https://www.scientificamerican.com/article/smile-you-are-about-to-lose/"In primates, baring teeth is used as a signal of submission or subordination – a means of avoiding potentially harmful and aggressive physical confrontations with others. In humans, another yearbook study showed that higher levels of testosterone are significantly correlated with smile intensity: the higher the testosterone, the fewer the smiles. And in a separate study people tended to smile more when they were lower in social status."
