Across the globe, ADHD prevalence is estimated around 5 per cent. It’s a figure that’s been rising for decades. For example, Sweden saw ADHD diagnoses among 10-year olds increase more than sevenfold from 1990 to 2007. Similar spikes have been reported from other countries, too, including Taiwan and the US, suggesting this may be a universal phenomenon. In fact, looking at dispensed ADHD medication as a proxy measure of ADHD prevalence, studies from the UK show an even steeper increase.
Does this mean that more people today really have ADHD than in the past? Not necessarily. For example, greater awareness by clinicians, teachers or parents could have simply captured more patients who had previously had been “under the radar”. Such a shift in awareness or diagnostic behaviour would inflate the rate of ADHD diagnoses without necessarily more people having clinical ADHD. However, if this is not the true or full explanation, then perhaps ADHD symptoms really have become more frequent or severe over the years. A new study in The Journal of Child Psychology and Psychiatry from Sweden with almost 20,000 participants has now provided a preliminary answer.
The researchers, led by Mina Rydell at Karolinska Institutet, examined data from participants in an ongoing study of all twins in Sweden that started in 2004 and aims to study their physical and mental health, with various measures taken the year that the children turn nine years of age.
Specifically, the researchers analysed A-TAC (Autism-Tics, ADHD and other Comorbidities Inventory) scores from 19,271 children from 9,673 families recorded between 2004 and 2014. The A-TAC is a telephone-based interview in which parents are quizzed about their kids’ behaviour and mental health, including sub-scales focused on attention deficits and hyperactivity. The questions are about symptoms with no mention of diagnostic categories and the wording has stayed the same over the years. A typical question is “Does he/she have difficulties keeping his/her hands and feet still or can he/she not stay seated?”.
The researchers used the A-TAC scores to classify the proportion of children in different years with diagnostic-level ADHD or subthreshold ADHD or no ADHD. Important to keep in mind here is that instruments like the A-TAC are restricted to assessing the severity of certain symptoms and cannot be used to diagnose children with ADHD (only clinicians and mental health experts can diagnose someone). For example, if a child fell in the diagnostic-level ADHD category, it would mean that the severity of his or her ADHD symptoms would likely result in a diagnosis by a specialist, but this couldn’t be known for sure. The authors calculated the changes in these categories, as well as in mean A-TAC scores, over time by comparing results from the parent interviews conducted in 1995-1998, 1992-2002 and 2003-2005.
Across the 10-year study period, 2.1 per cent of all participants (n=406) showed diagnostic-level ADHD and 10.7 per cent (n=2,058) showed subthreshold level ADHD. Interestingly, there was no statistically significant increase in diagnostic-level ADHD prevalence over time, fluctuating around 2 per cent in most years. On the other hand, the prevalence of sub-threshold ADHD increased significantly from 2004 to 2014, when at 14.76 per cent it reached its peak. Mean ADHD scores and inattention/ hyperactivity-impulsivity sub-scale scores also showed a similar increase from 1994 to 2004.
These symptom changes over time are probably not due to changes during the study in the status of the twin families who agreed to take part in the research and those who didn’t. The researchers accessed the National Patient Register and this showed that while participants in the twin study differed from non-participants in terms of having fewer ADHD diagnoses, this difference did not change over the years of the study, suggesting that it was unlikely to explain the results. Perhaps most important, the National Patient Register showed that prevalence of clinician-diagnosed ADHD had increased more than fivefold from 2004 to 2014, which is inconsistent with the fact that the twin study found diagnostic-level ADHD prevalence did not see a similar rise.
So while the diagnosis rates of clinical ADHD increased during the period of the study, the findings from the twin study suggest that only milder forms of ADHD-related symptoms became more frequent across the population during the same years. This suggests that the number of people who have such severe ADHD symptoms that it merits a diagnosis has actually remained stable, and that other factors are more probably the driving force behind an increased ADHD prevalence. While speculative, these could be related to changes in awareness among parents, teachers or clinicians; societal or medical norms; or better access to healthcare.
There are several caveats that need to be kept in mind when interpreting these findings. For example, as mentioned, A-TAC relies on parents’ reports, which might not be the most adequate source of information. In fact, a diagnosis of ADHD requires symptom impairments in at least two different contexts, such as at school or at home. Because only twins were enrolled in CATSS, it is also not clear whether these results also apply to only children. A similar argument could be made about the age of the participants.
Keeping its limitations in mind, this study highlights an important point by providing an alternative explanation for rising ADHD diagnoses. This demonstrates the effects that shifts in societal, political or medical opinion can have on the “prevalence” of an illness. Considering that more diagnoses are likely to go hand in hand with more (potentially unnecessary) medication, this study provides food for thought to clinical and political decision-makers.