Last week Jessica Alba was a guest on The Tonight Show with Jimmy Fallon. And although she's an accomplished actor, when "Jessica is just being Jessica" - she not quite as skilled at suspending our disbelief. The same is true for virtually all actors.
One particularly good example of this phenomenon is Alba's "smile" - for throughout much of this video segment her smile is insincere.
The lower teeth should not be exposed during a sincere smile (aka Duchenne Smile). There are exceptions to this however - such as when crescendoing to or decrescendoing from sincere laughter (fully expressed sincere laughter typically does display the lower teeth, but NOT during sincere smiles), with a downward directed camera angle, or when the person is tilting their head and neck forward or if the observer is significantly taller. Other than in these scenarios - the bottom teeth should not be exposed with smiles of sincerity.
During this variety of insincere smile (social smile) the corners of the mouth are vectored laterally (toward the sides) rather than being directed primarily upward. This allows the lower teeth to be uncovered.
Another telltale signal of insincerity seen here is Jessica's jaw jut. During sincere smiles as well in the midst of sincere laughter - the jaw is retracted (pulled inward) - NOT jutting outward as is displayed in these examples.
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