ADHD is caused by the combined effects of many genetic and environmental risk factors, not by any single thing. There is no one gene, one toxin, or one parenting style that flips a switch. Instead, dozens of small influences on brain development add up, shaping how the brain regulates attention, impulse control, and activity level. Here’s what researchers have identified so far.
Genetics Play the Largest Role
ADHD runs in families, and genetics account for the biggest share of risk. Twin studies consistently show that if one identical twin has ADHD, the other is far more likely to have it than a fraternal twin would be. Estimates of heritability from these studies typically land between 70% and 80%, meaning the majority of variation in ADHD risk across a population traces back to inherited DNA differences.
No single “ADHD gene” has been found. Instead, genome-wide studies point to many common genetic variants, each contributing a tiny amount of risk. A large meta-analysis of over 17,000 children found that the combined effect of common genetic variants explained roughly 8% to 34% of symptom variation, depending on how symptoms were measured and who reported them. One gene that emerged from that analysis, called WASL, is involved in how neurons develop. Another gene, ST3GAL3, showed up independently in both the largest genetic study of ADHD to date and in research on epigenetic markers at birth, reinforcing the idea that it plays a real biological role.
What this means practically: if you have ADHD, your biological children have a meaningfully higher chance of developing it too. But genetics load the gun without necessarily pulling the trigger. Environmental factors interact with that inherited vulnerability.
Brain Structure Differences in Children
Brain imaging research shows that children with ADHD tend to have slightly less cortical surface area, the folded outer layer of the brain responsible for higher-order thinking. A coordinated analysis published in the American Journal of Psychiatry compared brain scans across large clinical and population samples and found that 24 of 34 measured brain regions were smaller in children with ADHD. The most affected areas included the superior frontal gyrus, the orbitofrontal cortex, and the anterior cingulate cortex, all regions involved in planning, decision-making, and self-regulation. Four regions also showed thinner cortex in children with ADHD.
These differences are real but small, with effect sizes too modest to diagnose any individual from a brain scan. Interestingly, the study found no significant surface area or thickness differences in adolescents or adults with ADHD, suggesting the brain may partially catch up over time. This aligns with the clinical observation that some people’s symptoms ease as they get older, though many continue to experience ADHD into adulthood.
How Brain Chemistry Connects to Symptoms
Two chemical messengers in the brain, dopamine and norepinephrine, are central to ADHD. These molecules help regulate circuits connecting the front of the brain (where planning and impulse control happen) with deeper structures involved in motivation and movement. In people with ADHD, signaling through these pathways is less efficient. The result is difficulty sustaining attention on tasks that aren’t immediately rewarding, trouble inhibiting impulses, and challenges with working memory.
This isn’t a simple “chemical imbalance” in the way people sometimes imagine. It’s more like a volume knob turned slightly too low in specific circuits at specific times. That’s why someone with ADHD can hyperfocus on a video game for hours but struggle to read a textbook for ten minutes. The reward and novelty signals that amplify dopamine activity are doing the heavy lifting in one case but not the other. Medications used to treat ADHD work by increasing the availability of dopamine and norepinephrine in these circuits, which is part of the evidence that these pathways are involved.
Prenatal and Early Childhood Exposures
Several environmental exposures during pregnancy and early life are linked to higher ADHD risk. The two with the strongest evidence are prenatal tobacco exposure and childhood lead exposure. A study using nationally representative data from U.S. children found that prenatal tobacco exposure roughly doubled the odds of ADHD (adjusted odds ratio of 2.4), and children with the highest blood lead levels had similarly elevated risk (odds ratio of 2.3). When both exposures were present, the risk didn’t just add up; it multiplied. Children exposed to both tobacco in the womb and high lead levels were about eight times more likely to meet criteria for ADHD than children with neither exposure.
Other prenatal factors associated with increased risk include alcohol exposure during pregnancy, very low birth weight, and extreme prematurity. These exposures likely affect brain development during critical windows when neural circuits are forming. Reducing these common exposures may be one of the few actionable strategies for lowering ADHD rates at a population level.
Epigenetics: Where Genes and Environment Meet
Epigenetics offers a framework for understanding how environmental exposures get “under the skin” to affect gene activity without changing the DNA sequence itself. The most studied mechanism involves small chemical tags (methyl groups) that attach to DNA and dial genes up or down. These tags are influenced by both inherited genetics and environmental conditions, particularly during pregnancy.
A striking finding from population-based research is that epigenetic patterns measured in newborn blood samples at birth can distinguish children who will go on to develop persistent ADHD symptoms from those who won’t. In a meta-analysis of nearly 2,500 school-aged children, epigenetic patterns at birth predicted later ADHD symptoms, but patterns measured during childhood did not. This suggests that something is set in motion very early, potentially during fetal development.
Researchers have also found that specific epigenetic markers linked to ADHD overlap with markers affected by known risk factors like maternal smoking, maternal overweight before pregnancy, and early childhood nutrition. One particular gene region showed epigenetic changes tied to both early malnutrition and later attention deficits. This kind of convergence supports the idea that epigenetic changes may be one mechanism through which prenatal environments translate into ADHD risk.
Head Injuries and ADHD
Traumatic brain injuries in children have a complicated relationship with ADHD. A large study published in Pediatrics found that children who experienced a mild traumatic brain injury were 17% more likely to be diagnosed with ADHD during a four-year follow-up period, though this elevated risk only became statistically significant around year four. The association was strongest in children aged 10 to 13, who were 27% more likely to receive an ADHD diagnosis after a mild brain injury.
However, the researchers cautioned that this relationship lacks strong evidence for causality. Children who already have undiagnosed ADHD may be more prone to accidents and head injuries in the first place, and family and environmental factors complicate the picture further. Head injuries likely account for a small fraction of ADHD cases overall.
What About Sugar, Diet, and Screens?
Sugar is one of the most commonly blamed culprits, but the evidence doesn’t support it as a cause. A systematic review and meta-analysis found no relationship between dietary sugar consumption alone and ADHD symptoms. Sugar-sweetened beverages showed a positive association with ADHD symptoms in children over seven, but these drinks also contain artificial food colorings and preservatives, which may be the more relevant ingredients. Even so, the relationship is correlational. Children with ADHD may simply gravitate toward sugary drinks more than their peers.
Screen time is a more nuanced story. A longitudinal study from the American Academy of Pediatrics tracked children over time and found that social media use specifically predicted increases in inattention symptoms, and the relationship was one-directional: social media use led to more inattention, but inattention didn’t lead to more social media use. This suggests a potentially causal link, though the effect size was small at the individual level. Notably, playing video games and watching television were actually associated with slight decreases in hyperactivity-impulsivity symptoms over time, so “screen time” is not a single category with a single effect.
Even if social media use contributes to attention difficulties, researchers are careful to distinguish between worsening symptoms and causing the underlying disorder. A child genetically predisposed to ADHD may find that heavy social media use tips them over a diagnostic threshold, while the same exposure in a low-risk child produces no clinical effect. The most accurate way to think about these modern exposures is as potential amplifiers of existing vulnerability, not root causes on their own.