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.
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.
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.