Studies have documented that smiling is a universal and effective way to lift mood. 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 and highly aggressive teens referred to a youth program, either by educational authorities or the courts. About 70% of the teens already had a criminal record. In the first experiment, 40 healthy adults, aged 1830, looked at facial expressions on the computer ranging from happy to angry with increasingly difficult ones to _ 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 "unclear" or "unable to tell". From these ratings, the scientists could generate a score of their preference toward happiness or anger. Previous research found that aggressive people -- including violent offenders -- tend to consider even a common expression as an unfriendly one. "You looking at me?" can easily turn a simple question into a tragic fight. Based on their original scores, half of the participants were then told by the computer that some of the unclear faces that they had rated as angry should have been scored as happy. This was intended to guide them toward judging the inbetween faces more positively. The other 20 received feedback that simply proved their first 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 unclear 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 consider unclear faces as happier reported feeling less angry afterward. The researchers next focused on the 46 adolescents from the high risk youth program. 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 trained to interpret unclear facial expressions more positively were significantly less aggressive two weeks later. "The results of our experiments strongly suggest that the understanding of emotional facial expressions plays 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' understanding of clear facial signals, not unclear ones. Since unclear signals are more likely to be misunderstood, it may be that violent behavior in some youth exists with their constant misunderstanding of angry expressions where they don't exist, which push them to aggressive responses. The findings suggest that helping young people, particularly those easy to be violent, 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. The method Munafo and his colleagues used in their research is   _  .
field investigation case study observation and analysis experiment and comparison