Attention deficit hyperactivity disorder (ADHD) is a mental disorder of the neurodevelopmental type. It is characterized by difficulty paying attention, excessive activity, and acting without regards to consequences, which are otherwise not appropriate for a person's age. Some individuals with ADHD also display difficulty regulating emotions or problems with executive function. For a diagnosis, the symptoms should appear before a person is twelve years old, be 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 Despite being the most commonly studied and diagnosed mental disorder in children and adolescents, the exact cause is unknown in the majority of cases. Genetic factors are estimated to make up about 75% of the risk. Nicotine exposure during pregnancy may be an environmental risk. It does not appear to be related to the style of parenting or discipline It affects about 5–7% of children when diagnosed via the DSM-IV criteria[2][22] and 1–2% when diagnosed via the ICD-10 criteria.As of 2015, it was estimated to affect about 51.1 million people globally