Here's a bit from an article about current research on telling whether a smile is real or fake that uses a similar approach of "body feedback". I find this fascinating. This is from an article by Carl Zimmer in the NY Times on recent research:
Dr. Niedenthal herself is now testing the predictions of the model with her colleagues. In one study, she and her colleagues are testing the idea that mimicry lets people recognize authentic smiles. They showed pictures of smiling people to a group of students. Some of the smiles were genuine and others were fake. The students could readily tell the difference between them.I get a chuckle out of people who believe that we are purely rational calculating beings or "pure souls". We aren't. We are animals with all the complicated physiological interconnections with our brain and a complicated social life that make us an emotional beings. Sure we are rational calculators. But we are generally poor at rational calculation. The classic demonstration of this is the Wason Selection Task. Go read about it and try it out on yourself. You will discover that you aren't as "rational" as you thought you were. But you will also discover that you are very, very good at policing social rules! But to your shock and horror, you will discover that the two tasks are fundamentally the same, but when presented as a social rule you can ace it. On the other hand, when presented as a logical task, you will generally flub it. Amazing. We are fallible creatures!
Then Dr. Niedenthal and her colleagues asked the students to place a pencil between their lips. This simple action engaged muscles that could otherwise produce a smile. Unable to mimic the faces they saw, the students had a much harder time telling which smiles were real and which were fake.
The scientists then ran a variation on the experiment on another group of students. They showed the same faces to the second group, but had them imagine the smiling faces belonged to salesclerks in a shoe store. In some cases the salesclerks had just sold the students a pair of shoes — in which they might well have a genuine smile of satisfaction. In other trials, they imagined that the salesclerks were trying to sell them a pair of shoes — in which case they might be trying to woo the customer with a fake smile.
In reality, the scientists use a combination of real and fake smiles for both groups of salesclerks. When the students were free to mimic the smiles, their judgments were not affected by what the salesclerk was doing.
But if the students put a pencil in their mouth, they could no longer rely on their mimicry. Instead, they tended to believe that the salesclerks who were trying to sell them shoes were faking their smiles — even when their smiles were genuine. Likewise, they tended to say that the salesclerks who had finished the sale were smiling for real, even when they weren’t. In other words, they were forced to rely on the circumstances of the smile, rather than the smile itself.
Dr. Niedenthal and her colleagues have also been testing the importance of eye contact for smiles. They had students look at a series of portraits, like the “Laughing Cavalier” by the 17th-century artist Frans Hals. In some portraits the subject looked away from the viewer, while in others, the gaze was eye to eye. In some trials, the students looked at the paintings with bars masking the eyes.
The participants rated how emotional the impact of the painting was. Dr. Niedenthal and her colleagues found, as they had predicted, that people felt a bigger emotional impact when the eyes were unmasked than when they were masked. The smile was identical in each painting, but it was not enough on its own. What’s more, the differences were greater when the portrait face was making direct eye contact with the viewer.
Dr. Niedenthal suspects that she and other psychologists are just starting to learn secrets about smiles that artists figured out centuries ago. It may even be possible someday to understand why Mona Lisa’s smile is so powerful. “I would say the reason it was so successful is because you achieve eye contact with her,” said Dr. Niedenthal, “and so the fact that the meaning of her smile is complicated is doubly communicated, because your own simulation of it is mysterious and difficult.”