Perhaps the most famous theory, the study of body movement, was suggested by Professor Ray Birdwhistell. He believes that physical appearance is often culturally programmed. In other words, we learn our looks ---- we are not born with them. A baby has generally informed face features. A baby, according to Birdwhistell, learns where to set the eyebrows by looking at those around-family and friends. This helps explain why the people of some areas of the United States look so much alike, new Englanders or Southerners have certain common face features that cannot be explained by genetics . The exact shape of the mouth is not set at birth, it is learned after. In fact, the final mouth shape is not formed until well after new teeth are set. For many, this can be well into grown-ups. A husband and wife together for a long time often come to look alike. We learn our looks from those around us. This is perhaps why in a single country area people smile more than those in other areas. In the United States, for example, the South is the part of the country where the people smile most frequently. In New England they smile less, and in the western part of New York State still less. Many southerners find cities such as New York cold and unfriendly, partly because people on Madison Avenue smile less than people on Peachtree Street in Atlanta, Georgia. People in largely populated areas also smile and greet each other in public less than people in small towns do. According to the passage, the final mouth shape is formed   _  .
before birth as soon as one's teeth are newly set sometime after new teeth are set around 15 years old