ADHD seems to be caused by both genetic and environmental factors, but their interaction is unclear. This editorial summarizes research in this field.
In the 21st century, it has been generally established that attention deficit hyperactivity disorder (ADHD) has a heritability of about 80%, based on estimations from twin and family studies. At the same time, our understanding of specific genes that may cause ADHD is lacking, with hardly any gene candidates with high effect sizes for the risk of developing ADHD. More recently, detailed polygenic studies have shown some evidence that about 22% of the genetic liability of ADHD is caused by the heritability of single-nucleotide polymorphisms (SNPs). This editorial, published in the American Journal of Psychiatry, analyzes the relationship between genetic and environmental/developmental factors on the onset of ADHD, as presented in previous research.
The History of ADHD Research: Genes vs. Environment
Previous studies examining environmental risk factors on ADHD onset have had various limitations. Initial studies on the role of, for example, smoking during pregnancy, or formula feeding versus breastfeeding, did not take into account the mother’s psychopathology or genetic risk. These factors, in combination with environmental factors, could contribute to ADHD onset jointly but not in isolation. Studies have also shown that individuals with ADHD are more likely to smoke, drink alcohol, and use illegal drugs, thus making disentangling these risk factors difficult.
One specific example is preterm birth, which has a consistent correlation with increased ADHD risk. However, it is unclear if mothers with a genetic risk of ADHD are more prone to give birth preterm, whether something about preterm birth increases the risk of ADHD, or whether fetuses in whom the genetic risk of ADHD is already present tend to be born prematurely. The polygenic evidence related to ADHD makes it clear that a large number of other factors may relate to ADHD risk, including obesity, gestational diabetes mellitus, autoimmune disorders, and allergic disorders.
Avenues of New Research into ADHD: Polygenic Risk Factors and Gene–Environment Interaction
The author closes by highlighting a study by Brikell et al. that discusses the interaction between ADHD polygenic risk scores, ADHD diagnosis, and environmental risk. This study relied on data from 13,697 ADHD case subjects and 21,578 control subjects. The study analyzed 24 environmental risk factors and polygenic risk scores and showed significant correlations between ADHD polygenic risk scores and ADHD risk, as well as an association with environmental risk factors such as maternal autoimmune disorders, induction history, and mild traumatic brain injury. This seems to indicate that individuals with a higher ADHD polygenic risk score who are then exposed to additional environmental risk factors have a higher chance of developing ADHD. Additional research on the way that environmental factors can influence gene expression could provide additional insight into the pathomechanisms of ADHD.
Kittel-Schneider, S. (2023). ADHD: The Mammoth Task of Disentangling Genetic, Environmental, and Developmental Risk Factors. American Journal of Psychiatry, 180(1), 14-16. https://doi.org/10.1176/appi.ajp.20220916