The symptoms of ADHD vary in terms of severity and individual differences. Some people may have predominantly inattentive symptoms, while others may have predominantly hyperactive-impulsive symptoms, or a combination of both. The diagnosis for ADHD typically involves a thorough objective and subjective evaluation by a clinical psychologist, including a review of medical history, symptoms, and behavior. In some cases, additional testing may be needed to rule out other medical conditions that can cause similar symptoms.

Although the exact cause of ADHD is not fully understood, research suggests that it may be related to differences in brain chemistry and functioning. Specifically, some studies suggest that people with ADHD have lower levels of certain neurotransmitters (chemical messengers in the brain) such as dopamine and norepinephrine, which play a role in attention, motivation, and impulse control.