The exact cause of ADHD is unknown, and it is not fully understood. However, ADHD does tend to run in families. It is thought that the genes you inherit from your parents play a factor in whether or not you have ADHD. People with siblings with ADHD or autism are more likely to have ADHD. It is also suggested that there may be an imbalance in the neurotransmitters in the brain or that the brain area may be smaller in people with ADHD. It is also indicated that there may be a chemical imbalance.