal sign of reduced hostility and aggression, and thereby unintentionally communicate reduced physical dominance," the study concluded. "Two studies provide evidence in support of this prediction: Study 1 found that professional fighters who smiled more in a pre-fight photograph taken facing their opponent performed more poorly during the fight in relation to their less intensely smiling counterparts."
In the second study, observers looked at a variety of fighters, someone smiling and others who weren't. They found that the smiling fighters who less physically dominant because of their facial expressions.
"In Study 2, untrained observers judged a fighter as less hostile and aggressive, and thereby less physically dominant when the fighters' facial expression was manipulated to show a smiling expression in relation to the same fighter displaying a neutral expression," the study found.
Some excellent examples in 2013 include John Dodson smiling at Demetrious Johnson and Donald Cerrone smirking at Anthony Pettis.
Dodson and Cerrone both lost in lopsided fashion.