We know that ADHD is a highly heritable condition (leading to the common joke by parents, (“Yes ADHD is heritable. I got it from my kids.”). In actuality, studies on genetic influence on specific symptoms, informing the likelihood of ADHD symptom progression, are now under way.
An original investigation, from the July 2005 JAMA Psychiatry Online, considers the effects of genetics on the change of symptoms of ADHD in a group of children with ADHD between the ages of 8 and 16. These researchers (Pingault et al), between 2002 and 2013, assessed 8395 identical and non-identical twin pairs with ADHD, who were identified from the Twins Early Development Study in England and Wales. The study sample was fairly representative of the UK population.
As might be expected, the trend was for DSM (IV) ratings to decline over time. However, there was greater variability with inattention as compared to hyperactivity and impulsivity. In a minority, inattention even increased over time. By using what are called ACE and ADE statistical genetic modelling, they claim that inattention is mostly explained by genetic influences, and is independent of the baseline level of symptoms.
It should be noted that this is the first study in such a behavioral genetic design to look at the respective impact of genetic and environmental factors on the developmental course of ADHD. Therefore, other studies will be needed to test its hypotheses and conclusions. Moreover, there remains a limitation which these authors acknowledge, and with which all population studies must deal. That is the complex effects of “non-shared environmental factors.” It is hoped that in large studies, such effects even out and are less significant. Nonetheless, no one can apply these results to particular individuals.
In summary, this study does provide an approximate measure of the rate of decline of symptoms in ADHD, and postulates genetic reasons for the different developmental course of inattention over time. It also suggests that unchanging symptoms may be associated with a more persistent course.