Harold Robert Meyer | The ADD Resource Center
haroldmeyer@addrc.org http://www.addrc.org/
Reviewed 04/01/2026 – Published 04/10/2026
Listen to understand, not just to respond
If you’re the parent of twins and only one has been diagnosed with ADHD, you’re not alone — and your confusion makes perfect sense. ADHD is one of the most heritable conditions in psychiatry, with genetics accounting for roughly 70–80% of the variation in symptoms. So how can two children who share the same parents, the same home, and — in the case of identical twins — the same DNA end up with different diagnoses? The answer lies in the complex interplay between genes, environment, and a relatively new area of science called epigenetics. Understanding these factors won’t change the diagnosis, but it can relieve the guilt, confusion, and second-guessing that so many parents of twins experience.
It’s a fair question. ADHD has a heritability rate estimated between 77% and 88%, making it one of the most genetically influenced psychiatric conditions — more heritable, in fact, than height. Twin studies have been central to establishing this. When researchers compare identical twins (who share 100% of their DNA) with fraternal twins (who share about 50%), identical twins are far more likely to share an ADHD diagnosis. But “far more likely” is not the same as “always.”
Even among identical twins, concordance for ADHD is not 100%. Studies consistently show that one identical twin can meet the diagnostic criteria for ADHD while the other does not. If the condition were purely genetic, this would be impossible. The fact that it happens tells us something important: genes load the gun, but they don’t always pull the trigger.
When researchers talk about “environmental factors” in ADHD, they’re usually not referring to parenting style, screen time, or how much sugar a child eats. They’re referring to what scientists call the “non-shared environment” — experiences and exposures unique to each individual, even within the same family.
For twins, this can include subtle but significant differences such as:
Research following over 1,000 identical twin pairs (Livingstone et al., “Does the Environment Have an Enduring Effect on ADHD? A Longitudinal Study of Monozygotic Twin Differences in Children,” Journal of Abnormal Child Psychology, 2016) found that roughly 40–44% of the environmental influence on ADHD symptoms was stable and lasting, while the rest was transient — suggesting that some environmental exposures have enduring effects on attention and behavior, while others come and go.
Perhaps the most fascinating piece of this puzzle is epigenetics — the study of how genes get turned on or off without any change to the DNA sequence itself. Think of your DNA as a massive instruction manual. Epigenetics determines which pages of that manual actually get read.
The primary mechanism researchers have studied in relation to ADHD is DNA methylation — a chemical process that can silence or activate specific genes. Studies of identical twins who are “discordant” for ADHD (meaning one has it and the other doesn’t) have found measurable differences in methylation patterns, particularly in genes related to the dopamine and serotonin systems — the very neurotransmitter pathways most closely associated with ADHD.
One landmark study (Chen et al., “Neuroanatomic, epigenetic and genetic differences in monozygotic twins discordant for attention deficit hyperactivity disorder,” Molecular Psychiatry, 2018) examined 14 identical twin pairs discordant for ADHD and found that the twin with ADHD showed distinct epigenetic differences in brain regions including the striatum and cerebellum, along with altered methylation in genes tied to GABA, dopamine, and serotonin systems. These are not differences in the DNA itself — they are differences in how that DNA is being expressed.
What causes these epigenetic shifts? Researchers believe they can be triggered by prenatal exposures, early nutrition, stress, and even random biological variation during development. The epigenome is not fixed at birth; it continues to change throughout life in response to experience.
If your twins are fraternal rather than identical, the picture is more straightforward — though no less valid as a source of parental confusion. Fraternal twins share roughly 50% of their DNA, just like any other siblings. Given that ADHD involves the cumulative effect of many genes working together, it’s entirely expected that one fraternal twin could inherit a combination of gene variants that tips the scale toward ADHD while the other inherits a different combination that does not.
A large Swedish study examining over 8,000 identical twin pairs, 26,000 fraternal twin pairs, and millions of other sibling relationships confirmed this gradient clearly: the more DNA two people share, the more likely they are to share an ADHD diagnosis — but sharing DNA is never a guarantee.
If one of your twins has been diagnosed with ADHD, it’s worth paying careful attention to the other — not to look for problems where none exist, but because ADHD can present very differently from one person to the next. The predominantly inattentive presentation is frequently missed, particularly in children who are quiet, compliant, or academically capable. A child can struggle significantly with executive function, working memory, and internal restlessness without ever being disruptive in a classroom.
Research on families of children with ADHD shows that siblings are significantly more likely to meet criteria for ADHD than children in the general population. For twins, who share even more genetic material, the odds are higher still. A screening conversation with a qualified professional — not to label, but to understand — can be one of the most helpful things a parent does.
Understanding why one twin has ADHD and the other doesn’t can relieve a particular kind of parental guilt — the nagging worry that you somehow caused this, or that you treated one child differently. You didn’t. The science is clear: ADHD arises from a complex interaction of genetic predisposition, prenatal and early developmental factors, and epigenetic variation. Many of these factors were determined before your children ever took their first breath.
What you can control is how you respond. The twin with ADHD needs structure, understanding, and often professional support. The twin without ADHD needs something too — acknowledgment that living alongside a sibling with ADHD comes with its own challenges, including increased anxiety, shifts in parental attention, and the quiet pressure of being “the easy one.”
Both of your children deserve to be seen for who they are — not as a matched set, and not as a medical mystery, but as two individuals whose brains developed along slightly different paths. That’s not a failure of genetics or parenting. It’s simply how human biology works.
“The question isn’t why one twin has ADHD and the other doesn’t. The question is what each child needs from you right now — and the answer to that has nothing to do with matching.” — Harold Meyer
ADHD is among the most heritable psychiatric conditions, yet identical twins are not always concordant for a diagnosis. Roughly 70–80% of the variation in ADHD symptoms is attributable to genetic factors, with the remaining 20–30% driven by non-shared environmental influences and epigenetic variation. Prenatal differences, birth complications, and subtle variations in early experience can all contribute to divergent outcomes even in children who share the same DNA. Epigenetic research — particularly studies of DNA methylation in discordant identical twins — is revealing how the same genetic blueprint can produce different neurological results. Parents should be reassured that twin discordance for ADHD is well-documented and expected, not a reflection of differential treatment or parenting failure.
Harold Meyer and the ADD Resource Center – addrc.org – have been providing ADHD support nationally and internationally for over 30 years. Hal Meyer is a nationally recognized ADHD advocate, speaker, and author. He is the founder and managing director of the ADD Resource Center and co-founder of CHADD of New York City.
Disclaimer: The information provided in this article is for general educational purposes only and should not be construed as medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before making any changes to your or your family’s healthcare plan. The ADD Resource Center, its employees, and affiliates are not responsible for any consequences arising from the use of the information provided herein.
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Harold Robert Meyer The ADD Resource Center haroldmeyer@addrc.orgwww.addrc.org Reviewed: May 03, 2026Published: May 17, 2026 Listen to understand, not…
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