Young adult smokers may experience changes in the structures of their brains due to cigarette smoking, even with a relatively short smoking history, a study of adolescents suggests. It also suggests that smoking during this critical time period and the neurobiological changes that result may explain why adults who begin smoking at a young age stay hooked on cigarettes. By measuring cortical thickness of the insula in both groups, the researchers found that the amount of "pack-years" -- the time of cigarette exposure -- was negatively related to the thickness in the right side of the insula. That is, the more someone smoked, the thinner that part of the insula. The relationship also held true for the participants' level of dependence on cigarettes and the urge to smoke.