Happiness is considered the most universal facial expression with virtually the same meaning across all cultures. Indicated by a smile and crescent-shaped eye, this face is even innate in infants. Scientists and evolution experts have formulated many specific origins to the expression of happiness with particular regard to the smile. Unlike most of our other expressions, our smile does not have the same meaning as the “mouth smile” of other great apes, who use that expression for intimidation.
Many speculate that our smile stemmed from this in some way, and that by displaying our teeth we are indicating our health. While it may not carry with it an air of intimidation, it still indicates to others that you are well. In our complex social situations, the smile is typically inviting (perhaps because it shows some level of strength or self-sufficiency, either physically or emotionally), and people gain more acceptance when they display it genuinely.