Our social brain - what are smiles for?

In this past Tuesday's NYTimes science section, Carl Zimmer offers a brief review of some of the many social functions served by our smiling at each other (signaling happiness, social bonding, embarrassment, dominance, etc.). He focuses on the work of Paula Niedenthal. I found this particular bit interesting:

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....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.