An astonishing new study on attention deficit hyperactivity disorder shows that the brains of sufferers are slightly smaller than those who don’t have the condition. The study, published in the journal The Lancet Psychiatry, is the biggest review of ADHD sufferers and their brains so far, and it indicates that the disorder is caused by “structural difference” in the brain.

Scientists behind the study say it shows that ADHD isn’t just a label for difficult children or bad parenting. ADHD is a developmental disorder that begins in childhood and can last through a person’s life, with symptoms including a short attention span, bad personal organization and fidgeting.

The study focused on 1,713 people who had ADHD, and 1,529 who did not. The patients were between 4 and 63 years old. Researchers examined brain volume and the size of specific brain regions.