Research Digest, Blogging on brain and Behavior reports that apparently Smiling fighters are more likely to lose. They write that
"Consistent with the researchers' predictions, fighters who smiled more intensely prior to a fight were more likely to lose, to be knocked down in the clash, to be hit more times, and to be wrestled to the ground by their opponent (statistically speaking, the effect sizes here were small to medium). On the other hand, fighters with neutral facial expressions pre-match were more likely to excel and dominate in the fight the next day, including being more likely to win by knock-out or submission."
"These associations between facial expression and fighting performance held even after controlling for betting behaviour by fans, which suggests a fighter's smile reveals information about their lack of aggression beyond what is known by experts. Moreover, the psychological meaning of a pre-match smile appeared to be specific to that fight - no associations were found between pre-match smiles and performance in later, unrelated fights. Incidentally, smaller fighters smiled more often, consistent with the study's main thesis, but smiling was still linked with poorer fight performance after factoring out the role of size (in other words, smiling was more than just an indicator of physical inferiority)."
I found that link on Marginalrevolution