There is no conclusive answer to what causes ADHD. The source(s) is still being researched and studies continue to be done to try and pinpoint what factors contribute to its development. However, current research suggests that ADHD may be a result of any of the following: Genetics: ADHD may have a hereditary component, as there is a 25% chance that a child will develop ADHD if one of their biological parents also has it. Decreased activity levels in parts of the brain: This may cause imbalances that contribute to the signs and symptoms associated with ADHD. Premature birth: This may cause neural, structural, and/or chemical imbalances that increase the probability of developing ADHD. Toxic prenatal exposure: Those who were exposed to harmful substances in the womb (such as alcohol or nicotine) may be at more risk for developing ADHD. Head injurie: Changes in the physical and chemical structure of the brain may lead to the onset of ADHD.