ADHD is not really a single definable "illness," but rather a collection of common behaviors that we see in children and adults whose ability to concentrate is somehow blocked. There is almost certainly no one single cause. Instead, the set of behaviors is the result of various conditions, some physical and some psychological. Whatever the cause, the outcome is a child or adult who is easily distracted and not able to focus completely on the task at hand. Some patients have a clearly defined and even measurable (with the right equipment) problem in the right prefrontal cortex region of the brain. Others can't focus because of anxiety. Still others—probably the majority in my practice—are bright and creative children whose inner theater is so much richer and more interesting than what they are offered in the classroom that they turn off the outer world when they are in school. When these kids wake up to find that they have missed a lot that their peers have caught, they start to think of themselves as stupid, and the corrosive effect of these thoughts (and the need to conceal that secret) further disables their ability to focus.