Every night before bedtime, my daughter tilts back her head so that a pair of metal plates inside her mouth can be cranked apart another quarter of a millimeter. We turn a jackscrew with a wire tip; it spreads the bones within her upper jaw. At times she groans or even cries: she says that she can feel the pressure up into her nose. This is normal. My daughter is 9 years old. She has a palate expander. So does her best friend, and, by her count, so does nearly one in four of the kids in her fourth-grade class. On Reddit’s r/braces forum, a practitioner based in Frisco, Texas, said he was surprised by “how many parents ask me, ‘Hey, does my child need an expander? Everyone else seems to have one.’” His colleagues seemed to notice something similar. “Everybody’s being told they have a narrow jaw, and everyone’s being given an expander,” Neal Kravitz, the editor in chief of the Journal of Clinical Orthodontics, told me. A generation ago, getting braces was a …