Abstract Decades of research show that genes play an vital role in the etiology of attention deficit hyperactivity disorder (ADHD) and its comorbidity with other disorders. Family, twin, and adoption studies show that ADHD runs in families. ADHD’s high heritability of 74% motivated the search for ADHD susceptibility genes. Genetic linkage studies show that the effects of DNA risk variants on ADHD must, individually, be very small. Genome-wide association studies (GWAS) have implicated several genetic loci at the genome-wide level of statistical significance. These studies also show that about a third of ADHD’s heritability is due to a polygenic component comprising many common variants each having small effects. From studies of copy number variants we have also learned that the rare insertions or deletions account for part of ADHD’s heritability. These findings have implicated new biological pathways that may eventually have implications for treatment development.
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"ADHD has been clearly linked with numerous environmental risk factors, particularly around the prenatal and perinatal period. Some of the most robust risk factors identified are maternal prenatal health conditions and psychological distress (e.g. hypertension, obesity, pre-eclampsia, immune activation), in utero exposure to poor diet (with critical factors still being determined), teratogenic effects of certain medications (e.g. acetaminophen) and environmental exposures (e.g. lead), as well as neonatal factors such as prematurity and low birth weight [27]. Other extreme exposures in the postnatal environment (such as extreme infant emotional neglect) have also been…
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associated with an ADHD syndrome [28, 29]."
Cecil, C. A. M., & Nigg, J. T. (2022). Epigenetics and ADHD: Reflections on Current Knowledge, Research Priorities and Translational Potential. Molecular diagnosis & therapy, 26(6), 581–606. https://doi.org/10.1007/s40291-022-00609-y
"The convincing evidence for genes as risk factors for ADHD does not exclude the environment as a source of etiology. The fact that twin estimates of heritability are less than 100% asserts quite strongly that environmental factors must be involved. ADHD’s heritability is high, and that estimate encompasses gene by environment interaction. Thus, it is possible that such interactions will account for much of ADHD’s etiology. Environmental risk factors likely work through epigenetic mechanisms, which have barely been studied in ADHD [148]. The importance of the environment can also be seen in the fact that, as for other complex genetic disorders, much of ADHD’s heritability is explained by SNPs in regulatory regions rather than coding regions [149]."
Faraone, S. V., & Larsson, H. (2019). Genetics of attention deficit hyperactivity disorder. Molecular psychiatry, 24(4), 562–575. https://doi.org/10.1038/s41380-018-0070-0
I don't think ADHD in particular is caused by unrelated stuff. The heritability findings are pretty strong.
> A study of 894 ADHD probands and 1135 of their siblings aged 5–17 years old found a ninefold increased risk of ADHD in siblings of ADHD probands compared with siblings of controls [2]. Adoption studies suggest that the familial factors of ADHD are attributable to genetic factors rather than shared environmental factors [3, 4] with the most recent one reporting rates of ADHD to be greater among biological relatives of non-adopted ADHD children than adoptive relatives of adopted ADHD children. The adoptive relatives had a risk for ADHD like the risk in relatives of…
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control children [4].
> Twin studies rely on the difference between the within-pair similarities of monozygotic (MZ) twin pairs, who are genetically identical, and dizygotic (DZ) twin pairs, who share, on average, 50% of their segregating genes. The mean heritability across 37 twin studies of ADHD or measures of inattentiveness and hyperactivity is 74% (Fig. 1). A similar heritability estimate of around 80% was seen in a study of MZ and DZ twins, full siblings, and maternal and paternal half-siblings [5]. The heritability is similar in males and females and for the inattentive and hyperactive-impulsive components of ADHD [6,7,8].
https://www.nature.com/articles/s41380-018-0070-0
I find it interesting that the article you linked didn't mention the heritability of ADHD. In around 3/4ths[0] of cases ADHD is caused by genetics, which leaves plenty of space for other causes.
[0] https://www.nature.com/articles/s41380-018-0070-0
ADHD is certainly not Mendelian. There are likely multiple genes involved probably around specific pathways.
https://www.nature.com/articles/s41380-018-0070-0
> Do you have an explanation for this except the genes?
Yes of course - the environmental/experiential conditions in which the carrier of the genes find themselves.
Remember, ADHD is just a culturally defined description of behavioural tendencies, not a genetically hard-coded outcome like the shape of someone's face, so it's a mistake to think of them as being genetically determined in the same way.
Of course the genes someone inherits from their ancestors will influence their behaviour, but genes express differently depending on many conditions, and for behaviour it's highly contingent on one's own life experiences; indeed it's extremely important for survival and…
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continued evolution that one can adapt their behaviour to the environment in which they live.
Even recent twin studies [1] don't find anything more than about 75-80% heritability of "ADHD" diagnoses, so you still need to explain the other 20-25%.
[1] https://www.nature.com/articles/s41380-018-0070-0
It is wrong to claim that ADHD is exclusively hereditary. Also, epigenetics show that behavior changes what is inherited. This is a chance and a responsibility. [1]
More broadly speaking, anything that is claimed to be "hereditary" (which for ADHD the scientific consensus is that it is not exclusively but estimated at around 70%) could also be early nuture and prenatal environment. The way genetic inheritance is researched (identical twins vs. fraternal twins) cannot account for this. [2]
[1] e.g.
"ADHD has been clearly linked with numerous environmental risk factors, particularly around the prenatal and perinatal period. Some of the most robust risk factors identified…
Read the full comment
are maternal prenatal health conditions and psychological distress (e.g. hypertension, obesity, pre-eclampsia, immune activation), in utero exposure to poor diet (with critical factors still being determined), teratogenic effects of certain medications (e.g. acetaminophen) and environmental exposures (e.g. lead), as well as neonatal factors such as prematurity and low birth weight [27]. Other extreme exposures in the postnatal environment (such as extreme infant emotional neglect) have also been associated with an ADHD syndrome [28, 29]."
Cecil, C. A. M., & Nigg, J. T. (2022). Epigenetics and ADHD: Reflections on Current Knowledge, Research Priorities and Translational Potential. Molecular diagnosis & therapy, 26(6), 581–606. https://doi.org/10.1007/s40291-022-00609-y
"The convincing evidence for genes as risk factors for ADHD does not exclude the environment as a source of etiology. The fact that twin estimates of heritability are less than 100% asserts quite strongly that environmental factors must be involved. ADHD’s heritability is high, and that estimate encompasses gene by environment interaction. Thus, it is possible that such interactions will account for much of ADHD’s etiology. Environmental risk factors likely work through epigenetic mechanisms, which have barely been studied in ADHD [148]. The importance of the environment can also be seen in the fact that, as for other complex genetic disorders, much of ADHD’s heritability is explained by SNPs in regulatory regions rather than coding regions [149]."
Faraone, S. V., & Larsson, H. (2019). Genetics of attention deficit hyperactivity disorder. Molecular psychiatry, 24(4), 562–575. https://doi.org/10.1038/s41380-018-0070-0
"Parental stress and parental depression were significantly associated with increased risk for ADHD overall and both symptoms and diagnosis. Specifically, maternal stress and anxiety, maternal prenatal stress, maternal depression, maternal post-partum depression, and paternal depression were positively associated with ADHD. In addition, parental depression was associated with symptoms of ADHD inattentive and hyperactive/impulsive subtypes. Parental antisocial personality disorder was also positively associated with ADHD overall and specifically ADHD diagnosis. Prenatal antidepressant usage was associated with ADHD when measured dichotomously only. These findings raise the possibility that prevention strategies promoting parental mental health and addressing parental stress could have the potential for positive long-term impacts on child health, well-being, and behavioral outcomes."
Robinson, L. R., Bitsko, R. H., O'Masta, B., Holbrook, J. R., Ko, J., Barry, C. M., Maher, B., Cerles, A., Saadeh, K., MacMillan, L., Mahmooth, Z., Bloomfield, J., Rush, M., & Kaminski, J. W. (2024). A Systematic Review and Meta-analysis of Parental Depression, Antidepressant Usage, Antisocial Personality Disorder, and Stress and Anxiety as Risk Factors for Attention-Deficit/Hyperactivity Disorder (ADHD) in Children. Prevention science : the official journal of the Society for Prevention Research, 25(Suppl 2), 272–290. https://doi.org/10.1007/s11121-022-01383-3
[2]
"twin studies fail to separate the effects of genes and the prenatal environment. This failure casts doubt on claims of the relative effects of genes and environment on intelligence, psychiatric disorders, personality and other psychological variables, and other conditions."
https://www.psychologytoday.com/gb/blog/looking-in-the-cultu...
"Although many twin studies have been conducted (which is quite an understatement; there are almost 9,000 hits for “twin study” on PubMed!), there have long been critics who argue that they are scientifically worthless."
Smith, Jinkinson. (2020). The debate over twin studies: an overview. http://dx.doi.org/10.22541/au.159674847.78026661
"Because heritability is defined by both genetic and environmental influences, it is not a fixed characteristic of a disease or trait, but a population-specific estimate, analogous to, for example, the mean height, cholesterol level or life expectancy in a population. It also cannot be interpretated at the family or individual level."
Kaprio J. (2012). Twins and the mystery of missing heritability: the contribution of gene-environment interactions. Journal of internal medicine, 272(5), 440–448. https://doi.org/10.1111/j.1365-2796.2012.02587.x
More: https://en.wikipedia.org/wiki/Twin_study#Criticism
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