Some of the most common and highly heritable neurodevelopmental diseases in children are attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). Previous evidence has shown that there are genetic and molecular similarities between the mechanisms and manifestations of ADHD and ASD. In terms of heritability, some studies have shown an association between the two while others have questioned if such an association existed.
One way that this can be studied in further detail is by doing sibling studies. Recurrence of ADHD and/or ASD is common among siblings and is a good way to measure any shared genetic contributions. An important note in this situation is that studies have analyzed siblings born after a child with ASD/ADHD was diagnosed and the number of siblings in the family matters. For example, if there are fewer children born after the one with the prognosis, then the risk of recurrence is underestimated, so the best way to answer this limitation is to examine recurrence in families where at least 1 child was born after the diagnosis in an older sibling. This study explores cross-aggregation of ADHD and ASD among siblings born after previous diagnoses. The researchers extracted population data from families in the northwestern United States and included children ages 0 to 18 years. The researchers used an older sibling as the starting point (proband) of analysis for any given family and were either the sibling with a prognosis or the first-born child if there were no prognoses in that family.
After performing the appropriate statistical analyses, it was found that there was a higher recurrence of ASD in families where the older sibling had ASD as well compared to families with no prior diagnosis, which is to be expected. These results also applied to ADHD patients and families. One interesting find was that later-born children with ASD were more likely to develop ADHD compared to children who do not have that risk (meaning that the older child did not have any diagnosis). There could be multiple reasons for this phenomenon including the idea of refrigerator parenting with the second child. In other words, the first child could be receiving more attention and therefore the younger one may be overlooked. Another possibility is that parents are older when having later children. Maternal exposure to certain environmental factors or stress can also increase the risk of recurrence, which has been shown in other studies that analyze both ADHD and ASD. Another thing that this study found, which was surprising, was that boys had an increased risk overall to both disorders compared to girls. It would be interesting to see why this may be the case neurologically or biologically. Hopefully, in future research, both clinical and in the laboratory, it will be possible to further understand these differences and rid the world of such neurodevelopmental disorders.
Miller, M., Musser, E. D., Young, G. S., Olson, B., Steiner, R. D., & Nigg, J. T. (2019). Sibling Recurrence Risk and Cross-aggregation of Attention-Deficit/Hyperactivity Disorder and Autism Spectrum Disorder. JAMA pediatrics, 173(2), 147–152. https://doi.org/10.1001/jamapediatrics.2018.4076