When It Comes to ADHD, DNA May Be the Key
It’s well known that ADHD runs in families. But recent research shows that it goes even further, all the way down to DNA: the more DNA two people have in common, the more likely they are to share an ADHD diagnosis (or a lack of one).
September 2, 2016
Familial aggregation of ADHD — or the clustering of the disorder within genetically related groups of people — increases significantly as DNA overlaps more and more, according to a small new study out of Sweden. So while it’s been established that ADHD is often passed down between parents and children, the results of this study indicate that it’s even more likely to co-occur between blood-related siblings, who share more DNA. Among siblings, fraternal twins are even more likely to share the disorder, and identical twins are even likelier still.
The study, conducted by a team composed of researchers from Karolinska Institutet and Örebro University, gathered data from three major Swedish databases (The Medical Birth Register, The Multi-Generation Register, and The Swedish Twin Register) to examine sibling pairs born between 1985 and 2006. From the databases, they identified more than 8,000 identical twin pairs, approximately 26,000 fraternal twin pairs, and more than 2 million non-twin sibling pairs. To round out their data, they also looked at more than 600,000 half-siblings (both maternal and paternal), 4 million full cousin pairs, and almost 100,000 half-cousin pairs. Of these individuals, almost 32,000 had been diagnosed with ADHD.
The data showed that identical twins were almost 60 percent more likely than fraternal twins to share an ADHD diagnosis, but that fraternal twins were still significantly more likely to share a diagnosis than non-twin sibling pairs. And compared to cousins, siblings were more than twice as likely to share an ADHD diagnosis; cousins had just a 10 percent chance of sharing an ADHD diagnosis, while siblings (of any kind) had an overall chance of 25.3 percent.
Interestingly, maternal half-siblings were significantly more likely than paternal half-siblings to share an ADHD diagnosis. The researchers hypothesize that this is related to the pre-natal environment that maternal half-siblings share; many environmental factors that occur during pregnancy are thought to contribute to an ADHD diagnosis.
These findings do more than just get us closer to figuring out the root causes of ADHD — though they do reinforce that DNA is a huge contributing factor. They also encourage doctors to adopt new diagnostic processes based on familial relations, the study’s authors say.
“Close family members of individuals with persistent ADHD represent an important target group for diagnostic screening,” they write. If one sibling is diagnosed with ADHD, in other words, it may be prudent to explore the possibility that the other sibling might have ADHD too, even though there may still be confounding environmental factors at play.
The results of the study were published in The Journal of Child Psychology and Psychiatry on August 22, 2016.