The first place to start with the cause of ADD or ADHD is genetics. It is a very genetic disorder.

There is term called 'heritability', which refers to how genetic a disorder is. As an example - breast cancer is 30% heritable - meaning that about 30% of the possible causes of breast cancer can be attributed to genetics. Asthma is 40% heritable. Height is 80 to 85 percent heritable. Attention Deficit/Hyperactivity Disorder is about 80 percent heritable.

That means that ADHD is highly genetic. This research comes from twin studies and other family studies, and shows that ADHD is almost as genetic as height.

Now if genes cause 80 percent that means there is about 20 percent that is not caused by genetics. What are the other causes? When it comes to the non-genetic causes of ADHD, we can call these 'acquired' cases. When we look at acquired cases, we can separate these into: Prenatal (i.e. causes that affect the person before birth), and Postnatal (i.e. causes that affect the person after birth).

The Prenatal Causes of ADHD:

These are the acquired causes of Attention Deficit/ Hyperactivity Disroder that occur during the pregnancy.

* Mother smoking during pregnancy: if the mother smokes cigarettes during pregnancy this increases the risk by 2.5 times (so the child is 2.5 times more likely to develop ADHD than if the mother didn't smoke during pregnancy)

* Mother drinking alcohol during pregnancy: With alcohol during pregnancy, there is the same increased risk - 2.5 times.

* Prematurity: If there is prematurity at birth, there is a general increased risk for ADHD, however if there is bleeding in the baby's brain there is a 45 percent chance that that individual will have ADHD.

* Increase in pregnancy complications: If there is an overall increase in pregnancy complications there is a non specific increased risk for ADHD.

Interestingly cocaine or crack exposure is not a risk factor for ADHD once you account for smoking cigarettes and drinking alcohol. Generally speaking a pregnant mother who is using cocaine will also tend to use cigarettes and alcohol.

The Postnatal Causes of ADHD:

These are acquired causes of ADHD that occur after the pregnancy (i.e. after the child is born).

* Head injuries/head Trauma: ADHD can develop after a significant head injury - by significant I mean one that results in loss of consciousness, or a serious concussion

* Brain Hypoxia which means not enough oxygen getting to the brain for a period of time. An example of this could be a near drowning or some other situation like that.

* Brain tumours

* Brain infection.

* Lead poisoning in the preschool years (i.e. before 3 years old)

* Survival from acute lymphoblastic leukemia (ALL) - because the chemotherapy itself can lead to brain changes.

The importance of the cause of ADHD is that for many people, when they realize that it is a real medical condition that is caused by medical issues, then they can let go of guilt - which can get in the way of useful treatment progress.