David Adam is a writer and editor at the journal Nature and was a special correspondent at the Guardian, writing about science, medicine and the environment. Courtesy of Farrar, Straus and Giroux, LLC hide caption

itoggle caption Courtesy of Farrar, Straus and Giroux, LLC

Pathological nail biting may be a form of grooming on steroids, but it also makes the biter feel good, unlike fear-driven OCD. Andrea Kissack for KQED hide caption

itoggle caption Andrea Kissack for KQED