Defining the exact cause of ADHD is important for developing better and more targeted treatments. Though I am convinced that there is a neurological basis for many cases of ADHD, I strongly suspect that there is no one, single underlying cause. In fact, I think ADHD is a collection of similar behaviors and symptoms that all result from a multitude of different, unrelated, conditions, some biological and some psychological.

The strongest evidence for a neurobiological cause is that a genetically determined propensity to ADHD is clearly present in many of the children who display symptoms. (Some of the genes that contribute have already been identified.) Other evidence pointing to a biological mechanism comes from the observation that other physical causes (like perinatal stress, low birth weight, traumatic brain injury, and maternal smoking in pregnancy) increase the risk that a child will display the symptoms of ADHD. In addition, some brain-imaging methods have shown functional differences between some patients with ADHD and others who do not have the disorder. Psychological testing of children with ADHD often shows deficiencies in alertness, ability to plan, and ability to inhibit impulsive action. However, these findings are not at all universal, and it is unclear whether they are the result of a neurobiological dysfunction or psychological one. Nor does the fact that the condition often improves with medication tell us anything one way or the other.

Anxiety appears to play a significant role in this disorder, worsening symptoms. In addition, many patients with poor ability to focus in childhood show improvement as they grow older, though recent evidence shows that many have symptoms at least through adolescence.

In my own practice, I have seen that a disproportionate number of children labeled "ADHD without hyperactivity" are exceptionally bright and creative children. I've often thought that these kids find their own inner theater much richer and more interesting than the outer theater of the classroom. Shortly, these children realize that they've missed things that other children didn't. This realization often makes them doubt their own capacity and become anxious that their "secret" will be discovered. This anxiety makes it even harder for a child to concentrate and pay attention.