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A happy face can certainly lift spirits, but can it reduce rage?

Studies have documented that the physical act of smiling is a universal, and effective way to lift mood, if briefly. But in the latest research on the power of the smile, researchers led by Marcus Munafo of the University of Bristol in England found that even seeing smiles on the faces of others can have a profound effect on a person’s tendency toward violence or aggression— that is, as long as that person recognizes the smile as one of happiness, and not as a sneer.

Munafo and his colleagues conducted a series of experiments involving normal adults as well as highly aggressive teens who had been referred to a youth program, either by educational authorities or the courts. About 70% of the teens already had a criminal record.

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In the first experiment, 40 healthy adults, aged 18-30, looked at computer images of faces that had been morphed to show facial expressions that ranged from happy to angry with increasingly difficult to discern expressions in between. Participants were asked how angry they felt and then had to rate the images as displaying either happiness or anger — there was no option for “ambiguous” or “unable to tell.” From these ratings, the scientists were able to generate a score of their biases toward happiness or anger as reflected by where the volunteers decided that happiness ended and anger began.

Previous research found that aggressive people — including violent offenders — tend to interpret even neutral expressions as hostile: “You looking at me?” can easily turn what would have been a nonevent into a tragic confrontation, so preventing such misinterpretations could have important implications.

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Based on their initial scores, half of the healthy participants were then told by the computer that some of the ambiguous faces that they had rated as angry should have been scored as happy. This was intended to bias them toward judging the in-between faces more positively. The other 20 received feedback that simply validated their prior choices, creating a control group.

After this training, both groups were tested again and the group that received the biased feedback shifted its ratings of ambiguous faces toward the happy side. Participants were also asked to rate their level of angry feelings again after completing the second round of testing. Those who were trained to interpret ambiguous faces as happier actually reported feeling less angry afterward compared to the controls.

The researchers next focused on the 46 adolescents from the high risk youth program, ranging in age from 11 to 16. These teens completed the same testing, but both the youth and the staff reported on the teens’ levels of aggressive behavior before the testing started and for two weeks afterward. The teens who had been trained to interpret ambiguous facial expressions more positively were significantly less aggressive two weeks later, as rated by both the staff (who did not know which kids were in the intervention group) and by themselves.

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“The results of our experiments strongly suggest that biases in the perception of emotional facial expressions play a causal role in subjective anger and aggressive behavior,” the authors conclude.

That doesn’t mean that smiles alone are the answer to violence among adolescents — previous research in which antisocial youth were trained to better recognize emotions, for example, did not have any effect on their level of aggressive behavior. But this earlier study focused on improving teens’ perception of clear facial signals, not ambiguous ones. Since ambiguous signals are more prone to misinterpretation, it may be that violent behavior in some youth is perpetuated by their constant misintepretation of angry expressions where they don’t exist, that push them to combative responses. The findings suggest that helping young people, particularly those who are prone to violence, to learn to give others the benefit of the doubt when they see what they think is a threatening face could help end the vicious cycle of violence.