“You need a little bit of a sneer in there,” says Adrienne Wood, an assistant professor of psychology at the University of Virginia who studies social signals like smiles and laughter. Researchers have identified three main smile subtypes, each with its own morphology and social functions: reward smiles, affiliation smiles and dominance smiles. To make any smile requires upturned lip corners, resulting from the activation of the zygomaticus major muscle. Unlike the other two, a dominant smile is asymmetrical. “In other words,” Wood says, “it’s crooked.”