Children who are the youngest in their grade are more frequently diagnosed with attention-deficit/hyperactivity disorder (ADHD), a new study finds, raising concerns that some kids might be wrongly included in this category – and others might miss out on treatments that could improve their lives. That’s the conclusion of a large study of Finnish children published this month in the peer-reviewed medical journal Lancet Psychiatry. This same “age effect” on ADHD diagnosis has previously been found in other research studies, including those from Canada and the U.S. ADHD is a neurological disorder that appears to affect about 5 percent of children across the world, though diagnosis and treatment rates vary widely by country. The disorder is identified when a child’s difficulty with inattentiveness and impulsivity are developmentally out of the norm as compared to others of the same age. The symptoms also must cause problems for the child across multiple settings, such as at home, at school and at Scout meetings. The month you are born shouldn’t affect your chances of an ADHD diagnosis, of course. But because there is no objective test for ADHD, healthcare providers who diagnose it frequently rely on subjective descriptions of a child’s behavior from parents and teachers. Understandably, children’s inattention and impulsivity are measured against that of others in their class. However, children in the same grade can vary in age by as much as one year, depending on their birthday. This can make the younger children in the classroom, typically those born in the spring and summer, appear immature (squirrely, reactive) relative to children born in the fall and early winter. This wild variation in development will be more pronounced in the earliest elementary school years. This is exactly what was found in the Lancet Psychiatry study, where the birth month effect on ADHD diagnosis was greater for children aged 7 to 9 than it was for children aged 10 and over. The concern here is not just for overdiagnosis but also of missed diagnosis — and therefore treatment and help for younger children who truly do meet criteria for ADHD, but whose problems might be dismissed