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The Causes of ADHD

Margaret Austin, Ph.D., Natalie Staats Reiss, Ph.D., and Laura Burgdorf, Ph.D. Updated: Nov 5th 2007

A multitude of research studies have examined the causes of ADHD. Even with this wealth of data, experts remain uncertain as to the exact cause of the disorder. At this point, the research suggests that ADHD is primarily a neurological or brain-based disorder that is either present at birth, or that develops early on in childhood. Although environmental factors can play a role in increasing the severity of a person's symptoms, the environment does not seem to be the primary cause of the disorder.

Diathesis-Stress Theory

Although the consensus among the majority of mental health professionals is that ADHD is a neurological disorder, there are still some researchers who believe that environmental factors play a causal role. In mental health fields, the Diathesis-Stress Theory is used to explain the development of mental illnesses by focusing on the presence of an inherited weakness or vulnerability (the diathesis) and its interaction with environmental factors (i.e., stressors). According to this theory, some people have inherited a greater vulnerability, or a tendency towards the development of particular disorders than other people. Certain environmental stressors trigger the expression of the diathesis, and therefore, a person shows the symptoms of a particular illness.

The Diathesis-Stress Theory is a well-established theory used to explain why individuals develop mental illnesses such as depression and schizophrenia. However, this theory may be less relevant to the explanation of why someone develops ADHD. It does not seem to be the case that individuals inherit a tendency/predisposition toward developing ADHD. Rather, it seems that the disorder is present at birth or very early on in an individual's life. Environmental factors, rather than serving a causal or "triggering" role, can contribute to the severity of ADHD symptoms expressed.

Because biological factors seem to be most important in describing why someone develops ADHD, we present these variables first. We then follow up with a description of environmental factors that may contribute to the severity of symptoms.

Genetics

It is becoming increasingly evident that genes play a powerful role in the development of impulsiveness, hyperactivity, and inattention. The estimated heritability (the proportion of variance in a trait that can be attributed to genetics) of ADHD ranges from 75 to 91%. In other words, biological relatives seem to pass on the same or similar symptoms to their relatives. Twin studies also suggest a genetic basis for ADHD. Concordance rates (the occurrence of similar traits) are higher among monozygotic (identical twin) pairs (58-82%) than dizygotic (fraternal twin) pairs (31%-38%). In addition, in twin pairs where one person has ADHD, the "normal" twin also frequently shows problems in his or her ability to inhibit behavior (e.g, stop a response). Even though there is a clear genetic basis for ADHD, exactly which genes cause the disorder is unclear. Research suggests that most likely multiple genes are the culprit.

Brain Changes with ADHD

Researchers have studied both brain structures and levels of brain activity in individuals with ADHD. The studies suggest that people with ADHD have brains that are about 4% smaller than normal. Most of the size reduction occurs in the prefrontal cortex and the anterior temporal areas. Size reductions in the prefrontal cortex could impair an individual's self-awareness and his or her ability to manage emotions and behavior, as well as increase impulsive, angry, and hyperactive behavior. The anterior temporal areas influence hearing functions and language skills, including the ability to comprehend and/or communicate verbally, as well as the perception of non-language sounds. Decreases in this area of the brain could also influence memory and learning.

An overall smaller brain volume can also impair executive functioning, the term used by psychologists to refer to higher level skills such as organizing information, sustaining attention, and determining a reasonable course of action based on the information available. Whether or not decreased brain size is a cause or a consequence of ADHD remains to be seen, as more research on the topic is conducted.

Other researchers have found that children with lesser amounts of gray matter (the component of nerves composed of cell bodies that helps to send sensory or motor messages throughout the central nervous system) in the brain seem to have a harder time paying attention than do children with more gray matter. Research also suggests that there may be a subset of children with ADHD who have larger frontal lobes than normal. These children seem to have predominately hyperactive behavior. More research is necessary to determine whether there are truly brain-based differences between the three ADHD subtypes.

Brain activity can be measured by imaging technology such as Magnetic Resonance Imaging (MRI). An MRI allows researchers to use radio frequency waves and a strong magnetic field in order to produce images that are used to compare brain activity levels between individuals with and without ADHD. During tasks that require careful attention and the ability to inhibit an inappropriate response, individuals with ADHD show decreased activity in the striatum (the part of the brain responsible for modulating movement, making decisions, and responding to rewarding and unpleasant situations) and the prefrontal cortex (described above) when compared to individuals without the disorder. Research also suggests that frontal lobe connections to the basal ganglia may be affected by ADHD. The basal ganglia are brain structures that help to coordinate motor control, cognition, emotions and learning.

ADHD researchers are now focusing more attention on the cerebellum. Structural imaging studies in have repeatedly shown that the cerebellum (a region of the brain that plays a significant role in the communication between sensory perception and motor functions) of children with ADHD is notably smaller than their non-ADHD counterparts.

One line of brain-based research suggests that individuals with impulsive and hyperactive forms of ADHD are more sensitive to immediate rewards (rather than delayed rewards). So, these people are more likely to engage in a high amount of behavior (overactivity), rather than being able to wait patiently for something rewarding to occur later on (e.g., obtaining a good grade for a well-written research paper in school). Children with ADHD also seem to respond poorly to punishment.

Brain imaging techniques are currently not used to diagnose ADHD, but evidence collected from these types of studies are providing more detailed clues as to the causes of this disorder.

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Zinc supplementation as a solution for ADHD. - Hans de Rycke - Aug 27th 2009

In my profession as a Kinesiologist I have found many children with ADHD, or ADD suffering from allergies, in particular allergies to dairy, and, to a lesser extent, to chlorine. The allergic response causes a release of adrenaline to fight the perceived threat, causing a 'high'. This high can last up to 6 hours, after which time it reduces somewhat. With common foods like dairy it is often consumed more than once a day, leading to a constant allergic response. This, of course, affects the immune system which becomes depleted. The immune system includes the glandular system, including the glands of the brain, which are malfunctioning. Various studies have indicated supplementation with Zinc to be beneficial for brain function. One test found that alzheimer and other dementia's can and do benefit from Zinc. Doses up to 75 mg are used without any ill effects.

Conditions associated with Zinc deficiency:

Frequent and/or severe infections
Sleep and behavioral disturbances
Delayed wound healing
Psychiatric Illness
Inflammatory bowel disease
Impaired glucose tolerance
Malabsorption syndromes
Reduced appetite
Anorexia
Growth retardation
Loss of sense of smell or taste
Delayed sexual maturation
Night blindness
Impotence
Infertility
All dermatological disorders.
Abnormal menstruation
Dandruff and hair loss
Alcohol and drug abuse
Connective tissue disease
Diuretic usage
Rheumatoid arthritis

Most of these conditions are also related to allergies, hence the allergy connection.

Good luck

Hans

C-section - - Jul 19th 2009

I would like to know if there a nexus between iterative cesarean and ADD, and if you have some bibliography. Thanks.

Tried everything - Bobbi - Jun 18th 2009

I have tried every prescription medication for my son. He is now 9 years old and has been medicated since he was 3. As a last resort I did try herbal meds.....the omega fatty acids. It worked almost as well as the other prescribed medications which didnt really help all of his symptoms. I have now taken him off everything and he is doing about the same as when he was on them. I found while trying to help his ADHD symptoms ...the meds were causing other symptoms or side effects like not being able to sleep or feeling dopey. I often wonder why doctors dont do MRI as part of the diagnosis for the disorder.

Jaundyce factor - Silvio j Ponce MD - Jan 30th 2009

I had been studying this matter for a long period of time here in Venezuela south america... I have found out that the kids that develop hyperbillirrubine early at birth (from 9 to 15mg/100dc blood ) develop ADHD....the thing is that those afeccted kid develop intelligence higher lever... we had found too that the dopamine and the noradrenaline dis balance is the common cause of the brain functional disorder.....

We treat all this kids with no medication...we use the poli-insaturated greasy acid(linoleico - decahexanoico) Omega series....with excelent results....