While the majority of ADHD is believed to be genetic in nature, around one-fifth of all ADHD cases were considered to be acquired after conception due to brain injury caused by either toxins or physical trauma prenatally or postnatally. Some children who have suffered accidents leading to brain injury may show some signs of behavior similar to that of ADHD, but only a small percentage of children with ADHD have been found to have suffered a traumatic brain injury.