Not all smiles are made equal. In fact, scientists have discovered three types of smiles: rewarding smiles, which indicate encouragement; affiliation smiles, which indicate relatability; and dominant smiles, which indicate superiority. The average person can tell them apart easily, judging by the shape and size of the smile and the shape of the eyes.
Curious to see how people would subconsciously react to the three different smiles, researchers had 90 men deliver a short speech to a video camera. Immediately after, they watched a video of a male judge “react” to their speech, supposedly recorded while the speech was delivered. (In fact, the videos were prerecorded.) The judges flashed one of the three kinds of smiles. Levels of the stress hormone cortisol increased in all three cases, but surged about three times higher in participants who saw the dominant smiles, the team reports this month in Scientific Reports. That means the human brain is wired to react to facial expressions even without vocal cues—a dominant smile suggests a potential threat, so stress level increases.
But participants with smaller variations in their regular heart rates—associated with social anxiety disorder and depression, among other conditions—responded less dramatically to dominant smiles. That may mean, the scientists write, that people with these conditions respond less strongly to social signals like smiles—but far more work is needed before such a link could be shown.