Attention deficit hyperactivity disorder (ADHD) is a mental disorder of the neurodevelopmental type. It is characterized by difficulty paying attention, excessive activity, or difficulty controlling behavior which is not appropriate for a person's age. The symptoms appear before a person is twelve years old, are present for more than six months, and cause problems in at least two settings (such as school, home, or recreational activities). In children, problems paying attention may result in poor school performance. Additionally there is an association with other mental disorders and substance misuse. Although it causes impairment, particularly in modern society, many people with ADHD can have sustained attention for tasks they find interesting or rewarding (known as hyperfocus).