What causes ADHD?

Heredity, Brain Structures, and Neurotransmitters

The exact cause of ADHD is not know, but researchers have found links between ADHD and heredity, brain structures, and neurotransmitters.

Heredity: Heredity plays a strong role in the condition, meaning that ADHD tends to run in families. This does not mean there is an “ADHD gene”, rather it indicates that several genetic factrors together may predispose a person to have ADHD.

Brain Structures: Areas of the brain implicated in ADHD include the prefrontal cortex, certain portions of the basal gangia, and the cerebellum.

Neurotransmitters: Neurotransmitters are the chemicals that help relay information from one neuron to another neuron or cell. The primary neurotransmitters implicated in ADHD are dopamine and norepinephrine.

For more in depth information, continue reading below.

Heredity

As mentioned above, heredity plays a strong role in the development of ADHD. The statistic used when describing the influence of genetics on the development of a condition is called the heritability index. The closer the heritibility index is to 1.0, the stronger the genetic influence on the development and the weaker the influence of environmental factors. The heritibility index for ADHD is 0.77 in twin studies.

As a point of comparison, heritability of general twinsintelligence is estimated at 0.52, many psychologocal conditions are in the .5-.6 range, and the heritiability of a person’s height has been estimated to be as high as 0.95.

If your parent or sibling has ADHD, then there is a 30% chance you will have ADHD as well. If both parents have ADHD, the odds go up to 50%.

It is important to keep in mind the heritability index is an estimate of genetic influence, not an absolute measure.

How is the heritability index calculated?
Twin studies compare the similarity between identical twins (who share 100% of their genes), to that of dizygotic or fraternal twins (who share only 50% of their genes). The heritability index is the amount of statistical variability accounted for by genes.

In other words, the heritability index tells you how well you could predict a measurement obtained from one identical twin, given a measurement of the same variable obtained from the other twin. So a heritability index of .77 suggests that about three quarters of the difference in ADHD symptoms between twins is the result of genetic influence and the other quarter is related to environmental influences.

Brain Structures

prefrontal cortexPrefrontal cortex. (the area shaded green in the picture below)
prefrontal cortex

Function: involved in executive functions including decision-making, attention, planning, and working memory, the ability to exercise response inhibition, self-monitoring, verbal regulation, motor control, maintaining and changing mental set and emotional regulation.

Response inhibition is currently thought to be one of the core deficits in ADHD, which in turn affects the other aspects of executive functioning mentioned above. (Barkley, R. A. (1998). Attention-Deficit Hyperactivity Disorder: A Handbook for Diagnosis and Treatment (2nd ed.). New York: Guilford.)

In ADHD: Individuals with ADHD have lower activity in the prefrontal cortex.

Striatum: composed of the caudate nucleus and putamen and is part of a group of structures called the basal ganglia.

Basal ganglia

 

  • Caudate nucleus:
      • Function: gateway to areas of the brain involved in motor control, cognition, emotions, and learning.
      • In ADHD: This structure is somewhat smaller in persons with ADHD.
  • Putamen:
      • Function: Related to disinhibition.
      • In ADHD: Smaller size (as is seen in ADHD) is correlated with disinhibition
  • Globus pallidus
      • Function: It is believed the globus pallidus is involved in filtering out extraneous information (BBC News, 2007).
      • In ADHD: found to be somewhat smaller in boys with ADHD than in their non-ADHD peers.

Cerebellum

  • Function: involved in the integration of sensory perception, coordination and motor control.
  • In ADHD: somewhat smaller in individuals with ADHD.

 

 

Neurotransmitters

Neurotransmitters are the chemicals that help relay information from one neuron to another neuron or cell. The neurotransmitters implicated in ADHD include dopamine and norepinephrine. There is some data indicating that serotonin is involved to a lesser extent as well.

A more in depth description of the role of neurotransmitters in ADHD can be found on the medication page.

Miscellaneous

In addition, there are some environmental factors associated with increased risk for ADHD including prenatal exposure to nicotine, alcohol, illicit drugs, and PCB’s (polychlorinated biphenyls). Also, exposure to certain toxins during childhood (e.g., lead, PCB’s) has been correlated with ADHD.

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