This study aimed to determine the childhood antecedents and adolescent/adult outcomes of later (ages 12–25) ADHD diagnosis, compared to earlier (<12) and no diagnosis in females. Overall, the results indicate that although females with a later ADHD diagnosis generally had fewer difficulties than those with an earlier diagnosis (indicating a less severe phenotype in childhood), several childhood antecedents did not differ between these groups. Compared to females without ADHD, those receiving a later diagnosis already had clear evidence of childhood mental health and educational difficulties, increased healthcare usage, and socioeconomic deprivation. Several of these antecedents were more strongly associated in females than males. Despite these childhood differences indicating a less severe phenotype, by adolescence and early adulthood, females with a later diagnosis experienced more neurodevelopmental and mental health difficulties, teenage pregnancy, and healthcare contacts, compared to those diagnosed earlier. Many of these outcomes were exacerbated in females compared to males. All investigated adverse outcomes were more likely in females with a later diagnosis than no diagnosis, with generally stronger associations in females than males. Thus, our results demonstrate that a delay in ADHD diagnosis has profound and clear consequences by adolescence and disproportionately disadvantages females.

Our results support our first hypothesis that females with a later diagnosis have clear evidence of difficulties in childhood, which precede ADHD diagnosis. They had similar difficulties to females diagnosed earlier for anxiety, school absences, and emergency department visits. Compared to females without ADHD, they had a higher likelihood of recognised neurodevelopmental and mental health problems (autism, learning difficulties, conduct disorder, anxiety, anti-anxiety medication prescriptions), maternal ADHD and depression, educational problems (exam failure and school absences), and more contacts with healthcare services. As such, the presence of these childhood difficulties and healthcare service use, known to be associated with ADHD^39,40, can be considered as important early indicators that ADHD assessment may be warranted in primary school age females. Females with later diagnosed ADHD were also more likely to have experiences of the care system and socioeconomic deprivation compared to females without ADHD, consistent with previous research^41,42. These demographic groups are especially vulnerable to barriers to accessing care and to their health needs being overlooked. Overall, this evidence argues against the explanation of later diagnosis being a late-emerging ADHD phenotype that only has clinically significant impact later in life^43,44.

The generally elevated risks of childhood healthcare and educational difficulties in females with an earlier compared to a later diagnosis are consistent with previous research⁴⁵ and indicate that the most severely impacted females are more likely to reach ADHD services in Wales. However, our study clearly shows that those with fewer but nonetheless substantive difficulties are missing out on timely ADHD recognition and support.

The results generally support our second hypothesis, that females with later diagnosed ADHD exhibit worse healthcare, educational, and socioeconomic outcomes compared to both females with an earlier diagnosis and without ADHD, albeit with some exceptions, as discussed below. Despite the fact that the earlier diagnosed group had generally more difficulties in childhood, by adolescence, the later diagnosed group had more problems, especially with regards to mental health (including conduct disorder, anxiety, depression, self-harm, alcohol use, drug use, bipolar disorder, schizophrenia, anti-anxiety and antidepressant medication prescriptions). They were also at higher risk of teenage pregnancy, school absences, and more healthcare contacts. This suggests a potential protective effect of timely diagnosis and treatment on these later outcomes.

However, the later diagnosed group were not at higher risk for all outcomes, as the earlier diagnosed group continued to have poorer educational outcomes (more exam failure and low pass at key stage 4) and greater socioeconomic deprivation. These findings may reflect that those diagnosed later could have the impact of their ADHD symptoms somewhat buffered by relative advantages in academic ability and family socioeconomic resources⁴³, whereas the earlier diagnosed group have more learning difficulties and pre-existing family adversity. The groups also did not differ for several health outcomes (including adolescent neurodevelopmental conditions, eating disorders, antipsychotic medication prescriptions, or GP contacts). This suggests that even an earlier diagnosis cannot fully mitigate against the mental health risks conferred by ADHD, and that these emerging risks are not being sufficiently proactively identified in those diagnosed earlier. Whole-system care (encompassing healthcare, social care and educational services) needs to improve to address ADHD inequalities.

Females with later diagnosed ADHD had poorer outcomes than those without ADHD on every outcome examined, across mental health (including substance misuse) and education, and for teenage pregnancy. This is critically important. Educational failure and poor mental health (including substance misuse and severe and enduring mental illness such as schizophrenia and bipolar disorder) come at substantial and wide-ranging costs to individuals, their families, and the economy. The exceptionally high prevalence of depression (51.4%) anxiety (38.1%), and self-harm (36.2%) in the later diagnosed group should be noted. It is important to also recognise the impact on future generations as these adverse outcomes are likely to contribute to intergenerational transmission of adversity and inequality.

Although the focus of this paper was primarily on females with ADHD due to the historical neglect of this group (both clinically and in research studies), we also examined males. The pattern of results was very similar and therefore, many of the results discussed above are also relevant to later diagnosis of ADHD in males. Although females are more likely to be diagnosed later, there are still many males who are impacted by late diagnosis and thus experience consequences from delayed support. We found a number of key sex differences. Conduct disorder was a particularly strong indicator of earlier diagnosis in females, which has been shown before⁴⁶. Maternal ADHD was more likely among earlier than later diagnosed males, with no difference in females. This likely reflects small numbers with limited power to detect sex differences and also, we cannot differentiate between timing of maternal and offspring diagnoses. Childhood autism, exam failure and healthcare contacts were more strongly associated with later diagnosis compared to no diagnosis in females than males, indicating that these are especially important indicators of likely ADHD in females.

A key observation in our study is that where there were sex differences in adolescent outcomes, many of the associations were stronger in females. The risk of negative outcomes in later diagnosed females compared to earlier diagnosed females was greater than seen in males for depression, self-harm, drug use, bipolar disorder, antidepressant prescriptions, exam failure, and secondary healthcare contacts. In the comparison of later diagnosed females against those without ADHD, the associations were stronger than in males for autism, learning difficulties, anxiety, depression, self-harm, alcohol use, drug use, bipolar disorder, antidepressant prescriptions, school absences, and all healthcare contacts. This demonstrates that the consequences of later diagnosis are likely to have a larger impact on females, particularly in terms of mental health problems, leading to greater use of healthcare resources.

Our study has several notable strengths; we used a nationwide register of all individuals with ADHD in Wales, with good coverage, including both primary and secondary healthcare records, as well as linked educational and demographics variables, with detailed information on age at first diagnoses. In terms of limitations, routinely collected administrative data have variable accuracy and are not designed for research purposes, which may lead to biases. In particular, our study only includes individuals with ADHD who reached clinical services and received a diagnosis and so many individuals with undiagnosed ADHD will be among the non-ADHD group, although this misclassification would serve to reduce differences between our three cohorts, suggesting that group differences are robust. Other limitations are that the population coverage was lower for some of the linked databases (e.g. educational, maternal, and ethnicity data only from 2011 via ONS), we lacked linkage to biological fathers, and we were only able to look at biological sex and not gender identity.