The exact causes of ADHD are not known with certainty.
Experts do know that ADHD has a strong genetic component. In addition, they think that genes that control the levels of certain chemicals in the brain called neurotransmitters seem to be different in those with ADHD.
In some cases, though, there is no genetic link to ADHD, but other common behaviors, such as smoking or drinking during pregnancy, as well as other obstetrical complications have been linked to ADHD in children.
Babies with low birth weight may have an increased risk of ADHD. The same is true for children who have had head injuries, particularly an injury to the frontal lobe. Young children who are exposed to lead or other environmental toxins such as PCBs or pesticides early in life may also have a higher risk of ADHD.
ADHD always begins in childhood. For some people, though, ADHD is not diagnosed until adulthood. Read More: What Causes ADHD?