Babies start to smile at around five weeks old; an ability that can influence many things they’ll do for the rest of their lives. Social psychology research finds that the way we smile seriously affects how we’re perceived by others. Jurors are more likely to believe smiling defendants. Smiling waiters get more tips. And parents are likely to pay more attention to smiling children. Marianne La France is a social psychology professor who studies the effects of subtle communication. Her new book is called “Lip Service: Smiles in Life, Death, Trust, Lies, Work, Memory, Sex and Politics” and she discusses the hidden science behind the smile.