Causes under investigation include, but are not limited to:

  1. Brain differences: Brain scan technology has revealed differences in the size, symmetry, metabolism, and chemistry of the brain in those who have ADHD; however, it should be noted that there is yet no clear determination of the source of these differences.
  2. Genetic factors: It has been demonstrated that children who have at least one parent diagnosed with ADHD are more likely to be diagnosed as having ADHD themselves. Current research is examining which genes may be involved in ADHD. This investigative path also suggests an associated hypothesis that environmental factors, handed down from generation to generation, may trigger the symptoms associated with ADHD. There also exists a possiblity that a family with one diagnosed member may have a heightened awareness of the disorder, along with a willingness to seek formal diagnosis, which would make detection and diagnosis more likely, thus skewing the data on heritability.
  3. Brain development in utero and during the first year of life, possibly related to drug use during pregnancy or environmental toxins.

It has also been suggested that ADHD may result from a poor diet and other external factors rather than from any physiological source. Studies of changes in diets of children provide some anecdotal and scientific evidence for this, but current majority opinion seems to be that the available evidence is insufficient to either prove or disprove this. However, it has been noticed that a large portion of children with ADHD seem to be addicted to milk. It has been proposed by Norwegian and British scientists that this is due to the casomorphins, peptides formed by incomplete digestion of the whey protein.

While ADD/ADHD is a known psychiatric condition, there are various theories about the cause and some controversy over the number of persons diagnosed and the cost of medications. Some denial in families may also relate to the negative perception of the condition as a hereditary brain disorder.