What causes ADHD?

Attention-deficit hyperactivity disorder (ADHD) comes in a variety of forms. Common symptoms include difficulty focusing, being easily distracted, daydreaming, fidgeting, restlessness, and impulsivity. The exact cause of ADHD is not clear. First-degree relatives of people with ADHD are at increased risk of developing it, so genetics probably play a large role, but environmental factors may also contribute.