Show someone a piece of rotting food and their reaction will be visibly similar the world over. Their eyes will close, their noses will wrinkle and their mouths will tighten, all part of a universal expression of disgust. Darwin himself was struck by the universal nature of human facial expressions - from the busiest of cities to the most isolated of villages, smiles and frowns are recognisable, done in the same way and carry the same meaning.
Facial expressions are massively important for the social lives of humans and it should come as no surprise that some parts of our brain are dedicated to interpreting the small shifts in facial muscles that betray our emotions. In this light, it makes sense that expressions should be universal, but that doesn't tell us why they take the form they do. Why is it a smile that indicates happiness and not anger, or ...
