Nearly 1 million kids may have been wrongly diagnosed with attention deficit hyperactivity disorder, a new study says, because they were born later in the school year, not because they have behavior problems. The youngest children in class are 60 percent more likely to be diagnosed with ADHD than their older classmates, researchers at Michigan State University found. (A separate study at North Carolina State University got similar results.) The youngest are more than two times as likely to be taking Ritalin. About 4.5 million kids have been diagnosed with ADHD, and Ritalin has many side effects, including headaches, dizziness, and high blood pressure. With kindergartens using birthdays—often September 1—as a cutoff for entering school, there will always be children nearly a year apart in age in each class. The study’s author says that pediatricians should diagnose kids based on their age, not their grade in school.