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The exact cause of attention deficit hyperactivity disorder (ADHD) is not fully understood, although a combination of factors is thought to be responsible.

ADHD tends to run in families and, in most cases, it's thought the genes you inherit from your parents are a significant factor in developing the condition.

Research shows that parents and siblings of someone with ADHD are more likely to have ADHD themselves.

However, the way ADHD is inherited is likely to be complex and is not thought to be related to a single genetic fault.

Research has identified a number of possible differences in the brains of people with ADHD from those without the condition, although the exact significance of these is not clear.

For example, studies involving brain scans have suggested that certain areas of the brain may be smaller in people with ADHD, whereas other areas may be larger.

Other studies have suggested that people with ADHD may have an imbalance in the level of neurotransmitters in the brain, or that these chemicals may not work properly.

Certain people are also believed to be more at risk of ADHD, including people:

• who were born prematurely (before the 37th week of pregnancy) or with a low birthweight
• with epilepsy
• with brain damage – which happened either in the womb or after a severe head injury later in life