There are three basic types of human smile: “reward”, “affiliative” and “dominance” smiles. That’s according to a new paper by psychologists Magdalena Rychlowska and colleagues.
Here’s the authors’ illustration of the types, as posed by actors:
Reward smiles, the authors say, are used to signal enjoyment:
Reward smiles are displayed to reward the self or other people and to communicate positive experiences or intentions… the reward smile may have evolved from the play face of primates and canids.
Affiliative smiles have a more conciliatory purpose:
Affiliative smiles facilitate social bonding by communicating approachability, acknowledgment, and appeasement and thus may be functionally similar to the silent baredteeth display in chimpanzees that occurs during grooming, sexual solicitation, and submission.
Dominance smiles have a rather darker nature:
Dominance smiles serve to maintain and negotiate social or moral status and are associated with superiority or pride, defiance, derision, and contempt. Unlike reward and affiliative smiles, dominance smiles are assumed to elicit negative feelings in observers. No homologous primate facial expression is known; however, some facial expressions displayed by high-status animal aggressors involve smile components.
This trichotomy of smiles isn’t an entirely new idea, having been proposed by some of the current authors back in 2010. In the new study, Rychlowska et al. studied exactly what makes up these smiles in terms of facial muscle movements.
A group of volunteers were shown 2,400 randomly generated facial expression animations, built using a computer model of the human face. For each random expression, the participants had to say whether it was a reward, affiliative, or dominance smile, or none of the above. All of the expressions were constrained to be somewhat smile-like because they all involved the “Lip Corner Puller” muscle action.
This image shows the muscle movements most characteristic of each smile type:
The reward and affiliative smiles were fairly similar, but only the affiliative smile involved keeping the mouth closed and pressing the lips together. This is hard to see in the virtual faces, but can be seen in the actor photos. The dominance smile was very different from the others. In particular, it was a one-sided smile, with only one lip corned pulled up.
Further experiments showed that people were able to correctly tell apart (virtual) smiles of different types, although reward and affiliative smiles were the hardest to distinguish.
The authors conclude on a poetic note, writing that “our results highlight the versatile nature of the human smile, which can be used for multiple social tasks, including love, sympathy, and war.”
This study has a big limitation, however: all of the participants were white American college students, and all the virtual faces were white too. Does the three-smile model apply to other countries and cultural groups around the world? That remains to be seen.
Rychlowska et al. did publish a paper in 2015 that found three main “reasons for smiling” as reported by 726 people from 9 countries. These three factors matched the “reward”, “affiliative” and “dominance” model. However, I don’t think this establishes that people from every country would recognize all three types of smile, nor can we assume that the facial muscle patterns are the same everywhere.