The exact cause of attention deficit hyperactivity disorder (ADHD) is not fully understood. It is thought ADHD is caused by a mix of genetic (inherited) and environmental factors.

Genetics

ADHD tends to run in families and, in most cases, it is thought inheriting the condition is the most likely cause. Research shows that both parents and siblings of a child with ADHD are four to five times more likely to have ADHD themselves.

Brain function and anatomy

Research shows that the way the brain works in people with ADHD differs from that of people who do not have the condition. It is thought chemicals in the brain that carry messages, known as neurotransmitters, do not work properly in people with ADHD. Also, people with the condition seem to display less activity in the parts of their brains that control activity and attention.

Some research shows that the frontal lobes, the part of the brain that controls decision-making, do not work as they should in people with ADHD. Other research indicates they may have imbalances in the levels of certain chemicals, such as noradrenaline and dopamine.

Exposure to toxins during pregnancy

Women who drink alcohol when pregnant are more likely to have a child with ADHD. It is also thought that smoking and drug abuse can also increase the risks of ADHD in an unborn child. 

Being male

Boys are more commonly diagnosed with childhood ADHD than girls, and more men are diagnosed with the condition than women. Research suggests this could be because diagnosis tends to pinpoint loud, disruptive behaviour, which is more noticeable and more common in males than in females.

It could also be that ADHD is missed in girls because they tend to have the form of the condition defined by inattentiveness (ADHD mainly inattentive, or attention deficit disorder).

Excessive exposure to television

There have been several studies that have looked at the relationship between children watching a lot of television at a very young age and the development of ADHD in later childhood.

There is not enough evidence to say television is definitely a cause of ADHD, but allowing children up to the age of three to watch several hours a day could contribute to attention problems and ADHD in later life.

Other causes

Other possible causes of ADHD include:

• being born prematurely (before week 37 of the pregnancy)
• having a low birthweight
• brain damage either in the womb or in the first few years of life