Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, hyperactivity and impulsiveness. It is one of the most common mental disorders affecting children, with around 5% of children worldwide receiving an ADHD diagnosis. ADHD frequently persists into adulthood and is associated with difficulties in education, employment, relationships and mental health.
A key question in ADHD research is whether the brains of those with ADHD take longer to mature and develop than neurotypical brains. In this article, we will examine the evidence around brain development in ADHD and discuss what it may mean for the neurodiversity movement.
Theories around brain development in ADHD
There are several theories as to why brains with ADHD may show developmental delays:
Delayed cortical maturation
Some research shows that the brains of children with ADHD undergo cortical maturation at a slower rate. The cortex is the outer layer of the brain responsible for functions like attention, thought and language. Studies using brain imaging have found that cortical thickness and volume peak at a later age in ADHD brains versus controls. This suggests that the cortex may take longer to mature in those with ADHD.
Delayed development of executive functions
People with ADHD often struggle with executive functions like working memory, inhibition and planning. Brain regions linked to these processes, like the prefrontal cortex, develop later in ADHD. This delayed development of executive function brain circuitry could underpin the inattention, hyperactivity and impulsivity seen in ADHD.
Lagging development of brain connectivity
The brain regions implicated in ADHD don’t function alone, but work together in networks via connections called white matter tracts. Research suggests ADHD brains show immature and poorly synchronized connectivity between regions involved in attention, impulsivity and hyperactivity. This lagging brain connectivity could disrupt the smooth flow of information and underpin ADHD behaviors.
Delayed brain maturation combined with accelerated early brain growth
One theory suggests the brains of children with ADHD initially grow faster and peak in cortical thickness earlier before undergoing delayed maturation. This mismatch between early accelerated growth followed by delayed development may disrupt optimal brain function and connectivity.
Evidence for delayed brain development in ADHD
So what does the research evidence actually say about brain development in ADHD? Here we summarize some of the key findings:
Thinner cortex that normalizes later
Multiple MRI studies have shown children and adolescents with ADHD tend to have a thinner cerebral cortex across frontal, temporal and parietal regions critical for attention compared to peers. However, this cortical thinning normalizes beyond adolescence and is no longer seen in adults with ADHD.
For example, a 2021 meta-analysis of 98 studies confirmed cortex thickness is reduced in children with ADHD but not adults. The cortex of those with ADHD was around 0.09-0.15mm thinner on average – equivalent to a 3-4 year delay in peak cortical thickness.
Slower cortical thinning in adolescence
While cortical thickness peaks earlier in ADHD, the later thinning of the cortex during adolescent development occurs more slowly. Multiple longitudinal studies tracking brain development have found a lag of 2-5 years in cortical maturation in ADHD versus healthy controls.
An analysis of over 2000 brain scans found cortex thinning between ages 8-22 years occurred more gradually in ADHD, suggesting delayed brain maturation.
Altered trajectory of gray matter development
Gray matter volume in the brain, containing neuron cell bodies, dendrites and synapses, follows an inverted U-shape pattern from childhood to adulthood. Gray matter initially proliferates before undergoing pruning during adolescence.
While results vary across studies, research suggests gray matter development follows a different trajectory in ADHD, with delayed maturation leading to a more blunted adolescent decline in volume.
Delayed white matter development
White matter, made up of axons connecting different brain regions, increases in volume during childhood and adolescence. Though findings are mixed, several studies have found delayed white matter development in tracts linking cortical and subcortical regions relevant to ADHD.
For example, a 2021 study found 5 major white matter tracts showed maturational delays of 2-5 years in youth with ADHD versus controls.
Late stabilizing of structural networks
Advanced neuroimaging techniques can map the structural connections between brain regions into networks. Research finds core networks involved in executive functions and attentional control don’t fully mature until early adulthood in ADHD patients, lagging several years behind neurotypical development.
For instance, a 2016 study found structural brain networks continue changing for longer in ADHD, with delayed stabilization not achieved until age 30 versus 21 years in healthy controls.
Delayed functional connectivity
Studies examining functional connectivity in ADHD brains via techniques like fMRI also find immature patterns compared to neurotypical controls. Connectivity within and between major cortical networks involved in executive function appear underdeveloped for age.
A meta-analysis concluded functional connectivity in ADHD is consistently altered across default mode, ventral attention, dorsal attention, and motor networks, indicating delayed functional maturation.
What does delayed brain development mean for ADHD?
Research conclusively demonstrates that brain development is altered in ADHD, with cortical maturation and connectivity between regions critical for attention, impulse control and activity level lagging behind. But what are the implications of this delayed brain development for our understanding of ADHD?
ADHD as a disorder of delayed maturation
The evidence for delays in structural and functional brain development supports theories that view ADHD as a disorder of delayed maturation. The brains of children with ADHD may simply be “younger” than their age in key areas, explaining their difficulties with concentration, hyperactivity and impulsiveness.
ADHD persistence explained
The brain maturation delays in ADHD could explain why symptoms frequently persist from childhood into adulthood, as full cortical development is not achieved until individuals reach their 20s or 30s. Interventions that target boosting brain development may help remit symptoms.
Better understanding ADHD neurocognitive deficits
Immature development of brain regions and networks involved in executive functions helps explain the cognitive deficits in inhibition, working memory, attention, planning and organization associated with ADHD.
Windows of opportunity for early intervention
Understanding the timeline of altered trajectories of brain growth and maturation in ADHD provides windows of opportunity for early interventions to ameliorate symptoms and impairment before altered wiring becomes consolidated.
ADHD as a temporary delay rather than permanent deficit
Evidence that ADHD brains eventually catch up with maturation, combined with reduced symptoms in adulthood, supports it representing a temporary delay in development rather than a permanent neurological deficit.
Implications for the neurodiversity movement
Some argue that viewing ADHD as a disorder characterised by immaturity risks further stigmatizing neurodivergent individuals as having “younger brains”. However, delayed development does not mean brains with ADHD are inherently faulty or inferior, but implies they may benefit from more time to mature and develop optimal connectivity patterns. From a neurodiversity perspective, there is no ideal model of brain structure or connectivity – just an array of configurations that offer differing strengths and weaknesses.
Limitations of the research
While studies relatively consistently point to delays in structural and functional brain development in ADHD, there are limitations to consider:
- Most studies are cross-sectional rather than longitudinal, making it hard to track maturational trajectories over time within individuals.
- There is heterogeneity in ADHD, with variations in the extent and precise pattern of brain delays between patients.
- There are inconsistencies between studies, with some disputing the notion of simple linear developmental delay in ADHD.
- Brain imaging provides limited insights into the complex neurological dynamics underpinning cognition and behaviour.
Conclusion
In summary, considerable evidence from structural and functional neuroimaging indicates brains with ADHD take longer to mature in regions critical for functions like attention and executive control. This supports theories of ADHD as a disorder of delayed development rather than a fixed deficit. However, more longitudinal research is needed to precisely map maturational trajectories in ADHD patients. From a neurodiversity perspective, delayed cortical maturation should not be viewed as making ADHD brains intrinsically worse but as offering alternative configurations with both strengths and weaknesses. Understanding developmental timelines offers opportunities for targeted early interventions to get brain development back on track before altered wiring becomes engrained.