Asked what they know about attention-deficit hyperactivity disorder, or ADHD, many people will likely tell you that it mostly affects children, and mostly boys. However, recent research has shown that neither of these perceptions is entirely true.
There is a striking difference in the sex of children diagnosed with ADHD, with boys more likely to be diagnosed than girls (the ratios can be as high as 9:1 in some studies). However, these studies are of children who have an established diagnosis of ADHD, and such estimates are affected by referral patterns (for example, parents may be more likely to take their sons in for an ADHD assessment), so they may not reflect the true sex ratio.
Indeed, when we estimate the occurrence of ADHD in the population as a whole, rather than just in children at clinics, we find that a lot more girls meet diagnostic criteria than is reflected in the estimates from clinics. The same equalising trend between the sexes is visible when looking at adults with a diagnosis of ADHD. Taken together, this suggests that there are a substantial number of girls with ADHD going undiagnosed in childhood, with potentially serious implications for the effects of their untreated symptoms in childhood, adolescence and adulthood.
Why are girls less likely to be diagnosed?
One reason that fewer girls are diagnosed with ADHD is that girls may be more likely to have the inattentive-type ADHD symptoms, rather than the hyperactive and impulsive symptoms that are more common in boys. The issue is that while inattention and an inability to focus will cause problems for a child, such symptoms may be less disruptive and noticeable for parents or teachers, which means that these children’s ADHD may go unrecognised.
Considering that diagnostic criteria were created based on studies of boys, they are likely to be better geared towards the identification of ADHD in males. This has led to a stereotypical image of ADHD as a “disruptive boy”, even though it is becoming more widely recognised that ADHD also affects large numbers of females and adults.
If a male stereotype is seen as the norm, potentially only the girls with the most severe, or most “male-like”, symptoms that manifest as disruptive behaviour will be identified. We cannot definitively say that affected girls are not getting referred to clinics, but if they are, and if the symptoms of their ADHD are somewhat different to those seen in boys, they may well receive alternative diagnoses, such as anxiety or depression, instead.
In our study, published in the European Journal of Child and Adolescent Psychiatry, we aimed to identify which symptoms were the best predictors of an ADHD diagnosis and the likelihood of receiving medication, and whether these differed between boys and girls.
We used a large population dataset, the Child and Adolescent Twin Study from Sweden, which could be linked with Swedish registries holding information on individuals who had received a diagnosis of ADHD and been prescribed stimulant medication for ADHD. This means that we were able to link population data with clinical data, without needing to look only in clinics, where ADHD patients are more frequently boys.
True to our expectations, what we found was that hyperactivity, impulsivity and behaviour problems in girls were stronger predictors of clinical diagnosis and being prescribed medication than in boys.
This suggests that these sorts of behaviours are more likely to lead to clinical recognition of ADHD among girls. It supports the idea that unless girls with ADHD display more of these disruptive behaviours associated with the stereotypical image of the condition, they may be more likely to be missed. This highlights potential issues with the male-centric nature of the current ADHD diagnostic criteria and current clinical practice.
When we looked at the presentation of ADHD in the population, we found that the inattentive presentation was most common across both sexes. But among those that had been clinically diagnosed, a combination of both inattentive and hyperactive or impulsive symptoms was most common. What this again points to is that people with primarily inattentive symptoms may be less likely to be diagnosed with ADHD as children.
We also found that a greater percentage of girls than boys presented with predominately inattentive symptoms at the whole population level. Since children with inattentive symptoms are sometimes overlooked, this could partially explain why the ratio of boys to girls diagnosed with ADHD is higher than the estimated ratio for ADHD occurrence in the population as a whole.
Identifying undiagnosed ADHD
ADHD is associated with a wide-range of functional impairments, educational and occupational difficulties, family and social relationship problems, and problematic substance use. When it goes unrecognised, opportunities to provide treatment are lost, which can lead to worse long-term outcomes. As such, it is important to ensure that girls with ADHD are identified and treated in childhood.
It is clear that we need to work towards a better understanding of how ADHD manifests in girls as, while less visible or disruptive, inattentive symptoms can be very impairing, potentially over an entire lifespan. Given that the diagnostic criteria are primarily based on studies in boys, we need more studies to look at ADHD in girls, to develop better instruments to assess and diagnose it that are more sensitive to the way it affects females.
Why is ADHD more common in boys than girls?