What Causes ADHD? Genetics, Brain Injury, and Brain Structure

Last updated on:

January 19, 2026

When a child is diagnosed with ADHD – Attention Deficit Hyperactivity Disorder – one of the first questions parents, teachers, and caregivers ask is: why did this happen? Is it something in the genes? A complication during birth? Something going on inside the brain? The honest answer is that ADHD doesn’t have a single cause. It is a neurodevelopmental condition with multiple contributing factors, and decades of research have helped us understand three major ones: genetics, brain injury, and differences in brain structure. Understanding these causes doesn’t just satisfy curiosity – it changes how we support children and adults living with ADHD.

The role of genetics: ADHD runs in families
Siblings and family risk
It’s not one gene – it’s many
Brain injury as a cause of ADHD
Premature birth and low birth weight
Maternal smoking and alcohol use during pregnancy
Infections and oxygen deprivation
Brain structure differences in ADHD
Smaller brain volume in key regions
The prefrontal cortex: the command centre for attention
The basal ganglia and cerebellum
Slower brain maturation
How these causes connect

The role of genetics: ADHD runs in families

Of all the known causes of ADHD, genetics is the most significant. Family, twin, and adoption studies consistently show that ADHD runs in families, and the evidence for this is substantial. Across 37 twin studies, the average heritability of ADHD has been estimated at around 74-80%, making it one of the most heritable psychiatric conditions known – comparable to the genetic influence on height.

Twin studies have been particularly revealing. Identical twins (who share 100% of their genes) are far more likely to both have ADHD than fraternal twins (who share about 50%). Adoption studies add another layer: ADHD is transmitted only to biological relatives, not adoptive ones, which strongly confirms that family risk is driven by genes rather than shared environments like parenting styles or household routines.

Siblings and family risk

The sibling connection is striking. Research shows that children with siblings who have ADHD are roughly nine times more likely to have ADHD themselves compared to children without affected siblings. Separate estimates suggest that around 40% of parents with ADHD have children who also meet the diagnostic criteria. This doesn’t mean ADHD is inevitable if it runs in the family – the condition can skip a generation, and environmental factors always interact with genetic predisposition – but the familial pattern is one of the strongest signals in all of psychiatry.

It’s not one gene – it’s many

Importantly, ADHD is not caused by a single “ADHD gene.” Genome-wide association studies show that genetic susceptibility to ADHD is made up of many common DNA variants, each having very small individual effects. Copy number variants – small insertions or deletions in the DNA – also play a role. This complexity explains why there is currently no genetic test that can diagnose ADHD; the patterns are simply too intricate for any single marker to capture.

Brain injury as a cause of ADHD

While genetics accounts for a large proportion of ADHD cases, a meaningful share can be traced to brain injury – damage that occurs before, during, or shortly after birth. These are often referred to as perinatal or neonatal risk factors, and they point to how sensitive the developing brain is to disruption during pregnancy and the early weeks of life.

Premature birth and low birth weight

Preterm birth is one of the most consistently identified non-genetic risk factors for ADHD. Preterm birth is associated with an increased risk of neurodevelopmental and neurobehavioral impairments including ADHD, and the risk rises the earlier the birth occurs. A cohort study found a threefold increase in ADHD risk for births at 28 weeks’ gestation or earlier. Preterm birth and low birth weight together can heighten the risk of ADHD by at least twofold. The most common ADHD subtype seen in premature children is the inattentive subtype, which differs from patterns found in full-term children.

Maternal smoking and alcohol use during pregnancy

Exposure to nicotine and alcohol in the womb is another established risk pathway. Nicotine and other toxic substances in tobacco can cross the placenta and disrupt fetal brain development, affecting neurotransmitter systems – particularly dopamine and norepinephrine – that are central to attention regulation. Alcohol use during pregnancy is associated with Fetal Alcohol Spectrum Disorders (FASD), which overlap significantly with ADHD symptoms including hyperactivity, inattention, and impulsivity. Research indicates that maternal prenatal smoking increases the risk of ADHD in children by more than one and a half times, based on a review of 12 large studies.

It is worth noting that some of this association between smoking and ADHD may be partly explained by shared genetic factors – mothers with ADHD may be more prone to smoking – but the biological mechanisms through which nicotine disrupts fetal brain growth are well-documented and biologically plausible.

Infections and oxygen deprivation

Serious infections affecting the brain – such as meningitis – in infancy or early childhood can also contribute to ADHD risk by disrupting normal neural development. Oxygen deprivation at birth, low APGAR values, and emergency caesarean sections are all associated with an increased risk of ADHD. It is known that hypoxia (lack of oxygen) leads to biochemical changes that affect the dopaminergic system – the very neurotransmitter pathway implicated in ADHD symptoms. None of these factors, on their own, cause ADHD in every case, but each raises the probability, especially when combined with a genetic predisposition.

Brain structure differences in ADHD

Beyond genetics and injury, neuroimaging research has revealed something fundamental: the brains of people with ADHD are physically different in structure and development from those of people without the condition. These are not damage-related changes, but rather differences in how the brain grows and matures. Understanding them helps explain why ADHD symptoms aren’t simply about willpower or behavior – they reflect measurable neurological differences.

Smaller brain volume in key regions

Neuroimaging studies have pointed to a smaller prefrontal cortex and basal ganglia, and decreased volume of the posterior inferior vermis of the cerebellum in people with ADHD – all regions that play critical roles in focus, attention, and behavioral control. Studies suggest that the ADHD brain can lag behind neurotypical peers in development by up to three years, especially in the frontal cortex, which is critical for decision-making and impulse control. Crucially, these structural differences do not mean the brain is damaged – they reflect a different developmental trajectory.

The prefrontal cortex: the command centre for attention

The prefrontal cortex (PFC) is widely considered the most important brain region implicated in ADHD. Research shows that ADHD is associated with weaker function and structure of prefrontal cortex circuits, especially in the right hemisphere, which is specialized for behavioral inhibition. The PFC regulates attention, planning, and the suppression of inappropriate responses. When it is underdeveloped or under-activated, the result is exactly what we see in ADHD: distractibility, forgetfulness, impulsivity, and difficulty planning. The PFC also relies heavily on the neurotransmitters dopamine and norepinephrine for optimal functioning – both of which are known to be dysregulated in ADHD.

The basal ganglia and cerebellum

Two other regions – the basal ganglia and the cerebellum – are also consistently implicated. The basal ganglia is responsible for motor learning and helps regulate behavior, emotions, and the ability to plan, focus, and multitask – all of which are affected by ADHD. Imaging studies have reported reduced volume of the caudate nucleus and parts of the cerebellum in ADHD.

The cerebellum, traditionally associated with movement and coordination, plays a broader role than once thought. A smaller cerebellum in children with ADHD may contribute to difficulty with motor response inhibition – the ability to suppress actions that interfere with a current task, such as staying seated during a class lesson. The PFC, basal ganglia, and cerebellum are not isolated structures; they form an interconnected circuit, and disruption anywhere in this network can produce the constellation of symptoms we associate with ADHD.

Slower brain maturation

One of the most important findings from developmental neuroscience is that the ADHD brain doesn’t mature as quickly as a neurotypical brain. Research shows that in children with ADHD, the prefrontal cortex matures more slowly and is slightly smaller in size compared to typically developing children. This slower maturation may explain why ADHD symptoms often become less pronounced with age – as the brain continues to develop, function improves. It also helps explain why stimulant medications, which boost dopamine and norepinephrine activity in the PFC, can have such a pronounced positive effect on focus and impulse control in people with ADHD.

How these causes connect

It is tempting to see genetics, brain injury, and brain structure as three separate explanations, but they are deeply intertwined. Genetic variations can shape how the brain develops, making certain regions – particularly the prefrontal cortex – smaller or slower to mature. Brain injuries during or before birth can alter the same circuits that genes predispose a person to develop differently. The result is a condition that emerges from the interaction of biological inheritance and early developmental experience.

What is clear from the evidence is that ADHD is not caused by poor parenting, lack of discipline, excessive screen time, or diet. It is a condition with real, measurable neurological roots. The high heritability of ADHD is stable across the lifespan, and its neurological signatures – visible in brain scans and genetic studies – are among the most replicated findings in all of neurodevelopmental research.

What do you think? Given that ADHD has strong genetic and neurological roots, how should this change the way educators and schools respond to students who struggle with attention and impulse control? And if a child’s brain is simply developing more slowly in key areas, what does that mean for how we set expectations and design learning environments for them?

References

PDF 📄

What Causes ADHD? Genetics, Brain Injury, and Brain Structure

Table of Contents

The role of genetics: ADHD runs in families 🔗

Siblings and family risk 🔗

It’s not one gene – it’s many 🔗

Brain injury as a cause of ADHD 🔗

Premature birth and low birth weight 🔗

Maternal smoking and alcohol use during pregnancy 🔗

Infections and oxygen deprivation 🔗

Brain structure differences in ADHD 🔗

Smaller brain volume in key regions 🔗

The prefrontal cortex: the command centre for attention 🔗

The basal ganglia and cerebellum 🔗

Slower brain maturation 🔗

How these causes connect 🔗