Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, hyperactivity and impulsivity that begins in childhood and can persist into adulthood. It is estimated to affect around 5% of children and 2.5% of adults worldwide. ADHD has been surrounded by controversy, with some arguing it is overdiagnosed and overtreated while others say it is a legitimate brain disorder. So is ADHD truly a disability arising from differences in brain development and function? Or is it just normal childhood behavior mislabeled as a disorder? Here we review the scientific evidence surrounding ADHD and its brain basis.
What are the Core Symptoms of ADHD?
According to the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM-5), there are three core symptom domains in ADHD:
- Inattention – Difficulty sustaining focus, easy distractibility, lack of persistence, disorganization, failure to pay close attention to details.
- Hyperactivity – Excessive motor activity and restlessness when it is not appropriate, fidgeting, tapping, inability to sit still.
- Impulsivity – Hasty actions without forethought, poor planning, social intrusiveness, little self-control.
To receive a diagnosis of ADHD, patients must demonstrate at least six symptoms of inattention, hyperactivity or impulsivity that impair function and persist for at least six months. Symptoms must be present before age 12, occur in multiple settings, and cannot be better explained by another disorder.
Is There Solid Evidence ADHD has Biological Roots?
Considerable scientific evidence suggests underlying biological and genetic factors contribute significantly to ADHD:
- Family studies – ADHD strongly runs in families, with parents of children with ADHD more likely to have a history of the disorder.
- Twin studies – Identical twins share greater concordance rates for ADHD than fraternal twins, suggesting genetic influences.
- Genome-wide studies – Variants in several genes, including those involved in dopamine and norepinephrine neurotransmission, increase ADHD risk.
- Brain imaging – Structural and functional differences in frontostriatal networks implicated in attention, reward and inhibition are commonly found in ADHD.
- Neuropsychological testing – Individuals with ADHD tend to show deficits in executive functioning like working memory, planning and cognitive control.
These findings point to neurobiological and genetic underpinnings that distinguish ADHD as a legitimate brain disorder, not just a behavioral or moral failing.
Are There Physical Brain Differences in ADHD?
Brain imaging techniques have revealed several brain regions and networks that function atypically in people with ADHD. Some main findings include:
- Smaller volumes in the frontal lobe, temporal lobe, corpus callosum, cerebellum and other areas.
- Reduced connectivity between regions of the default mode network, involved in internal thought processes.
- Decreased activation of frontal and striatal areas during tasks of inhibitory control and attention.
- Atypical dopamine and norepinephrine receptor concentrations in multiple brain regions.
- Altered brain electrophysiology patterns measured by EEG.
These anatomical and functional brain differences likely underlie the deficits in executive functions like attention, planning and self-regulation that are characteristic of ADHD.
Key Brain Differences in ADHD
Some key brain differences frequently identified include:
- Frontal lobes – Important for paying attention, planning, organizing. Often smaller and less active in ADHD.
- Basal ganglia – Key for habit formation and motivation. Altered function impacts inhibition.
- Limbic system – Drives motivation and emotion. Underactivation may reduce reward from tasks.
- Cerebellum – Important for cognitive skills and coordination. Smaller in ADHD.
- Corpus callosum – Connects the hemispheres. Smaller size may impede communication.
These widespread brain differences support ADHD as a neurological disorder.
Is ADHD Associated with Cognitive Function Differences?
In addition to brain structure and activity differences measured by neuroimaging, abundant research shows ADHD is linked to atypical cognitive test performance:
- Sustained attention – Reduced ability to stay focused for prolonged periods.
- Distractibility – Difficulty screening out irrelevant external stimuli.
- Working memory – Deficits holding information in mind and manipulating it.
- Response inhibition – Impaired ability to deliberately suppress actions.
- Task planning – Struggles with developing and organizing goal-directed strategies.
- Task switching – Difficulty transitioning between different tasks.
These hallmark cognitive difficulties in ADHD arise from the brain differences and genetic factors mentioned earlier. They help account for the symptomatic behaviors of inattention, hyperactivity and impulsivity.
Are Brain Chemical Imbalances Involved in ADHD?
Imbalances in brain chemicals called neurotransmitters, especially dopamine and norepinephrine, are thought to play an important role in ADHD:
- Dopamine is key for motivation, reward, attention and movement control.
- Norepinephrine is involved in vigilance, focus, arousal and reaction time.
- ADHD brains may have insufficient signaling by these neurotransmitters.
- Medications boost dopamine and norepinephrine to improve symptoms.
- Genes linked to ADHD affect dopamine and norepinephrine activity.
While the neurochemistry is complex and not fully understood, dysregulation of dopamine and norepinephrine systems in the brain is likely a major factor underlying ADHD.
Are ADHD Brains Delayed in Maturation?
There is increasing evidence that ADHD involves a delay in brain maturation, particularly in frontal regions critical for controlling attention, behavior and impulses.
This delay hypothesis is supported by:
- Brain imaging showing frontal lobe volumes and connections develop later in ADHD.
- Symptoms decline for many in late adolescence as brains mature.
- ADHD medications stimulate development in dopamine signaling systems.
- Children with ADHD perform more like younger controls on cognitive tests.
The brains of those with ADHD may simply lag several years behind in reaching full maturity, especially in the prefrontal cortex.
Is ADHD Better Characterized as Executive Dysfunction?
Many experts believe the heart of ADHD impairment lies in altered executive functions – the cognitive processes that allow for organized, purposeful, goal-directed behavior.
Executive functions impaired in those with ADHD include:
- Response inhibition
- Interference control
- Working memory
- Cognitive flexibility
- Planning and organization
These executive function deficits, stemming from changes in brain structure and function, can account for the clinical symptoms of ADHD. Viewing ADHD as executive dysfunction driven by biological roots in the brain supports it as a legitimate disorder.
Is There a Link Between ADHD and Other Brain Disorders?
ADHD has significant comorbidity and overlap with other neurodevelopmental disorders including:
- Autism spectrum disorder
- Specific learning disabilities
- Developmental coordination disorder
- Tourette’s syndrome
- Mood and anxiety disorders
Up to 80% of those with ADHD meet criteria for another neurodevelopmental condition. This suggests shared genetic and neurological factors underlie these disorders. The overlap further reinforces the view of ADHD as originating from atypical brain development rather than just behavioral or environmental causes alone.
Conclusion
In summary, considerable scientific evidence from genetics, neuroimaging, neuropsychology, neurochemistry and developmental studies supports ADHD as a legitimate brain-based disorder, not simply misdiagnosed bad behavior. While the exact causes are still being investigated, differences in brain structure, function, connectivity and neurotransmission arising from complex gene-environment interactions appear central to the onset of ADHD. Though the mechanisms are still being uncovered, ADHD can be characterized as a neurodevelopmental disability originating from altered brain function rather than purely social or environmental forces. Moving forward, it will be important to further illuminate the biological underpinnings of ADHD while also providing support, accommodation and coping skills to those diagnosed with the disorder.