Smiling can teach us quite a bit about a person. A person who smiles tends to be in a good mood, easygoing and is fun to be around. People with a genuine smile (called the Duchenne smile) invite us to engage with them, start a conversation or even signal their romantic interest.

One of the hypotheses about the evolutionary origin of stems from studying smile and laughter in other animals, especially primates. Observations by primatologists reveal that many of our closest relatives use a facial expression that is similar to a human smile, usually referred to as the silent bared teeth display. This display is distinct from another display called the relaxed open mouth, which we equate with human laughter.

The silent bared teeth display among primates usually appears in fearful situations as a sign of submission after a fight between two individuals. The losing individual smiles to the winner and accepts his superiority to end the conflict.

Well, if this display is observed in chimpanzees, bonobos, orangutans and other apes, maybe it has a similar function among humans? A new study published in the journal Emotion wanted to test this by looking at whether smiles are signs of submission, and if individuals who do not smile are more likely to win fights. The researchers found a clever way to test this hypothesis by looking at pre-fight facial expressions of UFC fighters.

To test their hypothesis, the researchers coded the smile intensity of 152 UFC fighters one day prior to their fight. The photographs were taken during the traditional face-off between the fighters. Four independent judges, blind to the study hypothesis, coded the smiles for intensity. The researchers also controlled for potential confounding variables, such as the gambling odds of the fight.

The results showed that fighters who displayed a less intense smile won more fights. This was true for all fights, including the ones that were decided by a knockout. Interestingly, smile intensity predicted fight outcomes for the fights on the day after the photograph was taken but not for subsequent fights. This finding suggests that smile intensity does not necessarily reflect an individual trait, but rather is a situational cue that that has short-lived consequences. Think of it as the difference between having a humorous outlook that affects your life as a whole, compared to in response to a specific joke that does not necessarily reflect your humor preferences.

Having a neutral face increases the chances of winning a fight, possibly because the fighter is considered more dominant, while smiling indicates a lack of aggression. A follow up study confirmed these conclusions. In this study, subjects rated photographs of fighters for aggressiveness and hostility. Results showed that fighters who smiled were perceived as being less aggressive and hostile, which led to a judgment that they were less physically dominant. Fighters with neutral facial expressions were considered more dominant.

In all, the results of both studies show that smile intensity reveals the level of aggression and hostility of the fighter, which by itself puts him at a disadvantage, but also may increase the and the intensity level of his opponent during the fight. It is important to note, though, that we do not have direct access to the fighters’ internal state of mind, but rather we deduce it indirectly from his facial expression. It is also pretty clear that the fighters are not aware of such signals, otherwise they would want to regulate their emotions and smile less. It is also context-specific, as it does not predict future fights. Since the current study included only male fighters, it would be interesting to see if the same results hold for women.

The upshot of all of this? If you want to win a fight, don’t smile at your opponent.