Mary-Kate and Ashley taught us a thing or two about Boho chic, but there are more valuable lessons to be learned from twins. Using identical twins, doctors have long conducted scientific research to show just how dramatically sun exposure and smoking affects the aging process. Now, a recent comparison demonstrates the effects of Botox on preventing-not just relaxing-forehead lines. One twin had Botox injections in her forehead for 13 years, starting at age 25. The result at 38 was almost no deep wrinkles-just a few lines. Her twin, who’d had only two treatments in that time, had deeper and permanently etched wrinkles in the forehead, says William J. Binder, assistant clinical professor at UCLA Medical Center and the study’s author. Binder hopes this arresting finding will resonate with patients. “I’d never tell a woman she must get Botox, but it’s good that we now have examples of what happens with long-term usage to help her make her decision.”