Individuals with ADHD show both structural and functional differences in their brains when compared to those without. These individuals have smaller overall brain volume, which is specifically prominent in the frontal lobe. This area remains smaller throughout development, often falling two to three years behind in comparison to children without ADHD. In addition to being smaller, neural imaging and studies of electrical activity and blood flow show less activity in the frontal lobe of individuals with ADHD.
What Causes These Changes
Genetics have been implicated as the biggest factor in the development of ADHD. Twin and family studies show that 75% of the variation in ADHD symptoms can be accounted for by genetics. While many individual genes have been found to contribute, it is clear that a number of these are involved in dopamine production. Dopamine is a chemical in the brain involved in attention and impulsivity. Not coincidentally, stimulants which increase dopamine are often used to treat ADHD.
Environmental factors also play a critical role in accounting for the aforementioned brain changes. These factors include maternal alcohol and substance use while pregnant, maternal stress during pregnancy, premature birth, and low birth weight. Exposure to toxins (e.g., pesticides and lead) and neglect have been linked to ADHD as well.
Nature Versus Nurture
Generally, it is believed that certain children are genetically predisposed to ADHD, but an environmental stressor is needed to activate this risk, resulting in symptoms. This theory explains why two children can face the same stressor (e.g., both grow up near commercial farms which use high levels of pesticides), but only one develops ADHD due to this genetic predisposition.
Reference:
Sparrow, E.P., & Erhardt, D. (2014). Essentials of ADHD Assessment for Children and Adolescents. Hoboken, NJ: John Wiley & Sons, Inc.
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