From Early Behaviours to Later Diagnoses: Tracking ADHD in Children with Autism and Intellectual Disability

From Early Behaviours to Later Diagnoses: Tracking ADHD in Children with Autism and Intellectual Disability

Background

While autism and attention deficit/hyperactivity disorder (ADHD) are often discussed separately, 40% to 70% of autistic children also struggle with significant ADHD symptoms. Both conditions can impact social, academic, and daily life. Autism is characterised by difficulties in social connection and communication, along with repetitive behaviours and restricted interests, while ADHD involves challenges with attention and hyperactivity. The three subtypes of ADHD include the predominantly inattentive, the predominantly hyperactive/impulsive, and the combined types. As children mature, hyperactive/impulsive symptoms become less prominent, and attention problems come to the fore, especially in school settings. As a result, children with an initial predominantly hyperactive/impulsive diagnosis later fall into the combined category.

Before the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) was published in 2013, introducing the idea that autism and ADHD often go together, people could not be diagnosed with autism and ADHD simultaneously. Because of this old diagnostic rule, there is very little research into the early signs that predict if a young child with autism will go on to develop ADHD. A few studies, however, assessed ADHD symptoms in early childhood, when these behaviours may first appear in autistic children. The Childhood Autism Risks from Genetics and the Environment (CHARGE) study, for example, examined a large group of children aged 2-5 years. Researchers found that ADHD symptoms were most common in the autistic group, followed by those with developmental delays without autism, and the least common in typically developing children, highlighting the overlap of autism and ADHD. Since early intervention can effectively improve symptoms and prevent long-term difficulties in a child’s social and academic life, identifying the early developmental indicators that predict the specific type of ADHD a child may develop as they get older can be critical.

Approach

The Revisiting CHARGE (ReCHARGE) study, led by Ren and colleagues from the University of California, followed up on 645 children from the initial CHARGE study in middle childhood or adolescence. By revisiting the participants between the ages of 8 and 20, the researchers were able to track how their early childhood autism or developmental delay diagnosis predicted their probability of a formal ADHD diagnosis in later years.

The CHARGE project grouped children into the following four developmental categories:

1. Autism: This group included children diagnosed with autism using gold-standard clinical assessments (Autism Diagnostic Observation Schedule, ADOS, Autism Diagnostic Interview-Revised, ADI-R).

2. Developmental Delay (DD): These children did not meet the criteria for autism but displayed significant intellectual disability and challenges in daily functioning, measured with the Mullen Scales of Early Learning and the Vineland Adaptive Behaviour Scales.

3. Typical Development (TD): This group represented children without intellectual or functional disabilities and without autism.

4. Other Early Concerns (OEC): Children in this group were either identified as having an elevated likelihood of developmental differences or had a previous community diagnosis of autism or developmental delay. However, they did not meet the strict criteria for an autism or a developmental delay diagnosis, yet they also did not qualify as typically developing.

These groups allowed the researchers of the reCHARGE project to track how different early developmental paths influenced the likelihood of a child developing ADHD—whether inattentive, combined, or hyperactive/impulsive subtype—in middle childhood and adolescence. The authors used DSM-IV and DSM-5 criteria to establish an ADHD diagnosis, integrating information from parent clinical interviews (e.g., Mini-International Neuropsychiatric Interview), standardised rating scales (e.g., Conners’ Rating Scale-CPRS), and direct behavioural observations. Once the final diagnoses were reached, the team performed a series of statistical analyses to determine how a child’s early developmental status could predict a later diagnosis of ADHD and its specific subtypes.

To make the findings as reliable as possible, the statistical models also accounted for other important influences. These included socioeconomic factors, such as the family’s education, the mother’s health during pregnancy, and birth-related variables, including birth weight or being born prematurely. They also looked at parental histories of ADHD, since a family history of the condition can increase a child’s likelihood of a diagnosis.

Key Findings

The reCHARGE study found early childhood developmental status to be the strongest predictor of a later ADHD diagnosis. Among the 645 participants, 213 (33%) met the criteria for ADHD—a rate nearly triple the 12.9% typically seen in the general population. This overall high percentage is primarily due to focusing on groups with a higher probability of an ADHD diagnosis, such as children with autism and developmental delays. Notably, even the control group’s rate was higher than expected, likely because it had an overrepresentation of boys—who are generally diagnosed with ADHD more frequently than girls—and families who remained in the study possibly due to ongoing developmental concerns. Children with autism were over three times more likely to develop ADHD than their typically developing peers, with nearly half (46.2%) of autistic children affected. Children with developmental delay (DD) and other early concerns (OEC) both had a 2.4 times higher likelihood of ADHD.

The study showed that the children in each early childhood developmental category were prone to specific ADHD subtypes. The likelihood of presenting with the Combined subtype was the highest for the autism group. Autistic children were 5.4 times more likely to receive a diagnosis of this ADHD subtype, followed by the DD group, who had a 4.4 times higher chance of presenting with the Combined subtype of the condition.

The Inattentive subtype was most common in the OEC group, affecting 21.2%. The authors suggest that the early signs of attention issues in the OEC children may have been misinterpreted as autism or developmental delay symptoms when they were younger.

In addition to the early developmental status, the study revealed other factors that significantly influenced the likelihood of an ADHD diagnosis. For example, having a parent with a history of ADHD doubled the likelihood of a child developing the Combined subtype. The probability of presenting with the Combined and Hyperactive/Impulsive subtypes declined with age, showing about a 7% decrease per year. Moreover, girls were generally less likely than boys to be diagnosed with the Combined subtype.

The study further highlighted that children across all groups often face multiple challenges, including learning disabilities, anxiety, and depression. Learning disabilities were the most prevalent in the DD group (44%), followed by the autism group (38.7%). The OEC group showed unexpectedly high rates of depression (29.3%) and anxiety (22.2%).

Limitations and Future Directions

The authors identified several limitations to their research and proposed future directions to improve the understanding of ADHD in children with autism and intellectual disability. Due to resource constraints, a primary limitation was the absence of teacher input on school-related symptoms. The study relied on parent proxy reports rather than direct observations in the classroom, which is precisely where symptoms of inattention tend to become most apparent as children grow older. To address this, the authors recommend that future research combine reports from parents, teachers, and clinicians, to better distinguish between overlapping symptoms.

Two factors influenced data accuracy. First, 11.36% of the children were on ADHD medication at the time of the follow-up, which may have led parents to underreport current symptoms. However, the authors noted that since most children were on short-acting stimulants that wear off quickly, parents likely still observed and reported these symptoms. Second, data on learning disabilities relied on parent reports rather than verified school records, which may have led to overreporting or misinterpretation of the term.

Additionally, a large subset of participants with significant intellectual and functional disability were excluded because the standard clinical interviews are not designed to assess ADHD within this group accurately. Their exclusion may have introduced bias and highlights a critical need for assessment tools specifically tailored to this population.

The authors also acknowledged that the OEC and Hyperactive/Impulsive groups were relatively small and mixed, making specific results for those subgroups less reliable. Nevertheless, the OEC group showed unexpectedly high rates of depression and anxiety, suggesting this group requires more dedicated research to understand their unique mental health needs.

While the study followed the children over many years, it conducted only a single follow-up assessment per child. This approach meant the team could not observe how ADHD symptoms change year-by-year, underscoring the need for future longitudinal research to map out how different ADHD subtypes develop over time.

Finally, while the study confirmed that autistic children are at exceptionally high odds of developing ADHD, there is currently not enough research that explores how to treat both conditions at once. The authors recommend addressing this gap by developing novel interventions specifically designed for children who exhibit significant ADHD symptoms alongside autism or developmental delays.

Conclusion

The key takeaway of the reCHARGE study is that a child’s developmental status between the ages of 2 and 5 serves as the strongest predictor of a future ADHD diagnosis. The findings confirm that for children with autism or developmental delays, the likelihood of ADHD is not random, but can be identified years in advance. Given that autistic children are over three times more likely to develop ADHD than their typically developing peers, the authors recommend that clinicians move towards more proactive, periodic screenings for the condition. This kind of repeated evaluation is particularly important because inattentive symptoms—whether alone or part of the Combined subtype—are the most common in these children. Rather than waiting for academic or social struggles to peak in adolescence, clinicians should monitor for ADHD-related challenges throughout the child’s entire development. By identifying children with an elevated likelihood of ADHD early, families and educators can implement support strategies and interventions sooner, which is critical for improving daily functioning and long-term success.

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