One of my patients asked the question in the title of this post. He also asked if the genetic cause was a mutation. After I answered him, I thought that my readers might be interested in my reply, so I have cut and pasted it below.
ADHD is clearly inherited genetically since it is passed down from one generation to the next as reliably as height is. It is a general rule that boys grow to be about 4 inches taller than the average of their parents' heights and girls to be 4 inches shorter. In the case of height, there are known exceptions, some because of genetic issues (like dwarfism) and others because of developmental or environmental events like accidents or malnutrition.
Several genes have been associated with ADHD. They are not mutations any more than brown eyes are mutations of green eyes. The one gene that I remember is called DR4. It is one of at least 6 different sequences that code for a Dopamine Receptor. More people with ADHD have this particular gene than do people without ADHD. However, not everyone who has this gene also has ADHD and not everyone with ADHD has this gene.
This is probably where the developmental part comes in. It has been shown that young children have more symptoms of ADHD immediately after watching Spongebob Squarepants than they do after watching Sesame Street. It is possible that young children who are exposed to a lot of electronic media might be more likely to have enough symptoms to be diagnosed with ADHD than children who spend a lot of time reading or playing with blocks.
There is also the fact that while only 4.4% of adults can be diagnosed with ADHD, about 50% of the population in prison can be so diagnosed. This means more people who made the choice to do illegal activities (and got caught) exhibit ADHD than those who chose to obey the law. Most of these choices are made during adolescence and can be considered "late developments." It is clear that ADHD is associated with conduct disorder (a developmental issue that leads to problems with law enforcement) but conduct disorder can also occur without ADHD, so there may be reciprocal effects. This would be another example of development affecting the expression of a genetic tendency to ADHD.
To summarize, genes are responsible for the possibility of having ADHD symptoms. Environment and development probably determine whether an individual will have enough of those symptoms to match the full requirements of the diagnosis. There are
many genes involved and it is likely that certain variations of these genes are more common in ADHD. Therefore if more of these genes are the ADHD type in a given person, then that person will have more symptoms of ADHD.