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Scientific research has found there is a strong genetic link in ADHD. It is not a disorder that is learnt or passed on socially.

Studies have looked at identical and non-identical twins, adopted children and their non-adopted brothers and sisters, and family members of people with ADHD.

In identical twins, there is a 72 to 83 per cent probability that both will have ADHD, but in non-identical same-sex twins the probability is 21 to 45 per cent.

Research also shows parents, siblings and children of people with ADHD may be up to five times more likely to have the disorder than people who aren't related to someone with ADHD. More than half of all parents with ADHD will have a child with the disorder.

At one time, people thought there could be just one gene that makes someone develop ADHD. Now experts think the disorder is probably caused by several genes - each of which makes a small contribution.

The genes that experts have identified as potentially contributing to the development of ADHD are genes that control certain types of neurotransmitter.

The defects in one or more of these genes may contribute to a greater risk of developing ADHD symptoms, but they might not be severe enough to need treatment.

ADHD isn't entirely caused by genetic factors. Scientific studies have shown that people with ADHD can have abnormalities in some parts of the brain, including the prefrontal cortex. The prefrontal cortex is the area of the brain that's believed to control 'executive functions'.

Executive functions are specific mental activities that allow self-control. The core symptoms of ADHD - hyperactivity, impulsiveness and inattention - may all arise due to problems with executive functions.

Studies have also shown that people with ADHD can have different levels of certain 'neurotransmitters', such as dopamine and noradrenaline. Both these neurotransmitters are involved in executive functions.

Current thinking suggests that an imbalance of some of the neurotransmitter chemicals in the brain is important in ADHD.

The different levels of neurotransmitters may also have an effect on glucose levels in the brain, which affects activity in that area. Put simply, the more glucose used, the more energy and activity in the brain.

A study looked at glucose levels in adults who had been hyperactive since childhood and continued to have symptoms. It found that in people with ADHD, the areas of the brain that control impulses and attention used less glucose, which suggests they were less active.

It has been suggested that if someone has lower levels of noradrenaline and dopamine, their brain can't make use of the glucose, so less energy is available to some parts of the brain, causing the symptoms of ADHD.

Research suggests that when a baby's brain is damaged or negatively affected before or after birth, this could make the baby more likely to develop ADHD later on.

If a woman smokes, takes drugs, drinks a lot of alcohol, or is very stressed while she is pregnant, these things can affect the baby's brain.

Children with a low birth weight seem to be more likely to develop ADHD. Brain diseases, trauma during birth, head injury and passive smoking can also increase the chances of a child developing ADHD.

There isn't a great deal of evidence to show that eating particular types of food or food additives could play a role in causing ADHD.

Many parents of children with ADHD try changing their child's diet or giving food supplements, but there is little or no evidence this can benefit children with ADHD.

Some studies have found children with ADHD have lower levels of essential fatty acids than other children.

This suggests they may not absorb these fats from food as well as other people, or perhaps they lack the enzyme that converts the fatty acids to other substances in the body. However, it isn't clear what role these fatty acids play in the disorder.

There are certain aspects of the family environment that are found more often in children with ADHD. It isn't clear whether these factors can actually cause ADHD.

It may be the family environment just increases the likelihood of ADHD developing in a child who is already more at risk because of other reasons, eg genetic factors. A child's home life can also affect the severity of symptoms, how long ADHD lasts and how it develops.

It's possible that some of the factors linked with ADHD, such as family conflict and criminal behaviour, could be the result of the same genes that cause ADHD, rather than independent causes of the disorder.

It's a common misconception that ADHD is caused by 'bad' parenting. People who don't know anything about ADHD can be quick to blame the parents. But many experts think the causes of ADHD are largely genetic.

Research shows it's far more likely that good parents trying to cope with the behaviour of a child with ADHD sometimes seem like 'bad' parents, because they can appear short-tempered, stressed, depressed and poor at disciplining their child