When you beam at someone, a lot happens during that interaction without you knowing it.
It turns out that not just our grins, but all of our facial expressions are contagious, according to a paper published in the journal Trends in Cognitive Sciences on Thursday.
We tend to mimic the smiles or frowns of others because it helps us better understand what other people are feeling, allowing us to respond appropriately.
Adrienne Wood, a Ph.D. student in psychology at the University of Wisconsin-Madison and lead author of the paper, told The Huffington Post that "sensorimotor simulation" in our brains is what causes this bizarre mimicry to occur without us even realizing it.
"When you see a facial expression and you want to know what it means, you recreate that expression in your brain," Wood said. "In daily life, you rarely observe facial expressions in a vacuum, and we believe that you combine information from sensorimotor simulation with your understanding of the situation in order to fully comprehend other people's feelings."
For the paper, more than 120 previous studies were reviewed to help describe how exactly we simulate the facial expressions of others in social situations.
Based on their review, the researchers concluded that when we mimic someone else's facial expression, we trigger that same emotional state in ourselves, which then allows us to formulate an appropriate social response.
"Our own lab has shown that making eye contact is one of the easiest ways to elicit facial mimicry," Wood said. "It probably sounds obvious, but if you want someone to really understand how you feel, or you want to understand how someone else feels, look them in the eyes."
Social psychologist Dr. Paula Niedenthal, Wood's adviser and a co-author of the paper, said that the key aspect of this mimicry is that it helps us make appropriate decisions in our interactions with others.
"You reflect on your emotional feelings and then you generate some sort of recognition judgment, and the most important thing that results is that you take the appropriate action -- you approach the person or you avoid the person," she said in a statement. "Your own emotional reaction to the face changes your perception of how you see the face, in such a way that provides you more information about what it means."
Other scientists, however, warn that facial expressions are complex, and there's still much left to learn about how we display and perceive them in different contexts.
"Facial mimicry certainly plays some role, and perhaps a key role, in understanding other people's emotional states," Dr. Kevin Ochsner, director of the social cognitive neuroscience laboratory at Columbia University, told the Christian Science Monitor. But Ochsner, who was not part of the paper, also noted that "it may not be sufficient by itself to carry a full understanding of what other people are feeling in every context."
Niedenthal noted in the statement that a person's ability to recognize emotions can be inhibited when they can't mimic expressions, such as due to facial paralysis or other disorders, like autism. The researchers hope that their new paper and future studies can help lead to interventions.
"Emotion recognition is one of our fundamental social tasks. But it’s not easy for everyone -- people with certain developmental or neurological disorders show reduced social functioning and emotion recognition abilities," Wood told HuffPost. "We are trying to better understand how it is that healthy humans are so good at quickly detecting the subtle and complicated signals they send each other with their facial expressions."
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