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

Genetics

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

Research shows that both parents and siblings of a child with ADHD are four to five times more likely to have ADHD themselves.

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

Brain function and structure

Research has identified a number of possible differences in the brains of people with ADHD compared to those who don’t have the condition, although the exact significance of these is not clear.

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

Research has also shown that the brain may take an average of two to three years longer to mature in children with ADHD, compared to children who don’t have the condition.

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

Other possible causes

Various other causes have also been suggested as having a role in the development of ADHD, including:

• being born prematurely (before the 37th week of pregnancy)
• having a low birthweight
• brain damage either in the womb or in the first few years of life
• drinking alcohol, smoking or misusing drugs while pregnant
• exposure to high levels of toxic lead at a young age

However, the evidence for many of these factors is inconclusive, and more research is needed to determine if they do in fact contribute to ADHD.