Adhd Biology

ADHD is a complex neurobiological disorder that has roots in biological differences in the brain. What causes ADHD? Experts agree that the biology of ADHD is based in differences in brain structure and function. Both genetics and environmental factors can contribute to the development of the brain changes that lead to ADHD symptoms.


Family studies, twin studies and adoption studies have indicated that ADHD is a hereditary disorder and tends to run in families. In fact, approximately one quarter of individuals with ADHD have at least one family member with the disorder (New Zealand Online ADHD Support Group, 2009).

In a child with ADHD, changes in the genetic blueprint lead the brain to develop differently from the brain of an individual who doesn’t have ADHD. The specific genes involved in ADHD and the brain changes that cause the disorder have yet to be pinpointed, but current research continues to narrow the scope of genes involved in ADHD.

Environmental Influences

The fragile brain of a newborn or young child can be damaged when exposed to environmental toxins. Toxins can interfere with brain development, and contribute to the structural and functional changes in the brain associated with ADHD. If a mother is exposed to toxins before her baby is born, including smoking and drug use, these toxins can be transferred to the fetus. In childhood, exposure to toxins (such as lead) is associated with a higher risk of ADHD in children.

Brain Anatomy

Brain anatomy refers to the structure and form of the different parts of the brain. Some research suggests that the brains of individuals with ADHD are structurally different from the brains of other people. Some of the suggested differences include variations in the size of brain areas that mediate attention and impulse control, including the prefrontal cortex.

Brain Physiology

Brain physiology refers to the function of the brain. Differences in neurotransmitter activity have been indicated in the brains of those with ADHD. Neurotransmitters that may play a role in ADHD include:

  • Adrenaline
  • Dopamine
  • Serotonin.

ADHD medications work by changing neurotransmitter levels in the brain. For example, stimulants, the most common class of medications used to treat ADHD, work by increasing the amount of dopamine in the brain. This can help improve attention.

Imaging tests like functional magnetic resonance imaging (fMRI) can help identify changes in brain physiology. Unlike other imaging techniques, which portray static images, fMRI can measure changes in brain activation. Differences in activation have been observed in brains of individuals with ADHD when compared to typically functioning brains. Brain areas with functional differences in ADHD include the frontal lobe and the structures of the basal ganglia, a system of structures involved in planning and executing movement.


Mayo Clinic. (2009). Attention-deficit/hyperactivity disorder (ADHD). Retrieved August 10, 2010, from

Medline Plus. (2010). Attention deficit hyperactivity disorder (ADHD). Retrieved August 10, 2010, from

National Institute on Drug Abuse. (2009). NIDA infofacts: Stimulant ADHD medications—methylphenidate and amphetamines. Retrieved August 13, 2010, from

New Zealand Online ADHD Support Group. (2009) The neurobiology of ADHD. Retrieved August 12, 2010, from

Rosack, J. (2004). Brain scans reveal physiology of ADHD. Psychiatric News, 39(1), 26.