Research of twins has shown that about three in four cases of ADHD are caused by one or both parents having ADHD. Siblings of children with ADHD are three to four times more likely to develop the disorder than siblings of children without the disorder. Genetic factors are also believed to be involved in determining whether ADHD lasts into adulthood. Some scientists think that ADHD is common because when humans were first developing, it was good to be overly active. Others think that, because ADHD is more common in children when the mother is anxious, ADHD developed to help children in dangerous places.